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. Invited Review . February 2019 Vol.62 No. 2: 029506 Special Issue: The X-ray Timing and Polarimetry Frontier with eXTP https://doi.org/10.1007/s11433-017-9186-1

Observatory science with eXTP

Jean J. M. in ’t Zand1*, Enrico Bozzo2, JinLu Qu3, Xiang-Dong Li4, Lorenzo Amati5, Yang Chen4, Immacolata Donnarumma6,7, Victor Doroshenko8, Stephen A. Drake9, Margarita Hernanz10, Peter A. Jenke11, Thomas J. Maccarone12, Simin Mahmoodifar9, Domitilla de Martino13, Alessandra De Rosa7, Elena M. Rossi14, Antonia Rowlinson15,16, Gloria Sala17, Giulia Stratta18, Thomas M. Tauris19, Joern Wilms20, XueFeng Wu21, Ping Zhou15,4, Ivan´ Agudo22, Diego Altamirano23, Jean-Luc Atteia24, Nils A. Andersson25, M. Cristina Baglio26, David R. Ballantyne27, Altan Baykal28, Ehud Behar29, Tomaso Belloni30, Sudip Bhattacharyya31, Stefano Bianchi32, Anna Bilous15, Pere Blay33, Joao˜ Braga34, Søren Brandt35, Edward F. Brown36, Niccolo` Bucciantini37, Luciano Burderi38, Edward M. Cackett39, Riccardo Campana5, Sergio Campana30, Piergiorgio Casella40, Yuri Cavecchi41,25, Frank Chambers15, Liang Chen42, Yu-Peng Chen3,Jer´ omeˆ Chenevez35, Maria Chernyakova43, ChiChuan Jin44, Riccardo Ciolfi45,46, Elisa Costantini1,15, Andrew Cumming47, Antonino D’A`ı48, Zi-Gao Dai4, Filippo D’Ammando49, Massimiliano De Pasquale50, Nathalie Degenaar15, Melania Del Santo48, Valerio D’Elia40, Tiziana Di Salvo51, Gerry Doyle52, Maurizio Falanga53, XiLong Fan54,55, Robert D. Ferdman56, Marco Feroci7, Federico Fraschetti57, Duncan K. Galloway58, Angelo F. Gambino51, Poshak Gandhi59, MingYu Ge3, Bruce Gendre60, Ramandeep Gill61, Diego Gotz¨ 62, Christian Gouiffes` 62, Paola Grandi5, Jonathan Granot61, Manuel Gudel¨ 63, Alexander Heger58,64,121, Craig O. Heinke65, Jeroen Homan66,1, Rosario Iaria51, Kazushi Iwasawa67, Luca Izzo68, Long Ji8, Peter G. Jonker1,69, Jordi Jose´17, Jelle S. Kaastra1, Emrah Kalemci70, Oleg Kargaltsev71, Nobuyuki Kawai72, Laurens Keek73, Stefanie Komossa19, Ingo Kreykenbohm20, Lucien Kuiper1, Devaky Kunneriath74, Gang Li3, En-Wei Liang75, Manuel Linares17, Francesco Longo76, FangJun Lu3, Alexander A. Lutovinov77, Denys Malyshev8, Julien Malzac78, Antonios Manousakis79, Ian McHardy59, Missagh Mehdipour1, YunPeng Men80, Mariano Mendez´ 81, Roberto P. Mignani82, Romana Mikusincova83, M. Coleman Miller84, Giovanni Miniutti85, Christian Motch86, Joonas Nattil¨ a¨87, Emanuele Nardini88, Torsten Neubert35, Paul T. O’Brien89, Mauro Orlandini5, Julian P. Osborne89, Luigi Pacciani7, Stephane´ Paltani2, Maurizio Paolillo90, Iossif E. Papadakis91, Biswajit Paul92, Alberto Pellizzoni93, Uria Peretz29, Miguel A. Perez´ Torres94, Emanuele Perinati8, Chanda Prescod-Weinstein95, Pablo Reig96, Alessandro Riggio38, Jerome Rodriguez62, Pablo Rodr´ıguez-Gil33,97, Patrizia Romano30, Agata Ro´za˙ nska´ 98, Takanori Sakamoto99, Tuomo Salmi87, Ruben Salvaterra82, Andrea Sanna38, Andrea Santangelo8, Tuomas Savolainen100,101, Stephane´ Schanne62, Hendrik Schatz102, LiJing Shao19, Andy Shearer103, Steven N. Shore104,105, Ben W. Stappers106, Tod E. Strohmayer9, Valery F. Suleimanov8, Jir˘´ı Svoboda74, F.-K. Thielemann107, Francesco Tombesi9,

* Corresponding author (email: [email protected])

⃝c Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018 phys.scichina.com link.springer.com J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-2

Diego F. Torres10, Eleonora Torresi5, Sara Turriziani108, Andrea Vacchi109,110, Stefano Vercellone30, Jacco Vink15, Jian-Min Wang3, JunFeng Wang111, Anna L. Watts15, ShanShan Weng112, Nevin N. Weinberg113, Peter J. Wheatley114, Rudy Wijnands15, Tyrone E. Woods58, Stan E. Woosley115, ShaoLin Xiong3, YuPeng Xu3, Zhen Yan42, George Younes116, WenFei Yu42, Feng Yuan42, Luca Zampieri117, Silvia Zane118, Andrzej A. Zdziarski79, Shuang-Nan Zhang3, Shu Zhang3, Shuo Zhang119, Xiao Zhang4, and Michael Zingale120

1 SRON Netherlands Institute for Space Research, Utrecht 3584 CA, The Netherlands; 2 Department of Astronomy, University of Geneva, Versoix 1290, Switzerland; 3 Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China; 4 School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China; 5 INAF-IASF Bologna, via P. Gobetti 101, Bologna 40129, Italy; 6 ASI, Via del Politecnico snc, Roma 00133, Italy; 7 INAF-Istituto di Astrofisica e Planetologie Spaziali, Roma 00133, Italy; 8 Institut f¨urAstronomie und Astrophysik T¨ubingen, Universit¨atT¨ubingen, T¨ubingen 72076, Germany; 9 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; 10 ICREA & Institute of Space Sciences (IEEC-CSIC), Campus UAB, Carrer de Can Magrans, Barcelona 08193, Spain; 11 CSPAR, SPA University of Alabama in Huntsville, Huntsville, AL 35805, USA; 12 Department of Physics, Texas Tech University, Lubbock, TX 79409, USA; 13 INAF Osservatorio Astronomico di Capodimonte, Napoli 80131, Italy; 14 Leiden Observatory, Leiden University, Leiden 2300 RA, The Netherlands; 15 Anton Pannekoek Institute, University of Amsterdam, Amsterdam 1098 XH, The Netherlands; 16 ASTRON Netherlands Institute for Radio Astronomy, Dwingeloo 7990 AA, The Netherlands; 17 Universitat Politecnica de Catalunya (UPC-IEEC), Barcelona 08034, Spain; 18 Universit´adegli Studi di Urbino ‘Carlo Bo’, Urbino 61029, Italy; 19 Max-Planck-Institut f¨urRadioastronomie, Bonn 53121, Germany; 20 Dr. Karl-Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics (ECAP), Friedrich Alexander Universit¨atErlangen-Nurnberg, Bamberg 96049, Germany; 21 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China; 22 Instituto de Astrofys´ıcade Andalucia - CSIC, Granada 18008, Spain; 23 Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK; 24 IRAP, Universit´ede Toulouse, CNRS, UPS, CNES, Toulouse 31028, France; 25 Mathematical Sciences and STAG Research Centre, University of Southampton, Southampton SO17 1BJ, UK; 26 New York University Abu Dhabi, Abu Dhabi, UAE; 27 Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA; 28 Physics Department, Middle East Technical University, Ankara 06531, Turkey; 29 Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel; 30 INAF Osservatorio Astronomico di Brera, Merate 23807 (LC), Italy; 31 Tata Institute of Fundamental Research, Mumbai 400005, India; 32 Dipartimento di Matematica e Fisica, Universit`adegli Studi Roma Tre, Roma 00146, Italy; 33 Instituto de Astrof´ısicade Canarias, Tenerife 38205, Spain; 34 Instituto Nacional de Pesquisas Espaciais - INPE, S.J. Campos-SP 12227-010, Brazil; 35 National Space Institute, Technical University of Denmark (DTU Space), 2800 Kgs. Lyngby, Denmark; 36 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA; 37 INAF Osservatorio Astrofisico di Arcetri, Firenze 50125, Italy; 38 Dipartimento di Fisica, Universit´adegli Studi di Cagliari, Monserrato 09042, Italy; 39 Department of Physics and Astronomy, Wayne State University, Detroit, MI 48201, USA; 40 INAF Osservatorio Astronomico di Roma, Roma 00078, Italy; 41 Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA; 42 Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China; 43 School of Physical Sciences, Dublin City University, Dublin 9, Ireland; 44 Max-Planck-Institut f¨urExtraterrestrische Physik, Giessenbachstrasse, Garching 85748, Germany; 45 INAF, Osservatorio Astronomico di Padova, Padova 35122, Italy; 46 INFN-TIFPA, Trento Institute for Fundamental Physics and Applications, Trento 38123, Italy; 47 Department of Physics and McGill Space Institute, McGill University, Montreal, QC H3A 2T8, Canada; 48 INAF/IASF Palermo, Palermo 90146, Italy; 49 INAF - Istituto di Radioastronomia, Bologna 40129, Italy; 50 Department of Astronomy and Space Sciences, Istanbul University, Beyazit 34119, Turkey; J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-3

51 Universit´adegli Studi di Palermo, Dipartimento di Fisica e Chimica, Palermo 90123, Italy; 52 Armagh Observatory, Armagh BT61 9DG, N. Ireland; 53 International Space Science Institute (ISSI), Bern 3012, Switzerland; 54 School of Physics and Electronics Information, Hubei University of Education, Wuhan 430205, China; 55 SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK; 56 Faculty of Science, University of East Anglia, Norwich NR4 7TJ, UK; 57 Department of Planetary Sciences and Astronomy, University of Arizona, Tucson, AZ 85721, USA; 58 School of Physics and Astronomy, Monash University, Clayton VIC 3800, Australia; 59 Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK; 60 University of the Virgin Islands, Virgin Islands 00802-9990, USA; 61 Department of Natural Sciences, The Open University of Israel, Raanana 4353701, Israel; 62 CEA Paris-Saclay, DRF/IRFU/D´epartementd’Astrophysique, Laboratoire AIM (UMR 7158), Gif sur Yvette 91191, France; 63 Department of Astrophysics, University of Vienna, Vienna 1180, Austria; 64 School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA; 65 Department of Physics, University of Alberta, Edmonton AB T6G 2E1, Canada; 66 Eureka Scientific, Inc., Oakland, CA 94602, USA; 67 ICREA & Institut de Ci´enciesdel Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Barcelona 08028, Spain; 68 Instituto de Astrofisica de Andalucia (IAA-CSIC), Granada 18008, Spain; 69 Department of Astrophysics, IMAPP, Radboud University, Nijmegen 6500 GL, The Netherlands; 70 Faculty of Engineering and Natural Sciences, Sabanci University, Orhanli-Tuzla, Istanbul 34956, Turkey; 71 Department of Physics, The George Washington University, Washington, DC 20052, USA; 72 Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan; 73 Department of Astronomy, University of Maryland, Park, MD 20771, USA; 74 Astronomical Institute, Academy of Sciences, Prague 14131, Czech Republic; 75 GXU-NAOC Center for Astrophysics and Space Sciences, Department of Physics, Guangxi University, Nanning 530004, China; 76 Dip. di Fisica, Universit´adi Trieste and INFN, Trieste 34127, Italy; 77 Space Research Institute of the Russian Academy of Sciences, Moscow 117997, Russia; 78 IRAP, Universit´ede Toulouse, CNRS, UPS, CNES, Toulouse 31028, France; 79 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Warszawa 00-716, Poland; 80 Department of Astronomy, School of Physics, Peking University, Beijing 100871, China; 81 Kapteyn Astronomical Institute, University of Groningen, Groningen 9700 AV, The Netherlands; 82 INAF Istituto di Astrofisica Spaziale e Fisica Cosmica, Milano 20133, Italy; 83 Institute of Theoretical Physics, Faculty of Mathematics and Physics, Charles University, Praha 180 00, Czech Republic; 84 Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, Park, MD 20742-2421, USA; 85 Centro de Astrobiologia (CSIC-INTA), Dep. de Astrofsica, ESAC, Madrid 28850, Spain; 86 Universit´ede Strasbourg, CNRS, Observatoire Astronomique, UMR 7550, Strasbourg 67000, France; 87 Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Piikki¨o21500, Finland; 88 INAF Osservatorio Astrofisico di Arcetri, Firenze 50125, Italy; 89 Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK; 90 Dipartimento di Fisica Ettore Pancini, Universit´adi Napoli Federico II, via Cintia, Napoli 80126, Italy; 91 Department of Physics and Institute of Theoretical and Computational Physics, University of Crete, Heraklion 71003, Greece; 92 Raman Research Institute, Sadashivnagar, Bangalore 560080, India; 93 INAF, Osservatorio Astronomico di Cagliari, Selargius 09047, Italy; 94 Instituto de Astrof´ısicade Andaluc´ıa- CSIC, Granada 18008, Spain; 95 Department of Physics, University of Washington, Seattle, WA 98195-1560, USA; 96 IESL, Foundation for Research and Technology-Hellas, Heraklion 71110, Greece; 97 Departamento de Astrof´ısica,Universidad de La Laguna, La Laguna 38206, Spain; 98 N. Copernicus Astronomical Center, Warsaw 00-716, Poland; 99 Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara, Kanagawa 252-5258, Japan; 100 Aalto University Metsahovi Radio Observatory, Kylmala 02540, Finland; 101 Aalto University Department of Electronics and Nanoengineering, Aalto 00076, Finland; 102 National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA; 103 Centre for Astronomy, National University of Ireland, Galway H91 TK33, Ireland; 104 Dipartimento di Fisica ‘Enrico Fermi’, University of Pisa, Pisa 56127, Italy; 105 INFN Sezione di Pisa, Pisa 56127, Italy; 106 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK; 107 Department of Physics, University of Basel, Basel 4056, Switzerland; 108 Computational Astrophysics Laboratory - RIKEN, Saitama 351-0198, Japan; 109 Department of Mathematics, Computer Science and Physics, University of Udine, Udine 33100, Italy; 110 INFN Italian National Institute for Nuclear Physics, Trieste 34012, Italy; 111 Department of Astronomy and Institute of Theoretical Physics and Astrophysics, Xiamen University, Xiamen 361005, China; 112 Department of Physics and Institute of Theoretical Physics, Nanjing Normal University, Nanjing 210023, China; 113 Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; 114 Department of Physics, University of Warwick, Coventry CV4 7AL, UK; 115 Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA; J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-4

116 Department of Physics, The George Washington University, Washington, DC 20052, USA; 117 INAF Astronomical observatory of Padova, Padova 35122, Italy; 118 Mullard Space Science Laboratory, University College London, Surrey RH5 6NT, UK; 119 MIT Kavli Institute for Astrophysics and Space Research, Cambridge, MA 02139, USA; 120 Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA; 121 Astronomy Department, Shanghai Jiao Tong University, Shanghai 200240, China

Received December 6, 2017; accepted February 9, 2018; published online August 30, 2018

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.

space research instruments, nuclear astrophysics, flare stars, accretion and accretion disks, mass loss and stellar winds, cataclysmic binaries, X-ray binaries, supernova remnants, active galactic nuclei, X-ray bursts, gamma-ray bursts, gravi- tational waves PACS number(s): 07.87.+v, 26.30.ca, 97.30.Nr, 97.10.Gz, 97.10.Me, 97.30.Qt, 97.80.Jp, 98.38.Mz, 98.54.Cm, 98.70.Qy, 98.70.rz, 04.30.Db

Citation: J. J. M. in’t Zand, E. Bozzo, J. L. Qu, X.-D. Li, L. Amati, Y. Chen, I. Donnarumma, V. Doroshenko, S. A. Drake, M. Hernanz, P. A. Jenke, T. J. Maccarone, S. Mahmoodifar, D. de Martino, A. De Rosa, E. M. Rossi, A. Rowlinson, G. Sala, G. Stratta, T. M. Tauris, J. Wilms, X. F. Wu, P. Zhou, I. Agudo, D. Altamirano, J.-L. Atteia, N. A. Andersson, M. C. Baglio, D. R. Ballantyne, A. Baykal, E. Behar, T. Belloni, S. Bhattacharyya, S. Bianchi, A. Bilous, P. Blay, J. Braga, S. Brandt, E. F. Brown, N. Bucciantini, L. Burderi, E. M. Cackett, R. Campana, S. Campana, P. Casella, Y. Cavecchi, F. Chambers, L. Chen, Y.-P. Chen, J. Chenevez, M. Chernyakova, C. C Jin, R. Ciolfi, E. Costantini, A. Cumming, A. D’A`ı, Z.-G. Dai, F. D’Ammando, M. De Pasquale, N. Degenaar, M. Del Santo, V. D’Elia, T. Di Salvo, G. Doyle, M. Falanga, X. L. Fan, R. D. Ferdman, M. Feroci, F. Fraschetti, D. K. Galloway, A. F. Gambino, P. Gandhi, M. Y. Ge, B. Gendre, R. Gill, D. Gotz,¨ C. Gouiffes,` P. Grandi, J. Granot, M. Gudel,¨ A. Heger, C. O. Heinke, J. Homan, R. Iaria, K. Iwasawa, L. Izzo, L. Ji, P. G. Jonker, J. Jose,´ J. S. Kaastra, E. Kalemci, O. Kargaltsev, N. Kawai, L. Keek, S. Komossa, I. Kreykenbohm, L. Kuiper, D. Kunneriath, G. Li, E.-W. Liang, M. Linares, F. Longo, F. J. Lu, A. A. Lutovinov, D. Malyshev, J. Malzac, A. Manousakis, I. McHardy, M. Mehdipour, Y. P Men, M. Mendez,´ R. P. Mignani, R. Mikusincova, M. C. Miller, G. Miniutti, C. Motch, J. Nattil¨ a,¨ E. Nardini, T. Neubert, P. T. O’Brien, M. Orlandini, J. P. Osborne, L. Pacciani, S. Paltani, M. Paolillo, I. E. Papadakis, B. Paul, A. Pellizzoni, U. Peretz, M. A. P. Torres, E. Perinati, C. Prescod-Weinstein, P. Reig, A. Riggio, J. Rodriguez, P. Rodr´ıguez-Gil, P. Romano, A. Ro´za˙ nska,´ T. Sakamoto, T. Salmi, R. Salvaterra, A. Sanna, A. Santangelo, T. Savolainen, S. Schanne, H. Schatz, L. J. Shao, A. Shearer, S. N. Shore, B. W. Stappers, T. E. Strohmayer, V. F. Suleimanov, J. Svoboda, F.-K. Thielemann, F. Tombesi, D. F. Torres, E. Torresi, S. Turriziani, A. Vacchi, S. Vercellone, J. Vink, J.-M Wang, J. F Wang, A. L. Watts, S. S. Weng, N. N. Weinberg, P. J. Wheatley, R. Wijnands, T. E. Woods, S. E. Woosley, S. L. Xiong, Y. P. Xu, Z. Yan, G. Younes, W. F. Yu, F. Yuan, L. Zampieri, S. Zane, A. A. Zdziarski, S.-N. Zhang, S. Zhang, S. Zhang, X. Zhang, and M. Zingale, Observatory science with eXTP, Sci. China-Phys. Mech. Astron. 62, 029506 (2019), https://doi.org/10.1007/s11433-017-9186-1

Contents

1 Introduction 029506-5

2 Flare stars 029506-7

3 Supernova remnants 029506-8

4 Accreting white dwarf binaries 029506-10

5 Binary evolution 029506-12 J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-5

6 Thermonuclear X-ray bursts 029506-14

7 High-mass X-ray binaries 029506-18

8 Radio-quiet active galactic nuclei 029506-21

9 Radio-loud active galactic nuclei 029506-24

10 Tidal disruption events 029506-26

11 Gamma-ray bursts and supernovae 029506-28

12 Objects of uncertain nature 029506-31 12.1 Ultraluminous X-ray sources ...... 029506-31 12.2 Fast radio bursts ...... 029506-32

1 Introduction better than 30′′ (HPD) and a total effective area of ∼900 cm2 at 2 keV (including the detector efficiency). The PFA fea- tures Gas Pixel Detectors (GPDs) to allow polarization mea- The enhanced X-ray Timing and Polarimetry mission (eXTP) surements in the X-rays. It reaches a minimum detectable is a mission concept proposed by a consortium led by the In- polarization (MDP) of 5% in 100 ks for a source with a Crab- stitute of High-Energy Physics of the Chinese Academy of − − − like spectrum of flux 3 × 10 11 erg s 1 cm 2. The spectral Sciences and envisaged for a launch in the mid 2020s. It car- resolution is 1.1 keV at 6 keV (FWHM). ries 4 instrument packages for the 0.5-30 keV bandpass, with The LAD has a very large effective area of ∼3.4 m2 at the primary purpose to study conditions of extreme density, 8 keV, obtained with non-imaging silicon drift detectors, ac- gravity and magnetism in and around compact objects in the tive between 2 and 30 keV with a spectral resolution of universe. The mission concept provides for a low-Earth orbit about 260 eV and collimated to a field of view of 1 degree with a low inclination (< 15◦), incurring earth obscurations (FWHM). The LAD and the SFA together reach an unprece- every one and a half hour for targets near the celestial equator dented total effective area of more than 4 m2, compared to such as the Galactic center region. The scientific payload of 0.7 m2 for the largest flown instrument (Proportional Counter eXTP consists of: the Spectroscopic Focusing Array (SFA), Array on the Rossi X-ray Timing Explorer [2]). the Polarimetry Focusing Array (PFA), the Large Area De- The science payload is completed by the WFM, consist- tector (LAD) and the Wide Field Monitor (WFM). ing of 6 coded-mask cameras covering 4 sr of the sky at a The eXTP instrumentation is discussed in detail in ref. [1], sensitivity of 4 mCrab for an exposure time of 1 d in the which includes a comparison with other instruments. We 2 to 50 keV energy range, and for a typical sensitivity of summarize as follows. The SFA is an array of nine identi- 0.2 mCrab combining 1 yr of observations outside the Galac- cal X-ray telescopes covering the energy range 0.5-20 keV tic plane. The instrument will feature an angular resolution of with a spectral resolution of better than 180 eV (full width at a few arcminutes and will be endowed with an energy reso- half maximum, FWHM) at 6 keV, and featuring a total effec- lution of about 300 eV (FWHM). The WFM pairs the largest 2 2 tive area from ∼0.5 m at 6 keV to ∼0.7 m at 1 keV. The SFA duty cycle of 30% for an imaging monitoring 2-30 keV de- ′ angular resolution is required to be less than 1 (half-power vice (e.g., compared to 8% for BeppoSAX-WFC [3]) with −14 −1 −2 diameter, HPD) and its sensitivity reaches 10 erg s cm the largest spectral resolution (usually about 1 keV at 6 keV). 4 (0.5-20 keV) for an exposure time of 10 ks. In the current The baseline for the observatory response time to targets baseline, the SFA focal plane detectors are silicon drift de- of opportunity within the 50% part of the sky accessible to tectors that combine CCD-like spectral resolutions with very eXTP is 4-8 h. Depending on the the outcome of mission small dead times, and therefore are excellently suited for studies, this may improve to 1-3 h. The launch is currently studies of the brightest cosmic X-ray sources at the smallest foreseen in the mid 2020s. time scales. With a uniquely high throughput, good spectral resolution, The PFA consists of four identical X-ray telescopes that polarimetric capability and wide sky coverage, eXTP is an are sensitive between 2 and 8 keV, have an angular resolution observatory very well suited for a variety of studies comple- J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-6 menting the core science of strong gravity [4], dense matter targets are instead uniquely observed as part of the observa- [5] and strong magnetism [6]. The SFA will provide high tory science program (e.g., accreting white dwarfs, blazars, sensitivity in a soft but wide bandpass, the PFA will pro- high mass X-ray binaries), in turn providing useful compara- vide X-ray polarimetry capability, the LAD will provide the tive insights for the core science objectives. best timing and spectroscopic studies ever for a wide range eXTP will be a unique, powerful X-ray partner of other of high energy sources brighter than ∼1 mCrab in the 2 to new large-scale multi-messenger facilities across the spec- 30 keV band, and the WFM, with its unprecedented com- trum likely available in the 2020s, such as advanced grav- bination of field of view and imaging down to 2 keV, makes itational wave (GW) and neutrino experiments (Advanced eXTP a discovery machine of the variable and transient X-ray LIGO and Virgo, KAGRA, LIGO-India, Einstein Telescope sky. The WFM will reveal many new sources that need to be in GWs and KM3NeT and Icecube in neutrinos), SKA and followed up with the SFA, PFA, LAD and other facilities. Ex- pathfinders in the radio, LSST and E-ELT in the optical, and perience from the past decades shows that newly discovered, CTA at TeV energies (Figure 1). First eXTP and 4 years later unforeseen types of sources will provide unexpected insights Athena will be the major facilities in the X-ray regime and into fundamental questions. The WFM will also be moni- as such cover the hot and energetic non-thermal phenom- toring daily hundreds of sources, to catch unexpected events ena of any cosmic object, Athena focusing on diffuse con- and provide long-term records of their variability and spectro- stituents in the early Universe and eXTP focusing on bright, scopic evolution. Several targets of the eXTP core program variable and transient phenomena. eXTP will be particularly (e.g., low-mass X-ray binaries) allow one to address scien- suitable for electromagnetic (EM) studies of GW sources, an tific questions beyond those related to the key mission science exciting field of research whose birth we witnessed through goals. Thus, they can be used to fulfill the observatory sci- GW170817/GRB170817A [7, 8]. The combined EM/GW ence goals without requiring additional exposure time. Other signal is expected to provide us new insights of neutron stars

RadioAstron Spitzer Sofia JWST WFIRST/AFTA HST Kepler/K2 Gaia CHEOPS TESS Solar Orbiter Euclid PLATO XMM-Newton Chandra Swift MAXI NuSTAR ASTROSAT POLAR NICER Insight-HXMT MVN Spectrum-X-Gamma Kanazawa-Sat3 IXPE SVOM Einstein Probe ASO-S XARM eXTP Athena INTEGRAL Fermi DAMPE Lomonosov/MVL-300 M4 M5 LOFAR ASKAP/MWA MEERKAT FAST IRAM/NOEMA SKA ALMA APEX VLT LSST E-ELT H.E.S.S. CTA Advanced LIGO/Virgo KAGRA LIGO-India Einstein Telescope IceCube/PINGU KM3NeT 2018 2020 2022 2024 2026 2028 2030 Year Figure 1 Multi-wavelength and multi-messenger facilities relevant to eXTP with timelines as known in January 2018. The thin line separates space-based (top) from ground-based (bottom) facilities. Colors indicate similar wavebands from the radio (brown) via IR (red) and optical (green) to X-rays (blue) and gamma rays (purple). Grey bands: gravitational wave and neutrino detectors. Dark colors: funded lifetime. Light colors: expected lifetime, where known, independent of funding decisions. Missions might last longer. J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-7 through their merger events, see sect. 11 and ref. [5]. Steady high duty cycle monitoring devices such as the WFM. GW sources related to spinning neutron stars and ultracom- eXTP can contribute greatly to our understanding of stellar pact X-ray binaries may also be detected and identified in the flares. Expanding our knowledge of stellar flaring is crucial coming decades, providing new diagnostics for these multi- to examining the influence of transient sources of ionizing ra- wavelength sources that are particularly bright in the radio diation from a host star on exoplanet systems, important for and X-ray bands, see sect. 5 and ref. [5]. habitability concerns and space weather which other worlds To explain the science case for eXTP as an observatory might experience (e.g., ref. [27]). Such studies extend the in a kaleidoscopic manner in this White Paper, material is solar-stellar connection in determining the extent to which drawn from eleven White Papers that were written for the solar models for conversion of magnetic energy into plasma LOFT study by scientists from the community at large [9-19]. heating, particle acceleration and mass motions apply to the LOFT was an ESA mission concept with two of the three pri- more energetic stellar flares. Extrapolating from lower ef- mary science goals of eXTP and larger versions of the LAD ficiency high-energy monitoring observatories, we estimate and WFM. The outstanding capabilities of all four eXTP in- that eXTP will detect at least 35 superflares per year from struments will enable to answer key questions that could not stars exhibiting the extremes of magnetic activity. The antic- be addressed for many years, due to a lack of facilities with ipated source types include young stars, fully convective M sufficient sensitivity. These questions, across a broad swath dwarfs, and tidally locked active binary systems. The wide- of topics, will not be answered by any other planned future field monitoring capability of eXTP will also likely enable X-ray facility. The eXTP science involves a wide range of ob- the detection of flares from classes of stars hitherto not sys- jects, from normal stars to supermassive black holes in other tematically studied for their flaring, and will be important for galaxies. These objects have a wide range of fluxes. The expanding our understanding of plasma physics processes in wide range in capabilities of the instrument package allows non-degenerate stellar environments. Four important ques- one to deal with this range in fluxes: bright sources with the tions that eXTP can address are: LAD, WFM, and PFA; faint sources mainly with the SFA. • What are the properties of the non-thermal particles re- In this White Paper, the main questions are briefly discussed sponsible for the initial flare energy input? The study of the per category of objects, in order of approximate distance, that flare distribution with energy and of their temporal behav- can be addressed with eXTP and that are not dealt with in the ior, e.g., spikes, oscillations, etc., will be enabled by eXTP’s other eXTP White Papers on strong gravity, dense matter and sensitive hard X-ray capability provided by the LAD, far in strong magnetism. Due to the breadth of the science, this excess of previous missions. paper can only summarize those questions. • What are the physical conditions of the thermal plasma whose emission dominates the later stages of stellar flares? 2 Flare stars Through its soft X-ray capabilities, eXTP will enable us to study the variation of properties such as the temperature and On the Sun, flares occur in close proximity to active regions elemental abundances as a function of time during flares. that are characterized by boosted localized kG-strength mag- • What is the maximum energy that stellar flares can netic fields [20-23]. Loops from these regions extend into the reach? The wide-field, broad X-ray band and high-sensitivity solar corona and, as the footpoints of these loops are jostled capabilities of eXTP will enable us to detect the rare “super- by solar convective motions, they are twisted and distorted flares” that have been only sporadically detected by previous until magnetic reconnection occurs. There is then a sudden and current missions: better understanding of the prevalence release of energy, resulting in the acceleration of electrons of such powerful events is important both for the physics of and ions in these loops up to MeV energies, emitting non- stellar flares and for the implications of their effect on poten- thermal radio (gyrosynchrotron) and non-thermal hard X- tial planets orbiting these stars (see Figure 2). ray emission (bremsstrahlung and Compton back scattering). • What is the origin of flares from stars in nearby young These energetic particles both stream away from the Sun and associations? eXTP sensitivity will allow one to detect flares down the loops where they deposit substantial amounts of en- from stars in nearby young associations, such as the TW Hya ergy to the lower solar atmosphere (the chromosphere), pro- group, at distances up to 60 pc. As discussed by ref. [23], ducing the observed intense hard X-ray emission at the loop young stars with accretion disks like TW Hya are expected footpoints [23,24]. Flares are also observed on other late-type to have a second type of flares due to interactions between stars, with luminosities up to 1033 erg s−1 and durations from the magnetic fields of the active regions and the disk. It is an less than an hour to several days or longer [25, 26]. The high open question as to whether these flares have different proper- temperatures result in spectral peaks in the soft to hard X-ray ties than the better known in situ stellar flares (especially due regime, which make them particularly visible with wide-field to the very limited detections of these events in the X-rays). J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-8

(a) (b) 10

1 ) −1

keV 0.1 −1

0.01

10−3 2

normalized counts (s 0 χ

−2

2 5 10 20 Energy (keV)

Figure 2 (a) Artist’s impression of the 2014 April 23 “superflare” of one of the stars in the wide M dwarf binary DG CVn (credit Scott Wiessinger of the NASA/GSFC Scientific Visualization Studio). (b) Simulated LAD spectrum of a 1000 s time segment of a flare with similar flux and spectrum to the second, slightly less powerful flare of DG CVn 3 h after the first outburst. Notice the well-exposed He-like Fe lines at 6.7 and 7.9 keV which would enable an accurate coronal Fe abundance to be derived (e.g., ref. [28]).

3 Supernova remnants case of quasi-parallel injection [36]. eXTP can resolve the X-ray structures in the SNR (30’ in diameter) and provide Supernova remnants (SNRs), including pulsar wind nebu- spatially resolved physical information for them. The PFA lae (PWNe), are important Galactic producers of relativis- polarimeter can measure the polarization degree and angle tic particles and emitters of diffuse X-ray synchrotron radia- in the post-shock region of SN1006 with 15” angular res- tion [29, 30]. eXTP is expected to provide breakthroughs in olution (see Figure 3(a)). The measurement accuracy will σ some longstanding problems in the studies of shell SNRs and be about 2% for the polarization degree ( dop; assuming a ◦ PWNe via fine measurement of the distribution of magnetic polarization degree of 17%) and 3 for the position angle σ fields and polarized emission. ( PA) after a 0.5 Ms observation of filaments 1 and 2. With a The strong shocks driven by supernova explosions are 10 ks observation, the SFA can obtain spatially resolved spec- commonly regarded as the main factory of the cosmic rays tra of the non-thermal emission of the shell (see Figure 3(b) (CRs) with energy up to 3 × 1015 eV (the so called “knee”), for the simulated spectra of “filament 1” in the northeast of and accumulating evidence has also pointed to this notion the SNR). The eXTP polarization measurement and spectral [31, 32]. Despite the well established theory of diffusive analysis are thus expected to give a precise answer to a spe- shock acceleration for CRs, many crucial problems in the cific physical problem. acceleration process remain unsolved, such as the injection • What is the intensity of the synchrotron radiation and the mechanism of particles and energy, the efficiency of acceler- orientation of the magnetic field in the forward shock and, es- ation, the size of the acceleration region, and the upper limit pecially, the reverse shock in SNRs? While CR acceleration of particle energy [33, 34]. Clues to solve the problems can has been extensively studied at the forward shocks of SNRs, be provided by the non-thermal X-ray emission of the rela- how the reverse shocks accelerate CRs became an intriguing tivistic electrons arising from the strong shocks of SNRs. question in recent years. Using a 1 Ms Chandra observa- In particular, eXTP will be powerful in addressing the fol- tion towards Cas A, most of the non-thermal emission from lowing questions about SNRs and PWNe: the SNR was found in the reverse shock [37, 38]. eXTP will • What effect does the orientation of injection with respect be able to test this result and compare the polarization pa- to the direction of the magnetic field have on the shock accel- rameters between the forward and reverse shocks. With an eration efficiency in SNRs? In the acceleration mechanisms, exposure time of 2 Ms, the PFA polarimeter can measure the it remains unclear how the orientation of injection with re- polarization degree and angle of 0.5′×1′ sub-structures in Cas ◦ spect to the magnetic lines affects the acceleration efficiency A (4.2-6 keV) with accuracies of σdop∼3% and σPA∼10 for (“quasi-parallel scenario” or “quasi-perpendicular scenario” the forward shock (assuming a polarization degree of 10%), ◦ [35]). SN1006, a SNR with a bilateral-like non-thermal X- and σdop∼1.5% and σPA∼8 for the reverse shock (assuming ray morphology, is an ideal example to study this problem. a polarization degree of 5%). The spatially resolved polariza- The most recent radio polarization observation favors the tion measurements with eXTP will provide crucial informa- J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-9

(a) (b) 10 ) −1 1 keV −1

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2

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0.5 1 2 5 10 20 Energy (keV)

Figure 3 (a) The 2-8 keV image of SN1006 convolved with the PFA angular resolution (15′′). For the boxes around filaments 1 and 2, the polarization can ◦ ◦ be measured with accuracies of σdop = 1.8% and 2.1% and σPA = 2.9 and 3.2 for an exposure time of 0.5 Ms, respectively. (b) Simulated SFA spectrum of region “filament 1” in the northeast of SN1006 with a 10 ks observation. A model is employed of synchrotron radiation from an exponentially cutoff power-law 20 −2 distribution of electrons with the parameters hydrogen absorption column density NH = 6.8 × 10 cm , power-law photon index of −0.57 and cut-off energy of 0.5 keV. tion on the magnetic field at the reverse shock where CRs are the shock far exceeding the theoretically predicted shock- effectively accelerated. compressed strength. The joint capability of eXTP to mea- • What is the spatial distribution of the non-thermal X- sure X-ray emission and polarization will allow one to test ray emitting components with respect to that of the ther- current models of amplification of magnetic fields by SNR mally emitting regions in SNRs? Since the first detection in shocks. It is believed that such shocks propagate into a gen- SN1006 [39], non-thermal X-ray emission has been detected erally turbulent interstellar medium [42]. The turbulence can at the rims of several SNRs. This is believed to be due to be pre-existing or caused by the shock itself via, e.g., a non- synchrotron radiation of the CR electrons accelerated by the resonant streaming instability of cosmic-rays as first found SNR shocks (e.g., ref. [40]). by ref. [43]. In the former case, the density inhomogeneities In addition to the non-thermal X-ray dominant SNRs (such traversed by the shock front are expected to produce corru- as SN1006 and RX J1713-3946) and a few young SNRs (such gation of the shock surface; such a surface, as it propagates as Cas A and Tycho) in which non-thermal and thermal X-ray further, triggers differential rotation of over-densities during emission can be spatially/spectroscopically resolved, there their shock-crossing generating vorticity, therefore amplify- are several SNRs in which potential non-thermal X-ray emis- ing the magnetic field via a small-scale dynamo process in the sion can hardly be distinguished from the thermal emission. downstream fluid [44-46]. The field growth is exponentially These SNRs include N132D, G32.4+0.1, G28.6-0.1, CTB fast, as short as years or months (e.g., the rapid changes in hot 37B, W28, G156.2+5.7, etc. [41]. Thanks to the large col- spots brightness observed in the SNR RXJ1713.7-3496 and in lecting area of eXTP in the hard X-ray band, short exposures Cas A [47,48]), likely due to synchrotron electron cooling in of SFA towards these SNRs can collect enough photons for a strong magnetic field. spectral analysis to clarify whether the hard X-ray emission eXTP’s polarimetric capability will be a valuable asset is non-thermal. The enlarged sample of non-thermal X-ray here in the sense that it will provide, for the first time, the emitting SNRs in various environments and at different ages polarization in those hot spots, leading to estimates of the co- will allow one to study the mechanism of CR electron accel- herence scale of the magnetic turbulence. This will be of the eration and to constrain the leptonic emission, acceleration order of the Field length (i.e., about 0.01 pc [49]) if the vor- efficiency, magnetic field and other interstellar conditions. tical mechanism dominates. Indeed, the growth time scale of • Which are the mechanisms of amplification of the mag- the magnetic turbulence is approximately equal to the ratio of netic field at the SNR shocks? Several high-energy ob- the Field length to the local shock speed. As described above, servations of SNRs are consistent with a magnetic field at a 1 Ms exposure on Cas A would allow one to measure the po- J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-10 larization degree (angle) with an accuracy better than 1% (3◦) the question of whether accretion dominates over outflow, for the forward shock. The proposed analysis requires rela- possibly bringing the WD mass over the Chandrasekhar limit tively high SNR shock speeds (not too high to have a growth yielding type Ia supernovae. This is a key aspect for under- time at least of a few months) colliding with dense clouds standing Type Ia supernova progenitors. In addition, popu- or propagating into a region with significant clumpiness. By lation studies of galactic X-ray sources have recently recog- observing the X-ray emission during the early phases of the nized a crucial role of accreting WD binaries in the origin of shock/cloud collision, eXTP will allow one to constrain the the Galactic Ridge X-ray Emission (GRXE), one of the great growth time scale of the magnetic field. mysteries in X-rays [54-58]. • What is the configuration of the magnetic field in PWNe The X-ray domain is crucial to understand the physical and what physical insight can we infer from that? PWNe, conditions close to the compact star (e.g., review by ref. such as the Crab nebula, are high-energy nebulae composed [59]). Progress in many open questions is hampered by the 29-33 −1 of relativistic particles and magnetic fields created by the cen- low quiescent luminosities (LX ∼10 erg s ) and the un- tral pulsars. The energy input of pulsars, the particle accel- predictability of large-scale variations that can reach lumi- eration at the terminal shocks, the PWN structures (including nosities as low as ∼1032-1034 erg s−1 in dwarf novae (DNe) the equatorial ring and bipolar jets) and other physical prop- outbursts and as high as ∼1038 erg s−1 in nova explosions. erties (such as particle transport and magnetization factor) are The majority of persistent, steadily nuclear-burning accreting not fully understood yet (e.g., refs. [50, 51]). eXTP can per- white dwarfs thought to exist in the Milky Way, its satellites, form X-ray polarization measurements towards the Crab neb- and M 31 have yet to be detected [60, 61]. These “supersoft ula with high spatial resolution, and obtain the detailed dis- X-ray sources” are candidate Type Ia supernova progenitors, tribution of magnetic field and polarization degree. The PFA and can exhibit oscillations on very short timescales (. 100 s) polarimeter can measure the polarization degree and angle for as yet unknown reasons [62]. of the sub-structures (15′′) in the Crab nebula with a mean The unique combination in eXTP of large effective area, accuracy of about 1.7% (σdop, assuming an polarization de- fair spectral resolution and excellent sensitivity of the LAD ◦ gree of 19%) and 2.6 (σPA), respectively (we considered an and SFA instruments, as well as the wide field of the WFM exposure time of 10 ks). The unprecedented X-ray polariza- instrument, can uncover the wide range of variabilities to un- tion imaging of the Crab nebula will play an important role in derstand better the underlying physical processes. The three the study of PWNe. Another PWN worth following up with main questions that can be addressed are: eXTP is MSH 15-52 which has a size of 10′ and a total flux of • How does matter accrete onto white dwarfs and mix with ∼ 5.5×10−11 erg s−1cm−2. eXTP mapping will provide polar- the CO/CNe substrate? Fast (seconds to minutes) X-ray vari- ◦ ization accuracies of σdop=2% and σPA=3.7 for 10 pointings ability, both periodic and aperiodic, allow one to trace the with a 300 ks exposure (assuming a photon index of −2 and accretion flow close to the WD surface. In magnetic CVs, polarization degree of 15%). periodic coherent pulsations are a signature of magnetically funneled accretion and their spectral study allows one to in- fer the physical conditions of the post-shock region, the hot 4 Accreting white dwarf binaries spot over the WD surface as well as complex absorption from pre-shock material. Furthermore, quasi-periodic oscillations White dwarfs (WDs) accreting from a companion star in a bi- (QPOs) at a mean period of a few seconds are expected nary exhibit themselves as cataclysmic variables (CVs, e.g., to arise from shock oscillations [63], but so far these were review by ref. [52]). CVs represent the most common end only detected in a handful of systems at optical wavelengths. products of close binary evolution. These binaries display The high shock temperatures (∼20-30 keV) imply that QPO a wide range of time scales in variability (from seconds- searches need to be extended into the hard X-rays. minutes up to months-years) that make them ideal laborato- In non-magnetic CVs, when accreting at high rates, a va- ries for the study of accretion/ejection processes in the non- riety of low amplitude variability has been detected. DN os- relativistic regime. Disk instabilities, the disk-jet connec- cillations with a period of a few seconds were found in the tion and magnetic accretion are common processes occurring optical and soft X-rays, changing frequency during outburst. in a wide variety of astrophysical objects. Accreting WDs Slower (with a time scale of minutes) low-coherence QPOs provide an excellent perspective of these phenomena. Fur- were detected in a few systems in the hard X-rays during the thermore, thermonuclear runaways giving rise to novae (e.g., decline [64]. Their appearance is related to the transition of review by ref. [53]) have the potential to probe the accre- the boundary layer from optically thick to optically thin and tion/ignition mechanisms, outflows and subsequent chemical thus have a great potential to infer details of boundary layer enrichment of the interstellar medium, as well as to address regimes. Their frequencies stay in ratio, like the low and high J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-11 frequency QPOs seen in low-mass X-ray binaries. Further- X-ray emission from bright novae to be followed-up by the more, the presence of a break in the power spectra during SFA and LAD instruments to monitor over an unprecedented quiescence can allow one to infer disk truncation [65]. The energy range the X-ray spectral evolution including the onset striking similarities with X-ray binaries make CVs interesting of hard X-ray emission and of the super-soft X-ray phase as for a unified accretion scenario. well as to characterize for the first time fast variability over Peak temperatures reached during a nova explosion are eXTP’s unique broad range. An example of the eXTP poten- constrained by the chemical abundance pattern inferred from tial is shown in Figure 4. 8 the ejecta and do not seem to exceed 4 × 10 K, so it is un- • What causes DN outburst diversity and what are the con- likely that the large metallicities reported from the ejecta can ditions for disk-jet launching? DN outbursts are believed to be due to thermonuclear processes. Instead, mixing at the be due to disk instabilities but there is still lack of knowl- core-envelope interface is a more likely explanation. The true edge of what changes occur at the inner boundary layer. The mixing mechanism has remained elusive for decades. Mul- boundary layer is optically thin during quiescence and be- tidimensional models of the explosion suggest that mixing comes optically thick during outburst [74]. Hard X-ray sup- may result from Kelvin-Helmholtz instabilities at the core- pression during optical outbursts was believed to be a general envelope interface (e.g., ref. [66]). behavior. Instead, the few DNe observed so far showed great eXTP can address all this by studying selected samples of magnetic and non-magnetic systems, to detect for the first (a) time low-amplitude fast aperiodic and periodic variabilities 107 ff LAD, 10 ks over the broad energy range o ered by the LAD and SFA SFA, 10 ks and to perform time-resolved spectroscopy. 106 • How does mass ejection work in nova explosions? No- vae are, after thermonuclear supernovae, the second most lu- minous X-ray sources among accreting WDs with emission 105 ranging from the very soft to the hard X-rays. The soft X- Counts (keV) rays probe the continuous nuclear burning on the WD sur- 105 Day 3 face, while the hard X-rays are believed to trace the early Day 6 Day 10 shocks inside the ejecta and between the ejecta and the cir- Day 14 cumstellar material, as well as the onset of resumed accre- 103 Day 17 tion. The details of the mass outflow and the shaping and 0.5 1 25 10 20 Energy (keV) evolution of the ejected matter are still poorly known. A re- 0 cent challenge is the unprecedented detection by Fermi LAT (b) of high-energy gamma rays (E > 100 MeV) in an increasing WFM, 10 ks. number of novae [67,68] indicating that these systems are the 4 site of particle acceleration. This was predicted for RS Oph [69], a symbiotic recurrent nova where the early strong shock 3 between the nova ejecta and the red giant wind was traced in hard X-rays by RXTE [70]. In classical novae there is not a 2 red giant wind to interact with the nova mass outflow, because the companion is a main sequence star; therefore, particle ac- Normalized counts (s) 1 Day 3 celeration is more challenging to explain [71]. eXTP prompt Day 6 observations of nova explosions would be crucial to better Day 10 Day 14 understand such processes in both nova types (see Figure 4). 0 2 5 10 20 It is very important to observe over the broadest X-ray Energy (keV) range the spectral evolution of these explosions from the early hard X-ray onset to the later super-soft X-ray phase [70, 72]. Figure 4 Simulations of the spectral evolution of the symbiotic re- current nova RS Oph over its outburst, as seen by the LAD and SFA Furthermore X-ray variability and periodicities on many dif- (a) and by the WFM (b). The spectra include a thermal plasma model ferent time scales, from days to less than a minute are still to (mekal) with solar abundances, evolving from kT=8.2 keV and a flux of − − − be understood. Short oscillations in the soft X-rays can origi- 3.2 × 10 9 erg s 1 cm 2 (0.5-20 keV) on day 3, down to kT=2.5 keV and a . × −10 −1 −2 nate from H-burning instabilities [73] while those in the hard flux of 4 3 10 erg s cm (0.5-20 keV) on day 17 [70]. The high X-ray flux during the first 17 d would trigger the WFM in full resolution mode and X-rays still remain to be explored. a detailed study of the spectral evolution will be possible with the LAD and Thanks to the WFM, eXTP will be able to catch the early SFA. J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-12 diversity and not all of them show X-ray/optical anticorrela- good estimates of their system parameters. Because most X- tion [75]. The understanding of what fundamental parameters ray binaries, especially with black hole primaries, are tran- divide the different regimes of the boundary layer is crucial. sients, the ideal way to detect such objects is with all-sky A further challenge is the detection of radio emission from monitoring. The WFM represents a major step forward in the a DN and from high mass accretion rate CVs [76-78] inter- capabilities of all-sky monitors relative to present monitors. preted as the presence of a radio jet. CVs were believed to eXTP will make essential contributions to several major be unable to launch jets. It was recently shown that nova-like aspects of compact binary populations: the very faint X- CVs can have optically thin inefficient accretion disk bound- ray transient problem [85-87]; the evolutionary link between ary layers [79], allowing the possibility of retaining sufficient low-mass X-ray binaries and radio millisecond pulsars; the energy to power jets. If this were the case, a radio/X-ray cor- orbital period distribution of X-ray binaries, by finding the relation, similar to that observed in neutron star and black short period systems selected against by shallower all sky hole binaries [80], should be present in these systems. monitors [88, 89]; the “mass gap” between black holes and eXTP can address these challenging issues by following neutron stars that has been tentatively observed in existing a few DNe with adequate multi-outburst coverage through data [90-92] and not observed in microlensing data [93]; un- time-resolved spectroscopy and fast timing in unprecedented derstanding the formation of millisecond pulsars in compact detail. binaries [94] as well as the details of their spin evolution [95]. Thus eXTP will be crucial to measure the spectral and tem- eXTP will present a range of new opportunities to study poral properties of the poorly known hard X-ray tails in these binary stellar evolution processes. These are: systems and to correlate them with the soft X-ray emission. • What are the characteristics of the population of Very When eXTP is expected to be operational, wide field area Faint X-ray Binaries? The WFM will have the necessary sen- surveys will have provided statistically significant samples sitivity and large field of view to detect flares and outbursts to allow detailed investigation in coordination with ground- from the Very Faint X-ray Binaries with luminosities below based optical and radio facilities foreseen in the post-2020 1036 erg s−1, allowing a census of these sources across a large time frame (see Figure 1). fraction of the Galaxy. Furthermore, the WFM’s large field of view will easily pick up the rare X-ray bursts (see sect. 6) that very faint X-ray transients exhibit if the accretor is a neutron 5 Binary evolution star [85]. By establishing a substantial sample of such ob- jects through the wide field coverage and good sensitivity of The evolution of binary stars is one of the most important the WFM [14], it should be possible to engage optical follow- and challenging problems in modern stellar astronomy. Bi- up on enough of them to uncover their nature, which is still nary evolution—even when restricted merely to the evolu- poorly understood. tion of binaries with compact objects—touches on a range of • What is the evolutionary link between low-mass X-ray key questions [81-84]: the formation of Type Ia supernovae, binaries and radio millisecond pulsars? eXTP will provide the formation of merging neutron stars and black holes, the detailed timing studies of known low-mass and ultracompact rate of production of millisecond pulsars, and the production X-ray binaries (with an orbital period of less than ∼1 h, e.g., rate of a variety of classes of stars with abundance anoma- refs. [96, 97]) which will help us to better understand their lies. The challenges in understanding the populations of bi- evolutionary link to radio millisecond pulsars, such as the so- nary stars stem from the wide range of physics that goes into called redbacks and black widow systems [98]. them. In addition to normal stellar evolutionary processes, A particular class of objects of interest for eXTP studies the following must be understood: the stability and efficiency is that of the transitional millisecond pulsars. These repre- of mass transfer, the common envelope stage, the tides and sent the most direct evidence for an evolutionary link be- angular momentum loss mechanisms that keep systems tight tween radio millisecond pulsars and low-mass X-ray bina- (i.e., magnetic braking and circumbinary disks), the spin-up ries, since they are observed to transit between both states. of the accreting star, irradiation effects on the donor star, the Three, possibly four, are currently known [99-102]. Two of details of the supernova explosions of (ultra)stripped stars, these have relatively low luminosities in their X-ray states the kicks applied to neutron stars and black holes at birth, the of 1033-34 erg s−1 while the third had an ordinary X-ray out- spin-down due to the propeller effect, possible gravitational burst as an accretion-powered millisecond pulsar peaking at wave emission mechanisms, and the initial parameter distri- 1036 erg s−1. There may be many more of these systems. De- butions of binary and multiple star systems. tecting them in larger numbers, and mapping out the statisti- Proper testing of theories of binary evolution must come cal properties (e.g., pulse and orbital periods) of the resulting from exploration of substantial samples of close binaries, and sample will enable a better understanding of the binary evo- J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-13 lution. Good candidates may be found among the so-called example, Sco X-1, provided its neutron star spin rate is de- redbacks, which are radio millisecond pulsars in binaries with tected [110-112] which is where eXTP can help through the an ablated companion star of mass around 0.1 M⊙. Observa- detection of a weak pulsation. Recent calculations [113] sug- tions more sensitive than with XMM-Newton and Chandra gest that transiently accreting millisecond pulsars are promis- are needed to study the common flaring behavior of these ing as well. systems, for instance how the pulsar behaves on short time • How can eXTP help find sources for LISA? The very scales, to understand better the process behind switching be- tight X-ray binaries with orbital periods of less than 1 h, tween X-ray and radio states. eXTP would make this obser- the ultracompact X-ray binaries, are ideal sources to be de- vational progress possible. tected in GWs induced by the orbital motion by the upcom- It is worth mentioning the puzzling fact that fully recycled ing space-borne GW detector LISA [116] and LISA-like mis- radio millisecond pulsars are seen in systems with orbital pe- sions like TianQin [117] and TAIJI [118], missions with en- riods up to 200 d, but only 3 pulsar-containing low-mass X- visaged launch dates in the first half of the 2030s. It is ex- ray binaries are known with orbital periods longer than 1 d pected that eXTP will discover a number of new X-ray bi- and the neutron stars hosted are relatively slow pulsars (0.5- naries and may thus find additional ultracompact X-ray bina- 4.0 s). eXTP will be able to probe pulsations down to the ms ries (neutron star/white dwarf or double white dwarf binaries) range with an unprecedented sensitivity, hopefully allowing that will provide important and guaranteed sources of GWs one to reveal the currently missing fast rotators. for detection within the frequency band (∼mHz) of LISA and • What is the heaviest neutron star and the lightest black LISA-like missions. hole? eXTP will allow one to determine whether the orbital period distribution of black hole X-ray binaries follows the (a) predictions of binary evolution. At the present time, the se- vere selection biases against black hole X-ray binaries with orbital periods less than about 4 h (Figure 5(a)) lead to a deficit of what is likely to be the largest group of systems. Such systems are likely fainter, see Figure 5(b). Possibly, these contain the lighter black holes, especially if one in- cludes high-mass X-ray binaries (e.g., ref. [103]). By en- hancing the sample of objects for which masses can be esti- mated, combined with much more sensitive upcoming optical telescopes E-ELT and TMT to obtain dynamical mass esti- mates, eXTP will also help verify or refute the “mass gap” between neutron stars and black holes (Figure 6) which has (b) profound implications for understanding how supernovae ac- tually explode. • How efficient is the process of neutron star spin up? The process of spin-up of neutron stars in binaries (Figure 7) can be probed in two ways: by using LAD measurements to expand the sample of neutron stars with good spin pe- riod measurements and by using the combination of WFM and LAD to discover and study the population of objects that are in the act of transitioning from low mass X-ray bi- naries into millisecond radio pulsars. The study of accret- ing neutron stars and the torques acting on them is impor- tant for probing disk-magnetosphere interactions [104-107] Figure 5 (a) The orbital period distributions of dynamically confirmed black hole X-ray binaries (red curve), and cataclysmic variables (green as well as investigating continuous GW emission from the curve, with the numbers multiplied by 0.1 to fit the two distributions well spin of asymmetric neutron stars and the prospects of de- on the same scale), taken from the Ritter catalog [114]. It is apparent that tecting these sources using second (Advanced LIGO and the distributions are very different from one another. Ref. [89] find a proba- Virgo, KAGRA, LIGO-India) or third-generation (Einstein bility of 0.0012 that these two distributions are drawn from the same parent distribution. (b) Peak luminosity plotted versus orbital period for the black Telescope in Europe, Cosmic Explorer in USA [108]) GW hole X-ray transients seen with RXTE, with a linear fit. Reproduced by detectors [95, 109]. Among the promising sources for dual permission of the AAS from ref. [88]. Similar relations are also seen for observations in both X-rays and gravitational waves is, for cataclysmic variables [14, 115] and accreting neutron stars [88]. J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-14

Figure 6 Graphic showing neutron star and black hole mass measurements along the vertical axes (in units of solar mass) (from refs. [119-121]), including the 16 black holes and 2 probable neutron stars in GW150914, LVT151012, GW151226, GW170104, GW170608, GW170814, GW170817 [8, 122-127]. The GW measurements are indicated with curved line pointing to the merger event. There is a clear gap between 2 and 5 solar masses. Credit: Frank Elavsky, LIGO-Virgo, Northwestern University.

0.006 ing to the structure of the neutron star and exotic nuclear pro- cesses, while others are related to interesting physical phe- 0.005 nomena such as neutron star spins, convection and radiation transport under strong-gravity circumstances, the geometry 0.004 of accretion and magnetic field, and unusual stellar abun-

0.003 dances. Since eXTP is particularly well suited to observe

Density short bright phenomena, it will be able to answer many of

0.002 these questions. About 20% of all bursts exhibit oscillations with a fre- 0.001 quency very near that of the neutron stars spin (11-620 Hz). The oscillations during the burst rise can be explained by a 0.000 hot spot expanding from the point of ignition [132-137], al-

0 200 400 600 800 1000 though this still requires further study. The oscillations dur- Spin frequency ing the tail have not yet been explained. They could be caused Figure 7 The spin distribution for the fastest known (ν > 100 Hz) accreting by non-uniform emission during the cooling phase (e.g., be- neutron stars (adapted from ref. [128]). cause of different depths of fuel over the surface or cooling wakes [138]), but an interesting alternative involves global 6 Thermonuclear X-ray bursts surface modes that may be excited by the motion of the de- flagration front across the surface. This was first suggested While thermonuclear (type-I) X-ray bursts are well under- by ref. [139], and further theoretical work was performed by stood as a thermonuclear runaway in the upper freshly ac- (among others) refs. [140-144]. creted layers of a neutron star in a low-mass X-ray bi- The primary nuclear processes in thermonuclear bursts are nary, many important questions are unanswered (e.g., refs. the CNO cycle to burn hydrogen (β-limited “hot” CNO cy- [13, 53, 129-131]). Some of those are fundamental, pertain- cle above ∼ 8 × 107 K), the triple-α process to burn helium J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-15

(above several 108 K), the αp-process above 5 × 108 K to pro- 3.5 m2) and 5 times finer spectral resolution (i.e., 260 eV) duce elements like Ne, Na and Mg, and the rapid proton cap- than the previous burst workhorse RXTE-PCA [2] and the ture process (rp-process, above 109 K) to burn hydrogen into currently operational Astrosat-LAXPC [168] and about 50 even heavier elements (e.g., refs. [145-149]). In cases of times as much effective area at 6 keV than the currently op- hydrogen-free helium accretion or where the β-limited CNO erational NICER [169]. The coded-mask imaging WFM, cycle burns all hydrogen to helium before ignition, bursts are thanks to its 4 sr field of view (one third of the sky) will de- due to ignition in the helium layer. If the CNO burning is not tect as many as ten thousand X-ray bursts in 3 years, which β-limited, it can be unstable leading to a temperature increase is of the same order of magnitude as all bursts that were de- that subsequently ignites helium. The ignition of helium may tected throughout the history of X-ray astronomy. It is clear be delayed in that regime [150,151], resulting in pure hydro- that eXTP will provide a rich and novel data set on X-ray gen flashes. If the CNO burning is β-limited and this burning bursts. While the primary science motives for X-ray burst ob- has no time to burn away the hydrogen, helium ignition oc- servations with eXTP are dense matter physics and gravity in curs in the presence of hydrogen, a situation that leads to the the strong field regime, the secondary motives relevant to ob- rp-process. Hundreds of proton-rich isotopes are produced servatory science concern their fundamental understanding, during thermonuclear burning, particularly by the rp-process, in connection with nucleosynthesis, hydrodynamics, flame yielding rare isotopes, whose reaction rates have not yet been spreading and accretion flows (see ref. [13]). As a benefit, quantified in laboratories [152]. the outcome of these studies will help reduce the risk of any In the past one and a half decades, two new kinds of systematic errors in studies in the primary science goals. X-ray bursts have been discovered that are long and rare: eXTP will enable substantial advances in the understand- “superbursts” [153-155] and “intermediate duration bursts” ing of thermonuclear X-ray bursts, on particularly the follow- [156, 157] that ignite at 102 to 104 larger column depths than ing questions: ordinary bursts. It has been proposed that superbursts are fu- • What is the nature of burst oscillations? Oscillations in eled by carbon [155,158], but it is unclear how the carbon can burst tails can be studied at much better sensitivity than with survive the rp-process or ignite [159-163] and whether super- previous missions and at photon energies below 2 keV it will bursts are sometimes not fueled by carbon [164]. In contrast, improve on NICER [169] by having a four times larger effec- it seems clear that intermediate duration bursts are fueled by tive area. This will increase the percentage of bursts with de- helium on relatively cold neutron stars. The helium may ac- tected oscillations, enable the detection of shorter oscillation cumulate by slow and stable burning of hydrogen into helium trains and faster frequency drifts, probe fainter amplitudes as described above, or by direct helium accretion at low ac- and make possible spectro-timing analyses. Thus, eXTP will cretion rate (e.g., refs. [156, 157, 165, 166]). Due to their enlarge our knowledge about burst oscillations including the larger ignition depth, these long bursts can serve as probes possible persistence of hot spots during the burst decay and of the thermal properties of the crust, one of the outstanding the potential roles of surface r and g modes (see ref. [170]). questions of neutron star research. Superbursts have a prop- Simulations of measurements of burst oscillations are pro- erty which is intriguing in light of this dependence: they ig- vided in Figure 8, under different model assumptions (for de- nite sooner than expected on the basis of the measured mass tails, see ref. [171]). These simulations show that eXTP has accretion rate and presumed crust properties [161]. It looks a diagnostic capability for the cause of burst oscillations in as though the thermal balance in the crust (determined by nu- burst tails of a common type of X-ray bursts. clear reactions in the crust and neutrino cooling in the core) • Is there a preferred ignition latitude on the neutron star is different from expectations (see also ref. [162]). Better (as expected for fast spinning neutron stars with anisotropic measurements are needed to constrain more accurately the accretion); how do Coriolis, thermal and hydrodynamic ef- recurrence time and the time history of the accretion between fects combine to control flame spread? A thermonuclear burst superbursts. Currently, 26 superbursts have been detected in is expected to be ignited at one point on the neutron star. The the last 20 years [167]. eXTP can double that in a few years. latitude of that point may not be random, but influenced by eXTP will provide the opportunity to address the outstand- Coriolis forces. A thermonuclear flame should subsequently ing questions on X-ray bursts. A typical peak photon count spread over the stellar surface in a fashion that is determined rate with the LAD will be 105 s−1. Cases with ten times as again by rotation and by the thermal properties of the ocean high rates will also occur. These observations will provide in which this happens and the power of the thermonuclear unprecedented detailed insight at processes with sub-ms time flash [133, 135, 137]. Observational studies of the rising scales. With the anticipated observation program, it is ex- phases of X-ray bursts show diverse and interesting details pected that the LAD will detect at least several hundreds of (e.g., refs. [136, 172-174]). With the much larger area of the X-ray bursts, with about 5 times as much effective area (i.e., LAD, thermonuclear flame spreading can be studied at an J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-16

(a) (b) Counts (bin)

Figure 8 LAD simulations of X-ray burst oscillations for a bright burster similar to 4U 1636-536 that is bursting faithfully for at least 40 years already. For details on the model, see ref. [171]. The simulations were carried out for a neutron star of M = 1.4 M⊙, R=10 km and ν=400 Hz, for an observer’s inclination angle of i = 70◦ (measured from the spin vector) and with a time resolution of 0.125 ms. (a) The smoothed light curves for the rise and decay of the bursts (with 11 ms time bins). The red, green and blue curves show simulated light curves for ignition co-latitudes (i.e., also measured from the spin vector) ◦ ◦ ◦ θs = 30 , 85 and 150 , respectively. The hot spot temperature is assumed to be Th = 3 keV and the temperature outside the hot spot ∆T = 1.5 keV lower. ∝ / θ ff The flame spreading velocity goes as vflame 1 cos s, consistent with strong frictional√ coupling between di erent fuel layers [133]. The magenta and cyan ◦ curves are similar to the green curve (θs = 85 ) but with ∆T=2 keV and vflame ∝ 1/ cos θs, respectively, the latter referring to weak frictional coupling. (b) The simulated eXTP measurements of the fractional oscillation amplitudes for the same models as on the left panel. Note that the fractional amplitudes are shown on an expanded scale up to t = 1 s (burst rise). These simulations show that eXTP is capable of diagnosing ignition co-latitude and frictional coupling. They also indicate (see ref. [171]) that they can constrain the cause of the burst oscillations in burst tails. unprecedented level of detail and in many more bursts. For fective area one order of magnitude larger, at a fair spectral example, simulations show (see Figure 8) that eXTP mea- resolution to detect absorption edges. This provides excellent surements would enable constraining the latitude of the burst opportunity for the study of narrow spectral features, partic- ignition. ularly absorption edges for systems at known distance with • What is the composition of the ashes of nuclear burn- Eddington-limited bursts from a slowly rotating neutron star, ing; how large is the gravitational redshift on the neutron star like IGR J17480-2446 in the globular cluster Terzan 5 (e.g., surface? The SFA and LAD combined will be uniquely sensi- refs. [179, 180]). tive to details of the continuum spectrum that are induced by • What characterizes the transition from stable to unsta- Compton scattering in the atmosphere and to absorption fea- ble nuclear burning? A long standing issue in X-ray burst tures from dredged-up nuclear ashes such as iron and nickel research is the threshold between stable and unstable helium which may enable direct compositional studies (e.g., ref. burning [181-186]. Thermonuclear burning becomes stable [175]). Low-resolution evidence for such features has been once the temperature becomes so high that the temperature found in RXTE data of bursts that generated strongly super- dependence of the energy generation rate levels off to that of Eddington nuclear powers yielding strong photospheric ex- the radiative cooling rate (which is proportional to T 4). More pansion due to radiation pressure [176, 177], as predicted by than 99% of all X-ray bursts are due to ignition of helium ref. [178]. Surface absorption features will be gravitation- through 3α burning and α-captures. It is predicted that the ally redshifted, which can provide additional constraints on ambient temperatures in the burning layer only become high the compactness of neutron stars if rest wavelengths are iden- enough (& 5 × 108 K) for stable burning when the mass ac- tified. In this respect, the SFA would provide unique data. cretion rate is above the Eddington limit (e.g., ref. [187]). Most burst data currently have no coverage of sub-2 keV pho- However, one finds evidence of stable burning (i.e., absence ton energies. Those that do and are not susceptible to detec- of X-ray bursts and presence of mHz QPOs indicative of tor saturation, taken for instance with the grating spectrom- marginally stable burning) already at 10%-50% of Edding- eters on Chandra and XMM-Newton, are of low statistical ton (e.g., refs. [180, 182, 188-190]). A different perspective quality due to effective areas less than 150 cm2. The SFA on this issue may be provided by eXTP, by following exten- is anticipated to have pile-up fractions of a few percent for sively the aftermath of superbursts for about a week. The typical burst peak fluxes of a few Crab units and has an ef- superburst is another means to increase the ambient temper- J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-17 atures. While the neutron star ocean cools down in the hours duced by the burst interacting with the accretion flow (e.g., to days following a superburst, models predict [191] that it refs. [194,195]). The SFA and LAD will be able to detect re- is possible to follow the transition from stable to unstable flection features from bursts with integration times as short as burning on a convenient time scale, through sensitive mea- a few seconds (e.g., ref. [196]). As the evolution of the burst surements of low-amplitude (∼ 0.1%) mHz oscillations and is easily measured, changes in the reflection features can di- the return of normal, initially faint, bursting activity (see Fig- rectly map the changes in the accretion disk over the course ure 9). of the burst. • What burns in superbursts; what determines the thermo- X-ray burst science is also a high priority for the nuclear dynamic equilibrium of the crust and ocean; what determines science community. X-ray bursts are unique nuclear phe- the stability of nuclear burning; what is the nature of sur- nomena, and identified as key problems for nuclear science face nuclear burning at low accretion rates? WFM observa- in the US National Academies report NP2010 “An Assess- tions will measure more precisely the superburst recurrence ment and Outlook for Nuclear Physics”, the 2015 US Nu- time and time profile (e.g., ref. [167]). This will constrain clear Science Long Range Plan, and the 2017 NuPECC Eu- the thermodynamic balance in the ocean and crust, and probe ropean Long Range Plan. Major investments in a new gen- in unprecedented detail the boundary between stable and un- eration of radioactive beam accelerator facilities, in part mo- stable thermonuclear burning [157]. WFM will accumulate tivated by X-ray burst science, are now being made around large exposure times on all bursters (including yet unknown the world, including FRIB in the US, FAIR in Germany, and ones) and thus detect rare classes of events, such as the most HIAF in China. New instruments such as the recoil sepa- powerful intermediate-duration bursts at very low accretion rator SECAR at FRIB or storage rings at FAIR or Lanzhou rates. (China) are being developed to enable nuclear reaction rate • How do bursts impact and influence the surrounding ac- measurements for astrophysical explosions such as X-ray cretion flow; can bursts yield insights into the underlying ac- bursts. cretion physics? X-ray bursts can deliver a powerful impulse With a simultaneous advance in X-ray observational capa- to the surrounding accretion disk (e.g., review by ref. [192]). bilities through eXTP there is a unique opportunity to com- The response of the disk to the bursts could include (1) an in- bine accurate nuclear physics and precision light curve data crease in the accretion rate due to Poynting-Robertson drag, to determine stellar system parameters, to identify different (2) radiative driven outflows, (3) an increasing scale height nuclear burning regimes, to validate astrophysical 1D and 2D due to X-ray heating or (4) a combination of all these effects models, and to constrain neutron star properties. In addition, [193]. Unique insights into the properties of accretion disks the detection of spectral features from heavy elements on the could be obtained if this response can be measured for bursts neutron star surface would be of high importance for nuclear with a wide range of durations. The response of the disk can science, as it would provide direct insight into the nuclear be measured by detecting the X-ray reflection features pro- processes that power X-ray bursts. The Joint Institute for

Figure 9 Simulation of the ordinary bursting behavior (blue curve) around a superburst (red curve) in a hydrogen-rich low-mass X-ray binary (from ref. [191]). The superburst quenches ordinary bursts for one day after which a regime of marginal burning commences, characterized by mHz QPOs. The lower panel shows possible WFM and LAD measurements of such a superburst, if it is at 8 kpc, on-axis, placed at 45◦ from the Galactic center and if the accretion rate is 10% of the Eddington limit. J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-18

Nuclear Astrophysics (JINA-CEE) is now fostering the close ing material [204-206]. Extending the time scale of long- connections between observational and nuclear science com- term monitoring of the behavior of pulsars in HMXBs is es- munities that are needed for nuclear physics based model pre- sential to make progress in understanding this phenomenon. dictions to guide the interpretation of observations, and for Previous large field-of-view instruments provided the oppor- new observations to motivate nuclear experiments. tunity to monitor many X-ray pulsars in HMXBs, symbiotic X-ray binaries (SyXBs) in which the donor star is a red gi- ant, and in intermediate systems between HMXBs and low- 7 High-mass X-ray binaries mass X-ray binaries. Particularly striking were the findings for GX 1+4 and 4U 1626-67: after about 15 yr (for GX 1+4) Massive supergiant, hypergiant, and Wolf-Rayet stars and 20 yr (for 4U 1626-67) of spin-up, both systems showed (M &10 M⊙) have the densest, fastest, and most structured a torque reversal, which made them switch to a spin-down winds. The radiatively accelerated outflows trigger star for- phase [207, 208]. Other systems, such as Cen X-3, VelaX- mation and drive the chemical enrichment and evolution of 1, 4U 1907+09 and Her X-1, often showed torque reversals, Galaxies [197]. The amount of mass lost through these winds sometimes superimposed on a longer term trend of either has a large impact on the evolution of the star. The winds can spin-down or spin-up [207, 208]. give rise to an extremely variable X-ray flux when accreted At low mass inflow rates, the accretion might be inhibited onto an orbiting compact object in a high-mass X-ray binary completely through the onset of so-called centrifugal and/or (HMXB). The understanding of the relation between the X- magnetic barrier, which could drive the rapid variability ob- ray variability and stellar wind properties has been limited so served in the SFXTs and other HMXBs [205]. The observed far by the lack of simultaneous large collecting area and good variability is thus often used to probe the strength of the dipo- spectral and timing resolution. lar component of the neutron star magnetic field. This also In the past decades, observational evidence has been grow- shapes the geometry of the emission region in X-ray pulsars, ing that winds of massive stars are populated by dense which, in turn, influences their observed properties. Most no- “clumps”. The presence of these structures affect the mass tably, for luminosities above the critical value ∼1037 erg s−1, loss rates derived from the optical spectroscopy of stellar the local accretion rate at the neutron star polar areas might wind features, thus leading to uncertainties in our understand- exceed the Eddington limit, leading to the formation of an ing of their evolutionary paths [198]. HMXBs were long con- extended accretion columns [209]. The transitional luminos- sidered an interesting possibility to probe clumpiness [199]. ity setting the threshold between sub- and super-critical ac- As X-rays released by accretion trace the mass inflow rate cretion is directly related to the compactness of the neutron to the compact object, an HMXB provides a natural in-situ star and its magnetic field strength, and can thus be used to probe of the physical properties of the massive star wind in- constrain these parameters. The presence of an accretion col- cluding its clumpiness [200-202]. The so-called “Supergiant umn is expected to alter dramatically the intrinsic beaming of Fast X-ray Transients” (SFXTs) form the most convincing the pulsar X-ray emission, as well as its spectral energy dis- evidence for the presence of large clumps. In X-rays, the im- tribution and polarization properties [209]. Observing these print of clumps is two-fold: (1) clumps passing through the changes can thus be used to obtain a direct measurement of line of sight to the compact object cause (partial) obscura- the critical luminosity from the observations [210]. tion of the X-ray source and display photo-electric absorp- The propagation of X-ray photons emerging through the tion and photo-ionization; (2) clumps lead to temporarily in- strongly magnetized plasma within the neutron star emitting creased accretion and X-ray flares. A number of hours-long region also affects the observed spectra. The electron scatter- flares displayed by the SFXTs could be convincingly associ- ing cross section in strong magnetic field is largely enhanced ated with the accretion of dense clumps [203]. around the electrons gyro-frequency, leading to the so-called The compact objects in HMXBs are usually young neu- cyclotron resonance scattering features (CRSFs) often de- tron stars (typical age 106-7 yr) that are known to possess tected in the X-ray spectra of young accreting pulsars (for strong magnetic fields (&1012 G). These channel the accret- a recent review see ref. [201]). The scattering cross section ing material from distances as large as ∼103-104 km down to strongly depends on the angle between the direction of the the surface of the compact object, yielding X-ray pulsations, photon propagation and the magnetic field orientation. This and change the spin of the compact object. Large positive implies that the polarization degree and angle, the X-ray con- and negative accretion torques have been measured in both tinuum spectrum, and the CRSF parameters depend on pulse wind and disk-fed HMXBs on time scales between years and phase. Modeling these changes allows us to reconstruct the decades. These torques result from the yet unknown cou- configuration of the magnetic field and its orientation with pling between the neutron star magnetic field and the accret- respect to the observer and the neutron star. J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-19

The mass accretion rate for black holes in HMXBs tends study the spectral and temporal behavior of accreting neutron to be less variable compared to the NS case. However, most stars with strong magnetic fields. Follow-up observations ex- black holes in HMXB show some hard-to-soft state variabil- ploiting the high sensitivity of the SFA will allow us to ob- ity, similarly to what is observed in BH low-mass X-ray bi- serve the so-called propeller effect in dozens of X-ray pul- naries. The most studied and observed BH HMXBs are Cyg sars. For the sources with measured cyclotron energies it will X-1 [211, 212] and LMC X-3 [213, 214]. These sources make possible to determine the configuration of the neutron proved particularly useful to “X-ray” the stellar wind of their star magnetic field. Monitoring observations of long-period donor stars, complementing similar studies carried out on pulsars at low mass accretion rates will constrain the physi- NS HMXBs and discussed above. For example, the Galac- cal mechanisms regulating the interaction of plasma in a low tic black hole Cyg X-1 shows a periodic modulation of the ionization state with the neutron star strong magnetic field absorption column, which is due to the absorption of X-rays [220]. from the black hole in the massive stellar wind of its donor • What are the geometry and physical conditions within star, HDE 226868. As shown by ref. [215], the large scatter the emission region of X-ray pulsars, and their relation to in individual measurements of the absorption column to the the observed pulse profiles, spectra, and polarization in the black hole can be used to study the size distribution of clumps X-ray band? Pulse phase-resolved spectroscopy of accreting in the stellar wind, which is of great interest for understand- pulsars has been one of the main tools to diagnose their emis- ing the physics of the winds of massive stars [216]. sion region properties since decades. eXTP will bring such The instruments on-board eXTP will dramatically open up investigations to an entirely new level. The large effective perspectives for research in all above mentioned fields. In area of the SFA and the LAD will provide spectra and pulse particular the following questions can be addressed: profiles of unprecedented statistical quality even within com- • What are the physical properties of massive star wind paratively short integration times. It will be possible for the structures and how do these affect the mass loss rates from first time to study the spectral and pulse profile evolution on these objects? With current instruments, integration times of very short timescales comparable with the spin period of the several hundreds to thousands of seconds are needed to get a neutron star. The energy resolution of the LAD in a broad rough estimate of clump properties and only an average pic- energy band will allow us to perform detailed studies of the ture of the clump accretion process can be obtained. Simi- complex CRSF shapes observed in some sources [221], and lar observations performed with the LAD and SFA on-board investigate dependence of the CRSF parameters on the pulse eXTP will dramatically improve our present understanding of phase, which is essential to constrain the line formation mod- clumpy wind accretion and winds in massive stars in general, els and the emission region geometry. by studying (spectral) variability on time scales as short as a In addition, eXTP will provide for the first time highly few to tens of seconds Figure 10. This will permit a detailed complementary simultaneous X-ray polarization data. The investigation of the dynamics of the clump accretion process expected strong polarization and high X-ray fluxes of the ac- and obtain more reliable estimates of the clump mass, radius, creting pulsars make these sources a prime target for X-ray density, velocity, and photo-ionization state. In turn, it will polarimetry studies. The polarization degree and angle carry improve our understanding on the mass loss rates from mas- information on the geometry of their emission region, mag- sive stars. As structured winds are not spherically symmet- netic field configuration, and orientation of the pulsar with re- ric, they are also predicted to emit polarized X-ray radiation. spect to the observer [222]. A major change of the observed Measurements with the PFA can yield additional information polarization signatures is expected with the transition from to constrain the stellar mass loss rate [217]. The improved the sub- to super-critical accretion, i.e. at the onset of ex- capabilities of all eXTP instruments compared to the current tended accretion columns. eXTP observations can be used facilities will permit to conduct also similar studies on the to detect such transitions and the corresponding changes in SyXBs [218], thus exploring the structure and composition of the polarization properties of X-ray pulsars, as illustrated in the still poorly known winds of red giants stars (which might Figure 11. be accelerated through the absorption of the stellar radiation • What are the mechanisms triggering torque reversals in by dust grains rather than heavy ions). Finally, it is impor- HMXBs and what leads to orbital and super-orbital modula- tant to understand the consequences of inflated envelopes of tion of the X-ray emission from these systems? For systems massive giant stars [219] on the observational properties of with well known distances, simultaneous measurements of HMXBs in which the donor stars are filling, or close to fill- fluxes and spin-rates can provide constraints on the magnetic ing, their Roche lobes. The ability of the eXTP instruments field [223]. If the pulsar magnetic field is also known from (especially the WFM) to detect and monitor transient sources cyclotron line measurements, then the relationship between in a very broad range of mass accretion rates will allow us to the spin rate and the luminosity can be used to constrain the J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-20

(a)

(b) (c)

100 LAD

) SFA −1 keV −1 10

XMM

1 Normalized counts (s

20

χ 10

0

25 10 20 Time (s) Energy (keV) Figure 10 (a) Sketch of the accretion by the upper neutron star of a clump ejected by the lower supergiant star (credits: ESA). (b) Changes in the flux and spectral parameters during the flare recorded by XMM-Newton, from the SFXT IGR J18410-0535 [203]. Violet boxes represent the measurements obtained with XMM-Newton, while black points represent values obtained from the simulated LAD +SFA spectra with exposure times as short as 100 s. Compared to XMM-Newton, the dynamic process of the clump accretion can be studied in much more details and fast spectral changes can be revealed to an unprecedented accuracy (we remark also that the pile-up and dead-time free data provided by the LAD and SFA greatly reduce the uncertainties affecting the XMM-Newton, data obtained so far from HMXBs during bright flares). (c) Shown, for comparison, is an example of a XMM-Newton, spectrum extracted in a 1 ks-long interval (source flux 3×10−10 erg s−1 cm−2) during the decay from the flare shown on the left and the corresponding simulated SFA and LAD spectra. The Fe-K line at 6.5 keV, used to probe the clump material ionized by the high X-ray flux, is barely visible in XMM-Newton, but very prominently detected in both the SFA and LAD spectra. neutron star parameters, probing not only accretion physics accreting X-ray pulsars will contribute to the solution of the but also fundamental physics in these highly magnetized ac- questions mentioned above, and it will also trigger target of creting systems. The WFM is a perfectly suited instrument opportunity observations with the LAD/SFA at the onset of for these studies in that it is a wide field monitor capable of torque reversals. Early detections of these state changes are simultaneously measuring the source flux and accurately de- critical to initiate early observations by the LAD/SFA in or- termining its spin frequency. Its lower energy threshold will der to understand the associated spectral/timing variation As make it the most sensitive monitor for these types of systems a byproduct of the monitoring observations, it will be possi- that has ever flown. Measurements from the WFM of pulsed ble to carry out detailed spectroscopic analyses as a function flux, unpulsed flux, frequency and frequency derivatives of of orbital, superorbital and spin phase. The orbital depen- J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-21

(a) +90 30.25 eXTP WFM 30.00

29.75 +180 −180 29.50 (keV)

cyc 29.25 E

29.00

28.75 −90

28.50 Figure 12 Comparison of WFM’s field of view with the 2-20 keV map, 2 4 6 8 10 12 14 in Galactic coordinates, of the X-ray sky as observed with MAXI (courtesy L 37 of T. Mihara, RIKEN, JAXA, and the MAXI team). The WFM will have x/10 (erg/s) the largest field of view and monitor simultaneously a large fraction of all currently known HMXBs and low-mass X-ray binaries. 60 (b) Sub-critical Super-critical 50 8 Radio-quiet active galactic nuclei 40

30 The standard model of active galactic nuclei (AGN) predicts “Fan” a supermassive black hole (SMBH) surrounded by a geomet- 20 rically thin, optically thick accretion disk. Like the case of Polarizationdegree X-ray binaries, the seed photons from the disk are Compton “Pencil” 10 scattered into the X-ray band in a hot corona, the main differ- ence being that the accretion disk in AGN emits at optical/UV 0 102 103 104 105 106 instead of X-ray temperatures. The UV and optical seed pho- Requiredexposure (s) tons from the disk are Compton scattered into the X-ray band in a hot corona above the accretion disk which produces the Figure 11 The combination of the LAD large effective area and energy hard power-law spectrum extending to energies determined resolution will allow us to monitor the luminosity-related changes of the observed CRSF energy in transient X-ray pulsars with unprecedented accu- by the electron temperature in the hot corona, with a cut-off racy. (a) A simulation of thirty 10 ks LAD observations of the X-ray pulsar energy ranging from 100 to 200 keV [224-228]. This primary V 0332+53, where a transition from super- to sub-critical accretion has been X-ray radiation in turn illuminates the inner disk and is partly recently detected through a luminosity-dependent change of the observed CRSF energy [210]. eXTP will allow us to investigate the same effect with reflected towards the observer’s line of sight [229], producing a much larger accuracy and to investigate changes in pulse profile shape and a Fe-K line that will be heavily distorted, with a pronounced polarization signatures associated with similar transitions. (b) The expo- red wing—a clear signature of strong field gravity effects on σ sure required to measure the polarization of a given strength at a 3 signifi- the emitted radiation. The phenomenology of AGN is quite cance level for the typical super-critical and sub-critical fluxes observed from V 0332+53. The polarization signal strength predicted by ref. [222] in the similar to stellar black holes accreting from companion stars two cases are also shown. The eXTP PFA will allow us to detect and com- in X-ray binaries, except for a difference in time scales and a pare the polarized emission in both the sub- and super-critical regime using difference in typical temperatures of spectral thermal com- relatively short exposure times, performing also studies of the dependence ponents from the accretion disk that scales with the black of the polarization properties as a function of the pulse phase. hole mass because the size of the innermost stable circular orbit scales with mass. The inner structure in AGN is sur- dence, for instance, can be used to map long-lived structures rounded by rapidly rotating clouds of gas—the Broad Line surrounding the neutron stars in these systems, like accretion Region (BLR). A toroidal absorber is located at a distance wakes. of a parsec from the black hole and within the Narrow Line Last but not least, it is worth mentioning that many Region (NLR) clouds. These regions contribute with addi- HMXBs are transient and thus the discovery of new sources tional features to the primary continuum: (1) a narrow Fe-K or new outbursts from previously known objects in this line at 6.4 keV produced in the BLR, in external regions of class will greatly benefit from the capabilities of the WFM. the accretion disc or in the molecular torus; (2) a reflection Figure 12 shows that a large fraction of all these sources can hump above 10 keV, due to the reprocessing of the continuum be efficiently monitored during each single pointing of the from the optically-thick medium (such as the torus and/or the WFM, let alone in sequential different pointings. accretion disk); and (3) the absorption and variable features J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-22 produced in the ionized and cold material around the inner re- • What are the emitting materials around the central su- gion (up to the NLR distance scales). The typical broadband permassive black hole? The origin of the narrow Fe emission spectrum as may be observed by eXTP is shown in Figure 13. line observed in AGN is not completely clear, while this is Thanks to the large effective area and the CCD-class a fundamental piece of information about the SMBH central spectral resolution of the LAD and SFA, coupled with the engine and its environment. The width of the line is less than large sky coverage of the WFM, eXTP will make a deci- few thousand km/s (in terms of full width at half maximum, sive step forward in our understanding of the inner struc- [231]) and suggests an origin within the BLR or the puta- ture of AGN. The unprecedented SFA spectral sensitivity tive molecular torus (at pc scale) or within the BLR closer to will allow one to observe hundreds of AGN down to a the BH. A powerful method to study the origin of the narrow flux of 10−13 erg s−1 cm−2 in the local universe with a very Fe line is to measure the time lag between the variation of good accuracy (i.e., a signal-to-noise ratio in excess of 100). the continuum and the line [232]. The reverberation mapping Thanks to the LAD effective area, the brightest AGN (3 × methodology is used with the optical and UV broad emis- 10−11 erg s−1 cm−2) can be characterized with extreme ac- sion lines with great success [233] and, recently, in X-rays to curacy. Moreover, the WFM will be capable of continuous investigate the origin of the broad Fe line [234, 235]. How- X-ray monitoring of large numbers (&100) of AGN on time ever, the uncertainties on the Fe line flux (about 10%) and the scales of a few days to weeks. The WFM will thus provide low sampling frequency of the X-ray continuum, prevent us an essential synergy with those observatories that will, in par- applying this method to a large sample of AGN, especially allel to eXTP, be opening up AGN time domain science on on the daily time scale expected for a BLR origin [236-238]. those time scales at other wavebands, for example with CTA eXTP will allow one, for the first time, to perform X-ray re- (TeV energy domain), Meerkat/ASKAP/SKA (radio wave- verberation of the narrow core and broad component (see the lengths) and LSST (optical band, see Figure 1). Here, the eXTP white paper on strong gravity [4]) of the Fe line with a focus is on three key aspects of AGN physics (related to the single mission. accretion properties and variability), which will be specifi- The very large field of view of the WFM is a powerful cally addressed by the unique capabilities of eXTP: tool to monitor the continuum intensity of virtually all the sources in the sky. To demonstrate the unique capabilities of the WFM we used the 1-year exposure sky map shown

) (a) −1 10 in Figure 14 as a good approximation of the adopted point- keV

−1 ing strategy. The WFM can produce continuous light curves 1 with daily 3σ detections for bright sources (i.e., brighter than ∼ 1 × 10−10 erg s−1 cm−2 in the 7-50 keV band, i.e., above

Counts (s 0.1 the Fe K edge). Weaker objects (∼ 5 × 10−11 erg s−1 cm−2) σ 4 (b) 2 will be detectable in weekly exposures. These time scales are 0 perfectly suited for the narrow Fe line reverberation analy- −2 −4 sis, since the expected time scales are from days to weeks (data−model)/ 1 2 5 10 20 to years [239], depending on the location and geometry ) −1 (c) of the material and the intrinsic variability of the sources

keV 0.01 −1 (therefore, on the black hole mass and the accretion rate). s −2 At the same time, the large area available with the SFA 10−3 cm 2 and LAD will allow one to perform well-sampled (weekly

(keV 1 10 100 to monthly) monitoring of the Fe-K line and Compton re- Energy (keV) flection component with short targeted observations. The

Figure 13 (a) eXTP (SFA in black, LAD in red) spectrum. (b) Residu- CCD-class energy resolution will easily allow one to disen- als as obtained by a 100 ks integration of a bright 1 mCrab AGN composed tangle the broad Fe line component from the narrow core. of: the continuum, three components of ionized absorbers, the cold reflec- In fact, in only 1 ks a narrow Fe line of equivalent width α β α tion and narrow Fe line K , Fe K and Ni K , the ionized lines FeXXV 100 eV can be recovered with an uncertainty of 1%-2% for and FeXXVI and the blurred ionized reflection component (black hole spin ∼ × −10 −1 −2 ∼ 3 a = 0.7). (c) The model complexity we used for the simulation is similar bright sources ( 1 10 erg s cm with 10 SFA plus to the case of the AGN SWIFT J2127.4+5654 [227,230] and where separate LAD counts in the line). The same uncertainties can be re- spectral components are indicated with different colors (in blue the primary covered for weaker sources (∼ 5 × 10−11 erg s−1 cm−2) with ff cut-o power-law, in cyan the blurred reflection component originated in the 5 ks exposures. Thanks to the broad bandpass of the LAD, innermost regions nearby the central BH and in magenta the contribution of the a cold reflection component originated in a medium distant from the eXTP will be able to recover the reflection continuum with central BH. 10% accuracy in a 1 ks exposure for the brightest sources J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-23

Figure 14 Number of AGN per 4π sr as a function of the position in the sky that are bright enough to be accessible to the WFM for studies of X-ray variability. For these sources, a light curve with at least 10 time bins in a 1-year period can be built, achieving in each bin a signal-to-noise ratio of at least 3. The lateral bar reports the total number of variable AGN observed on the sky, assuming an example of a 1-yr pointing plan. Only regions of the sky covered with more than 10 cm2 of net detector area have been considered.

(∼1 × 10−10 erg s−1 cm−2). This measurement, together with density and of the covering factor over timescales of a few ks the WFM daily monitoring of the continuum above 7 keV will provide unprecedented information on the structure, dis- (even with a low signal-to-noise ratio), will be a instrumen- tance and kinematics of the obscuring clouds. In particular, tal in identifying the origin of the narrow distant reflection close to the lower end of the BH mass range in AGN, with components in AGN. a single 100 ks observation it will be possible to fully char- • What are the absorbing regions in the environment of su- acterize the circumnuclear absorbing medium by sampling permassive black holes, from kpc to sub-pc scale? There is eclipsing events from multiple clouds (e.g., ref. [248]). which strong evidence of at least three absorption components on is now precluded by the short timescales involved. Moreover, very different scales: on scales of hundreds of parsecs, on the the broad-band energy range offered by eXTP will allow one parsec scale, and within the dust sublimation radius, on sub- to explore in detail also those clouds with larger column den- parsec scale (e.g., refs. [225, 238, 240]). The most effective sities in Compton thick AGN, whose variability mostly af- way to estimate the distance of the absorber is by means of fects the spectrum at high energies. the analysis of the variability of its column density. In par- • How does the central engine produce the observed vari- ticular, rapid (from a few hours to a few days) NH variability ability? Our knowledge of the X-ray variability in AGN has has been observed in most bright AGN in the local Universe advanced substantially in the last 15 years, thanks mainly [241-247], suggesting that obscuration in X-rays could due, to monitoring campaigns with RXTE and day-long XMM- at least in part, to BLR clouds or in the the inner regions of Newton observations of a few X-ray bright AGN [249-253]. infrared-emitting dusty tori. These observations with XMM- AGN show red noise in their power spectral density func- Newton and NuSTAR have achieved a precision for the cover- tions (PSD) with a power law that decreases at high frequen- ing factor of ∼5%-10%. Detailed simulations show that such cies as a power law with index −2. Below some character- a measurement (assuming the same observed fluxes) will be istic frequency the PSDs flatten and the break frequencies obtained with eXTP with significantly higher precision (1%- scale approximately inversely with the BH mass, from BH 2%), on even shorter time scales (few ks instead of ∼100 ks). X-ray binaries to AGN [254]. These characteristic frequen- This would allow, for the first time, an extensive study of X- cies seem linked to both the BH mass and the properties of ray eclipses, which are frequently seen in local bright AGN the accretion flow. Although all the relevant accretion disk (e.g., ref. [246]). Following the time evolution of the column time scales depend on the mass of the central object, only a J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-24 few of them may depend (indirectly) on the accretion rate as We estimate this to be a few thousand with SFA and a few well. So far, our knowledge is based on the power-spectral hundred simultaneously with the LAD. These numbers are analysis of a few objects. A substantial improvement of the derived from the 30-50 keV log N − log S reported in ref. analysis of X-ray variability in AGN would be possible by [258], after applying a flux rescaling to match the LAD and avoiding the problems introduced by large statistical noise SFA energy bands. This extrapolation is well justified by the and irregular time sampling. The WFM plays a crucial role fact that a hard X-ray survey preferentially selects Flat Spec- in our understanding of the X-ray variability in AGN. The trum Radio Quasars and low energy peaked BL Lacs due to WFM will survey large areas of the sky, allowing one to study the hardness of their X-ray spectra, which are generally well long-term X-ray variability of bright/nearby AGN. Using the described by a single power law in this entire energy range. In AGN number counts observed by previous missions, we com- addition, the WFM can provide monitoring of bright AGN on puted the number density of sources that will be accessible to time scales of days to weeks thanks to its large sky coverage. eXTP in each region of the sky (assuming an average AGN Among the radio-loud AGN, blazars are certainly the spectrum and Galactic absorption). Using the typical AGN most promising in the era of the time domain astronomy. flux distribution, we derived the number of sources that are They are among the most powerful persistent [259] cosmic bright enough to build a light curve with a worst resolution sources, able to release (apparent) luminosities in excess of of 1 month at signal-to-noise ratio of 3. Figure 14 shows the 1048 erg s−1 over the entire EM spectrum, from the radio to number density map of AGN that fulfill these criteria. Ref. the very high energy γ-ray band. This intense non-thermal [255] demonstrated that it is possible to retrieve the intrin- emission is produced within a relativistic (bulk Lorentz factor sic variability (within a factor of 2) even when using sparse Γ = 10-20) jet pointing toward the Earth. Relativistic effects light curves with low signal-to-noise ratio, such as the ones lead to the beaming of the radiation within a narrow cone of expected here for most of the faint AGN population in the semi aperture ∼1/Γ rad with strong apparent amplification of WFM sample. Depending on the exact monitoring pattern, the luminosity and shortening of the variability time scales in much smaller uncertainties are expected for the brightest 20 the cases in which (as in blazars) the jet is closely aligned to sources. The WFM, even in its first year of operations, is the line of sight. expected to return light curves with at least 10 data points Blazars come in two flavors: those that show optical broad per year for several hundreds of variable AGN, allowing both emission lines (typically observed in quasars that are called individual (for the brightest) and ensemble studies of AGN Flat Spectrum Radio Quasars or FSRQs) and those that in variability on long (monthly) time scales. One will therefore general display rather weak or even absent emission lines, be able to determine accurately the intrinsic variability am- collectively grouped in the BL Lac object class. The lat- plitude (on long time scales) for hundreds of AGN and study ter generally show the most extreme variability, with varia- the variability-luminosity relation with an unprecedented ac- tions as fast as a few minutes, and the most energetic photons curacy. This relation will then be compared with similar re- (above 100 GeV). lations for AGN at higher redshifts (see, e.g., that derived The conventional scenario (e.g., ref. [260]) foresees a from the Chandra Deep Field-South survey [256]), obtained, single portion of the jet dominating the overall emission, however, with much higher statistical uncertainties, due to at least during high activity states. However, this simple the limited sample size, that prevent us to probe effectively idea has recently been challenged by the observation of very the luminosity, redshift and timescale dependence of the in- fast (minutes) variability, which requires the existence of trinsic AGN variability (e.g., ref. [257]). very compact emission sites. The most extreme example is PKS 2155-304, which experienced two exceptional flares in 2006—with a recorded luminosity of the order of several 9 Radio-loud active galactic nuclei times 1047 erg s−1—on top of which events with rise times of ∼100 s have been detected. Even assuming that these strong eXTP is planned in the same time frame as other observato- events originated in very compact emission regions, the phys- ries that are opening up a window to short time scale variabil- ical conditions should be rather extreme and Lorentz factors ity for AGN, such as the Cherenkov Telescope Array (CTA) of Γ∼50-100 seem unavoidable [261]. Physical mechanisms at TeV energies and the Square Kilometer Array (SKA) and possibly triggering the formation of these compact emission Atacama Large Millimeter Array (ALMA) in the radio and regions include magnetic reconnection events [262] and rela- (sub-)mm bands, respectively (see Figure 1). This offers a tivistic turbulence [263, 264]. unique opportunity to interpret short time scale variability Alternatively, narrow beams of electrons can attain ultra- and connect accretion and jet physics in radio-loud AGN. relativistic energies (Γ∼106) in the black hole vicinity through eXTP will be able to follow up a large number of AGN. processes involving the black hole magnetosphere [265,266]. J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-25

In this case, it is expected that the emission shows up only in 0.5-10 keV flux & 2 × 10−13 erg s−1 cm−2. For these objects, the high energy γ-ray region, giving no signals at lower fre- eXTP will probe the rise of the inverse Compton (IC) emis- quencies (thus resembling the case of the so-called “orphan” sion whereas new generation radio and (sub-)mm instruments flares; different from the simultaneous X-ray and TeV flaring will provide excellent timing capabilities to study short time- observed from PKS 2155−304 in 2006). scale correlations with the synchrotron spectral domain. The eXTP can address the following questions with regards to nature of the photon seeds for the IC emission responsible radio-loud AGN: for the high-energy emission is still strongly debated. Tight • What causes the fast γ-ray flares? Until now, ultra-fast constraints are expected to come from the investigation of variability has been recorded only in the γ-ray band, espe- time scales of variability and polarimetry. In this respect, the cially at TeV energies, at which the Cherenkov arrays are SFA will be crucial in a multi-frequency context providing: characterized by gigantic collection areas, required to probe (1) temporal investigation on second time scales; (2) spectral such short time scales. Current instruments do not provide an trend investigation on minute time scales. With CTA, this analogous sensitivity in the key X-ray band, at which the low may provide a multi-wavelength perspective to understand energy component of TeV emitting BL Lacs (the so-called better the rapid (unexpected) TeV emission recently detected HBLs) peaks and thus it is not possible to study potential in some FSRQs. The WFM will provide not only an excel- ultra-fast variability events at these frequencies. Moreover, lent trigger for both CTA and (sub-)mm observatories but also such events appear to be quite rare, with a duty cycle of less long-term monitoring of a large sample of blazars. than 1%. Therefore, monitoring of the sky is required to catch Moreover, polarimetric measurements performed in the these events. 2-10 keV band, in comparison with polarization at lower The LAD, thanks to its timing capability, will allow one frequencies, represent a powerful tool to solve the long- to detect the X-ray counterpart of the very fast variabil- standing problem of the origin of the seed photons and ity observed at TeV energies for 2-10 keV fluxes above therefore constrain the high-energy dissipation region for the 2 × 10−11 erg s−1 cm−2, shedding light on the nature of the brightest objects and flares on daily time scale. The aver- X-TeV connection in HBLs [267]. Moreover, eXTP polari- age X-ray fluxes of FSRQs are actually much lower than metric measurements of HBLs will constrain the nature of those of high synchrotron-peak blazars (in the range 10−12- the electron population(s) responsible for the X-ray emis- 10−11 erg cm−2 s−1) and small amplitude variations have been sion, providing us with the magnetic field orientation and its detected during the flaring activity. This implies much longer change during the flaring activity as compared with the aver- integration times to achieve MDP values of the order of a few age emission. In these blazars, the polarimeter energy band %, important to disentangle among different leptonic models will cover the region of the synchrotron peak, where flux and (external Compton versus synchrotron self Compton, SSC) polarization degree demand sub-daily integration times. If by modeling the X-ray emission. In particular, in the case one assumes a 2-10 keV flux of 2 × 10−10 erg s−1 cm−2, one of FSRQs with flux ∼2 × 10−11 erg s−1 cm−2, a minimum de- can measure a minimum detectable polarization of ∼ 10% in tectable polarization of 7% will be obtainable in 100 ks. 5 ks with the PFA. This value is in the range of polarization • What is the disk-jet connection in misaligned AGN? measurements for synchrotron emission as derived from opti- WFM long term monitoring (monthly time scale) of radio cal measurements (between 3% and 30%). A highly ordered galaxies hosting efficient accretion disks (mainly Fanaroff- magnetic field would imply higher polarization degrees. In Riley II sources, hereafter FRIIs; Figure 15) will provide an this respect, eXTP will be able to probe the evolution of the optimal tool to investigate the disk-jet connection by trigger- magnetic field lines and their role in powering fast variations ing both radio and LAD X-ray follow-ups. eXTP will inves- as detected by the LAD (flare rise over 100 s as detected in tigate the disk-jet connection in a similar manner as in X-ray the TeV range). binaries, that exhibit the same behavior [268]. • What accelerates highly energetic electrons? A fur- Recent detections of radio galaxies hosting inefficient ac- ther potential of eXTP in the study of HBLs is measuring cretion flows (Fanaroff-Riley I, hereafter FRI) at high and the change in spectral curvature of their synchrotron spectra very high energies (e.g., IC 310 [271]) are providing a fur- which will enable a direct study of the mechanism of acceler- ther sample of radio-loud sources well suited for follow- ation of highly energetic electrons. These studies will benefit up observations with eXTP. Combining SFA and LAD with from the simultaneous observations of the LAD and SFA that CTA simultaneous observations will be crucial in constrain- provide the broad band coverage (0.5-30 keV) of the syn- ing the TeV emitting region, likely connected with the mag- chrotron emission. netosphere surrounding the central engine, as derived by the • Where is the high-energy dissipation region? The SFA rapid variability observed by MAGIC [271]. FRI behave like sensitivity is very promising for the study of FSRQs with a high synchrotron-peak blazars (with lower luminosities) and J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-26

Figure 15 Chandra 0.3-7 keV image of the FRII radio galaxy 3C 111 with 3.6 cm radio contours overlayed [269]. In the inset the multi-wavelength light curve from mm to γ-rays is shown (from ref. [270]). The simultaneity of the flare is impressive: the core luminosity was increasing from millimeter to X-ray frequencies exactly when the greatest flux of γ-ray photons occurred. This is a clear indication of the cospatiality of the event. The outburst of photons is directly connected to the ejection of a new radio knot (the time of the ejection is indicated by the black arrow). therefore simultaneous observations in X-rays and at TeV ray spectra of NLSy1. This is to investigate the possible link energies are the key to interpret their spectral energy dis- between accretion and jet properties. tribution in terms of SSC rather than hadronic models (not ruled out so far). Given the typical fluxes of TeV FRI (a few ×10−12 erg s−1 cm−2), SFA (combined with LAD for the 10 Tidal disruption events brightest flares) observations will allow one to detect the X- ray counterpart of the fast TeV variations, providing tight Observational evidence is mounting to support a scenario constraints on the location of the TeV emitting region and where most galactic nuclei host SMBHs. Gas inflow causes the particle acceleration process [272]. a small (∼1%) fraction of SMBHs to accrete continuously for • How important is reflection in spectra of radio-loud millions of years and shine as AGN. Most are expected to narrow-line Seyfert 1 galaxies (NLSy1s)? NLSy1s are of- be quiescent, accreting if at all at a very sub-Eddington rate ten considered as a third class of AGN with relativistic jets [275]. Observationally, it is therefore hard to assess the pres- [273, 274]. LAD observations will be very promising not ence and mass of most SMBHs beyond the local universe. only for studying the physical mechanisms responsible for Occasionally, a sudden increase of the accretion rate may oc- the higher energy emission (in γ-rays or at TeV energies) and cur if a large mass of gas, for instance a star, falls into the tidal then the blazar-like behavior, but also for determining the role sphere of influence of the black hole and finds itself torn apart of the thermal component. The combination of SFA and LAD and accreted. One calls these events ’tidal disruption events’ follow-up of the high energy flares from NLSy1 detected in (TDEs, see ref. [276]). TDEs can result in a sudden increase γ-rays or at higher energy γ-rays will allow one to follow of EM emission. They can reach the luminosity of a quasar the X-ray spectral variations of these sources in the broad en- but they are rare (∼10−4 yr−1 per galaxy; in particular types of ergy range (0.5-30 keV) down to a 10−12 erg s−1 cm−2, unveil- galaxies, it may be ∼10−3 yr−1 per galaxy [277,278]) and last ing the interplay between the Comptonized radiation from the several months or years. TDEs are a multi wavelength phe- corona and the non-thermal emission from the jet. The LAD nomenon, that has so far been detected in radio, optical, UV, observations extending up to 30 keV will play an important soft and hard X-rays. They display both thermal and non- role in the detection and characterization of the Fe-K line, and thermal emission from relativistic and non-relativistic matter test with high accuracy the reflection model applied to the X- (for a recent review on observations, see ref. [279]). J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-27

The detection and study of these flares can deliver other A few years ago, the Swift Burst Alert Telescope (BAT) important astrophysical information beyond probing the pres- triggered on two TDE candidates in the hard X-ray band ence of a SMBH. First, TDEs can allow one to discover in- [291,300]. A multi-frequency follow-up from radio to γ-rays termediate mass black holes, as their peak bolometric lumi- revealed a new class of non-thermal TDEs. It is widely be- nosity is expected to be inversely proportional to the black lieved that emission from a relativistic jet (bulk Lorentz factor hole mass. Second, TDEs are signposts of supermassive bi- Γ ≈ 2 [301, 302]) is responsible for the hard X-ray spectrum nary black holes, as their light curves look characteristically (with a negative power-law photon index between −1.6 and different in the presence of a second black hole, which acts as −1.8) and the increasing radio activity [303] detected a few a perturber on the stellar stream [280]. Recently, a disruption days after the trigger. More recently, ref. [304] reported an- by a SMBH binary candidate was claimed [281]. Thirdly, other candidate relativistic TDE (Swift J1112.2-8238). Ref. as also mentioned below, TDEs give us the opportunity to [305] found nine TDE candidates in BAT data after searching study accretion and jet formation in transient (∼month to year the data from 53000 galaxies out to 100 Mpc. Finally, refs. time scale) accretion episodes around SMBHs in contrast to [306,307] discovered the first radio outflow associated with a AGN. Fourthly, dormant SMBHs represent the majority of thermal TDE, suggesting that jets may be a common feature the SMBHs in our universe. X-ray flares from TDEs provide of TDEs. us with the opportunity to probe the mass and spin distribu- The best studied of the relativistic events is Swift tion of previously dormant SMBHs at the centers of normal J1644+57 (Sw J1644 in short [303,308]). The two main fea- galaxies as well as the geometry of the debris flow of the ac- tures that support the claim that Sw J1644 is a TDE are (1) −5/3 cretion flare through X-ray line reverberation [282]. Finally, the X-ray light curve behavior, that follows LX ∝ M˙ (t−τ) in X-rays TDEs represent a new probe of strong gravity, for after a few days (τ ≈ 3 d) from the trigger [309], and (2) the instance tracing precession effects in the Kerr metric (see refs. radio localization of the event within 150 pc from the center [283,284]). of a known quiescent galactic nucleus [310]. The first TDEs were discovered in ROSAT surveys of the If one uses the Sw J1644 radio and X-ray light curves to es- X-ray sky (e.g., refs. [285, 286]), see ref. [287] for a re- timate prospects for any future planned missions, one obtains view. Later, GALEX allowed for the selection of TDEs at that non-thermal TDEs can be discovered at a higher rate UV frequencies [288, 289]. Many of the most recent TDE thanks to triggers provided by radio surveys and to follow- candidates are found in optical transient surveys [290-292]. ups at higher energies. The wide field-of-view survey of An alternative method to select TDE candidates is to look SKA will effectively detect TDEs in the very early phase, al- for optical spectra with extreme high-ionization and Balmer lowing follow-up X-ray observations in the very early phase lines [293, 294]. This class of TDEs is called “thermal”, and [311, 312]. In synergy with SKA in survey mode, the SFA the spectral energy distribution is believed to be associated can in principle successfully repoint to each radio candidate with phenomena that lead to mass accretion onto the black in the sky accessible at any moment to eXTP (i.e., 50% of the hole. time), and measure its light curve decay index and spectral Currently, the theoretical picture proposed to interpret the properties. The X-ray counterpart of radio-triggered events observational properties of TDEs is as follows. After stel- can be followed up with the LAD too (see Figure 6 in ref. lar disruption, part of the stellar material is accreted onto [309]), thus extending the energy coverage up to 30 keV the black hole, causing a luminous flare of radiation. If the with a better characterization of the non-thermal process and star is completely disrupted, its debris is accreted at a de- of the jet energy budget. The X-ray polarimeter will def- creasing rate according to M˙ ∝ t−5/3 [276, 295]. Therefore, initely play the major role. For events followed up within TDEs allow one to study the formation of a transient accre- a few weeks from the beginning, a fraction of ∼10% of the tion disk and its continuous transition through different accre- radio-triggered events will have a 2-10 keV flux greater than tion states. The super-Eddington phase—which occurs only 10−10 erg s−1 cm−2. For these events, it is possible to derive 7 for SMBH masses M < 10 M⊙—is theoretically uncertain, a minimum detectable polarization of ∼10% over a 5 ks time but it may be associated with a powerful radiatively driven scale. This will allow one to test the variation of magnetic wind [296] that thermally emits 1041-1043 erg s−1 mainly at field over a time scale of one hour and derive the time needed optical frequencies [297, 298]. The disk luminosity (1044- for the transient jet to build up a large scale coherent magnetic 1046 erg s−1) peaks instead in the far-UV/soft X-rays [298]. field. Moreover, if eXTP were able to follow-up TDEs within As an alternative scenario to thermal emission from an accre- 1 d, with the trigger provided by LSST, SKA, the Einstein tion disk, the optical emission has been suggested to come Probe or WFM and if the source is bright enough in 2-10 keV from shocks as the stellar debris self-crosses or from repro- (> 10−10 erg s−1 cm−2), it will be possible to investigate also cessed X-ray light (e.g., ref. [299]). polarization angle variations aimed at assessing the role of jet J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-28 precession in the early phase of the event. In this case, one future X-ray bright relativistic TDEs or thermal TDEs, it is foresees a set of daily (50 ks) observations [313, 314] spread possible to detect spectral features with 1 ks SFA and LAD over 10 d in order to appreciate angle variations within a few exposures. degrees. eXTP is therefore very well suited for the X-ray follow-up of jetted TDEs in the SKA era. The expected rate is between 11 Gamma-ray bursts and supernovae a few to several tens per year (depending on the value of Γ). The attractive prospect is that of building for the first time Gamma-ray bursts (GRBs, for reviews, see refs. [316-318]) a sample of well studied TDEs associated with non-thermal are short (∼10 s), bright (up to 10−5 erg s−1 in 10 keV-1 MeV), emission from jets. This will allow one to study disk-jet energetic (with fluences up to more than 10−4 erg cm−2) and formation and their connection, in a way complementary to frequent (about one per day). Observational efforts since the other more persistent sources such as AGN and X-ray bina- 1990s have provided, among others, (1) the accurate local- ries. In addition, jetted TDEs can uniquely probe the pres- ization and characterization of multi-wavelength afterglows ence of SMBHs in quiescent galaxies well beyond the local [319, 320], (2) the establishment of cosmological distances universe (redshift range 1.5-2). and supernova-like radiated energies [321], (3) the identifica- Moreover, the SFA energy band and sensitivity will enable tion of two classes of GRBs, long and short, with a division the follow-up of thermal TDEs, that are expected to be trig- at 2 s [322], and (4) the connection of long GRBs with Type gered in a large number with LSST. These will be studied Ib/c supernovae [323]. The most important open questions with eXTP in a tight connection with the X-ray binary tran- in our current understanding of GRBs involve: the physical sients. The combination of the soft telescopes, the broad en- origin of sub-classes of GRBs (i.e., short, X-ray rich, sub- ergy coverage and the polarimetric capabilities makes eXTP energetic and ultra-long GRBs), the physics and geometry of a great X-ray mission to study all the variety of tidal disrup- the prompt emission, some unexpected early afterglow phe- tion events and to provide the unique opportunity to answer nomenology (e.g., plateaus and flares), the connection be- the following questions: tween GRBs and particular kinds of supernovae, the nature • What is the rate of non-thermal TDEs? The SFA sensi- of the “central engine” and the use of GRBs as cosmological tivity will allow one to follow up and identify any radio trig- probes. gered TDEs. The WFM can allow the serendipitous detection Short GRBs were long suspected to be connected to bi- of non-thermal TDEs in X-rays at a higher rate (between one nary neutron-star merger events [322, 324]. The detec- and a few tens per year) than any current missions. The com- tion of short GRBs associated with old stellar populations, bination of the two will provide a unique tool to establish the their offsets from their host galaxies and the identification rate of non-thermal TDEs and study the mechanisms power- of a candidate kilonova explosion supported this associa- ing the hard X-ray emission. tion (e.g., refs. [325-327]). The suspicion received unprece- • Do TDEs exhibit similar spectral states as X-ray binary dented support through the coincident detection of the short transients? The SFA energy band and sensitivity will enable GRB170817A [7] and the binary merger gravitational-wave the follow-up thermal TDEs that are expected to be triggered event GW170817 [8, 328]. The masses of the two binary in large numbers with LSST. Several thousands per year are components measured through the GWs are more consis- expected to be triggered up to z∼1 [297]. The X-ray follow- tent with a set of dynamically measured neutron star masses up with SFA will make it possible to study the evolution of than with that of black holes. The tidal disruption of neu- the thermal TDEs and possibly see the transition from soft tron stars (see Figure 16 for stills from a simulation) is ex- to hard spectra as expected at later stages due to the coronal pected to yield EM radiation because of the delayed accre- Comptonized component [315]. tion of part of the expelled matter onto the merger product, • What are the masses and spins of the previously dormant the strong radio-active decay of r-process elements produced SMBHs at the centers of normal galaxies? AGN cover only during the merger, the shocks resulting from an expanding a small portion of the SMBHs in our universe. The distribu- cocoon and/or jets. tion of the masses and the spins of these SMBHs is impor- The prospect of GW detections with Advanced LIGO and tant to our understanding of the cosmological growth of mass Virgo, KAGRA and LIGO-India in the 2020s have been cal- and evolution of the spin of the majority SMBHs. At present culated to be rather promising (tens to hundreds per year) for and in the near future, we are only able to study the proper- distances of up to ∼300-500 Mpc [330,331]. So far, very few ties of these dormant SMBHs (also some intermediate-mass (4-5, see, e.g., refs. [332, 333]) supernova-less GRBs (in- black holes in dwarf galaxies) through these flaring events. cluding the so-called long-short GRBs and short GRBs) have Among them, X-ray flaring TDEs allow us to probe the mass been localized within this distance regime. This may improve and spin with X-ray line reverberation (e.g., ref. [282]). For with the softer response by the WFM and its larger field of J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-29

0.5 0.5

0 0

−0.5 −0.5 log temperature Me(V) log temperature Me(V)

−1.0 −1.0

1.0 1.0

0.5 0.5

0 0 log temperature Me(V) log temperature Me(V)

−0.5 −0.5

Figure 16 Simulation of a neutron star - neutron star merger. The four frames are timed within 6.8 ms. From ref. [329]. view. It was thought that, due to the beaming effect in spe- for such a supernova event is SN 2005ek [346]. Further- cial relativity, only relativistic jets from the mergers of two more, as a result of severe mass transfer in the X-ray phase neutron stars or of a black hole and a neutron star beamed just prior to these supernovae (via so-called Case BB Roche- toward the Earth can be detected as short GRBs. The detec- lobe overflow), such systems are also likely to be observable tion of the off-axis GRB170817A [7], without any noticeable with eXTP as pulsating ultra-luminous X-ray sources (see signal of a relativistic jet in our line of sight, suggests better sect. 12.1). prospects for the detection of off-axis X-ray emission, possi- Another interesting class of soft GRBs may arrive from su- bly produced as a result of the shock propagation or by the pernova shock breakouts. Wide-field soft X-ray surveys are mildly relativistic cocoon (see, e.g., refs. [334, 335]). This predicted to detect each year hundreds of supernovae in the emission has a much larger opening angle, and despite the act of exploding through the supernova shock breakout phe- much lower luminosity, it might still be observable by eXTP nomenology [347, 348]. Supernova shock breakout events within the LIGO-Virgo detection horizon. Moreover, inde- occur in X-rays at the very beginning of the supernova ex- pendently from short GRBs, some models predict detectable plosions, allowing for follow-up at other wavelengths to start and nearly-isotropic X-ray emission powered by the merger extremely early on in the supernova, and possibly shedding remnant [336-342]. With its unprecedented large field of light on the nature of the progenitor of some types of super- view, WFM may help to confirm or exclude these models novae. To date, only one supernova shock breakout has been by possibly detecting EM signals from the remnant on the unambiguously detected, in Swift observations of NGC 2770 needed time scale (from ∼ 1 s to longer). GRB170817A [349]. It has a fast-rise, exponential decay light curve with would have been detectable with the WFM with a signal-to- a rise time of 72 s, and an exponential decay time scale of noise ratio during the peak of up to about 10 if its spectrum 129 s, and a peak X-ray luminosity of 6.1 × 1043 ergs s−1. would be on the 1σ soft side of the error margins of the spec- An interesting target for eXTP are the recently discovered tral shape parameters measured with Fermi-GBM [7]. ultra-long GRBs [350,351]. These events are soft gamma-ray The nature of the second supernova in tight binaries, which bursts lasting hours (instead of seconds). Only a few events are responsible for producing the binary neutron star progen- are known, and most of their properties remain to be studied. itors of short GRBs and GW sources, is an ultra-stripped su- We know however that these events present a possible faint pernova [343]. These supernovae produce relatively faint and thermal component, and that they may be linked to progeni- rapidly decaying light curves [344, 345]. A prime candidate tors very similar to population III stars (i.e., extremely mas- J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-30 sive, metal poor stars [352]). These objects are not brighter 15 than normal long GRBs (albeit the total energy budget is far eXTP-WFM larger), but last for so long that it is possible to re-point eXTP Swift-BAT 10 and still to observe the very bright prompt phase. This bright- BATSE ) Fermi-GBM −1 ness, combined with the unprecedented large collecting area s SVOM −2 of the LAD, would allow spectral studies impossible with any

other mission to date, studying in detail the evolution of the (ph cm 5 thermal component and its still debated nature [353]. Thanks to its unique combination of field of view, broad peak,1–1000 keV bandpass, spectral resolution, large effective area, and polar- E ization capabilities, the instruments on board eXTP, particu- larly the WFM (see Figure 17), can address questions of fun- 2 damental importance to understanding the above phenomena: • 1 10 100 What are X-ray Flashes? eXTP can enable a substan- E peak (keV) tial increase (with respect to past and current missions) of the Figure 17 GRB detection sensitivity in terms of peak flux sensitivity as a detection rate of X-Ray Flashes (XRFs), events commonly function of the spectral peak energy Ep [359] of the WFM (red) for M4 con- interpreted as a softer sub-class of GRBs (but see also ref. figuration compared to those of CGRO BATSE (green), Swift BAT (blue), [354]) which likely constitutes the bulk of the GRB pop- Fermi GBM (brown), and SVOM ECLAIRs (cyan). ulation but still lacking a good body of observational data [355-357]. With respect to HETE-2 WXM (2-25 keV), the most efficient XRF detector flown so far, WFM will increase (a)

the XRF detection rate 7-fold, with an expected rate of 30-40 ) XRF per year (e.g., ref. [358]). See also Figure 17. −1

100

• −1

What are the main radiative mechanisms at the basis of keV s GRB prompt emission? Measurement of the GRB spectra −2 and their evolution down to 2 keV are an indispensable tool for testing models of GRB prompt emission and will be stud-

(Photons cm 10 ied with unprecedented detail thanks to the WFM’s unique 2 capabilities (see Figure 18(a)). keV • What is the nature of GRB progenitors? Transient ab- sorption edges and emission lines in the prompt and afterglow 10 100 X-ray spectra of long GRBs are expected in several scenar- Energy (keV) ios for their origin. Firm detections or deep limits on such (b) features would be essential for the understanding of the prop- erties of the circum burst matter (CBM) and hence the nature ) of GRB progenitors. In addition, they could enable us to de- −1

termine the GRB redshift directly from X-ray observations. −1 keV s

Up to now, only a few marginal detections were provided by −2 100 BeppoSAX, XMM-Newton and Chandra, while no signifi- cant evidence for line emission was found in Swift afterglow data. As illustrated in Figures 18 and 19(b), eXTP, thanks Counts ( cm to the good energy resolution of both WFM and SFA, com- 10 bined with a large field of view and a large area, respectively, 2 5 10 20 can greatly contribute to solve this issue and possibly open a Energy (keV) new discovery space in the understanding of the GRB phe- Figure 18 Illustrative plots of WFM capabilities and of their impact on nomenon. GRB science. (a) Simulated WFM spectra of the first ∼50 s of GRB 090618 • Can supernovae be discovered promptly? Supernova ex- (from ref. [360]) obtained by assuming either the Band-function extrapo- lation (black) or the power-law plus black-body model extrapolation (red) plosions can be detected at their onsets thanks to the shock to the Fermi GBM measured spectrum (blue). The black body and power- breakout effect. According to recent supernova rate estimates law model components best-fitting the Fermi GBM spectrum are also shown [347], about one to two supernova breakouts per year should in black dashed lines. (b) Simulation of the transient absorption feature be detectable with the WFM out to 20 Mpc. If one takes the in the X-ray energy band detected by BeppoSAX WFC in the first 8 s of GRB 990705 as it would be measured by the WFM [361]. J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-31

10 (a) cay indices in the three phases—steep, shallow, normal— respectively), we computed the average flux for an exposure ff 1 of 100 ks starting at di erent epochs after the trigger. We find )

−1 that sensitive polarimetric studies with the PFA (i.e., average fluxes above 1 mCrab) can be performed for a typical X-ray keV 0.1 −1 afterglow only if it is observed no later than a few minutes after the GRB trigger. However, when considering X-ray af- 0.01 Counts (s terglows with a bright plateau phase (i.e., 10 times brighter at the start of the plateau), an average flux of few mCrab can be 10−3 reached for a 100 ks exposure even starting the observations 0.5 1 2 5 10 after 6 h. Energy (keV) • What is the end product of a neutron star merger? The 0.01 (b) end product of a merger of two neutron stars can either be ) −1 5×10−3 a black hole or a heavy neutron star that may be hypermas- keV

−1 2×10−3 sive (temporarily supported by for example fast spins) with 15 1×10−3 magnetic fields of order 10 G (thus a magnetar, e.g., ref. 5×10−4 [338]). This probes a very interesting question: what is the Counts (s threshold mass above which a neutron star cannot exist (see

2.0 also sect. 5)? This is intimately related to the neutron star equation of state. A possible observational means to distin- 1.5 Ratio guish between both kind of compact objects is, apart from 1.0 the GW signal [5], the detection of a pulsed EM signal after 5 Energy (keV) the merger event, for instance in the so-called plateau phase of GRB afterglows that is suggested to be due to rapidly ro- Figure 19 As an example, we considered the redshifted Fe-K line that was tating magnetar (e.g. refs. [370, 371]). eXTP would be measured by BeppoSAX in the X-ray afterglow spectrum of GRB000210 well equipped to hunt for such a signal, provided the LAD [369]. (a) A simulated SFA spectrum as obtained considering the same af- terglow intensity and line equivalent width as detected by BeppoSAX 12 h is pointed quick enough and the prompt signal is very lumi- after the onset of the GRB. An exposure time of 50 ks is assumed (average nous (roughly in the neighborhood of its Eddington limit for −13 −1 −2 flux of 8 × 10 erg s cm ). The detection of the iron line with the eXTP a distance of 40 Mpc and a signal duration of 104 s, or more). instrument would be outstanding. (b) The same simulation for Swift/XRT. No line is detected. This confirms the low sensitivity of XRT to similar fea- Finally we note that, as a public community service, eXTP tures. These unique and unprecedented results would provide an important will include a capability to localize short transients automati- step forward in the investigation of the circumburst material composition and cally onboard using data from the WFM. These positions, or distribution, with important consequences for shedding light on the nature of a subset of them, will be disseminated via a VHF network GRB progenitors and emission physics. and the internet in a similar fashion as for the SVOM mission [1,372]. case of NGC 2770 as a template, the optimum integration time to detect such events out to 20 Mpc is 240 s (the WFM sensitivity for the detection of impulsive events within this 12 Objects of uncertain nature time interval is ∼30 mCrab). • What is the structure and evolution of GRB jets? The 12.1 Ultraluminous X-ray sources contribution to GRB science by the LAD and especially the PFA may provide breakthrough information on the jet struc- Ultraluminous X-ray sources (ULXs) have observed fluxes 39 −1 ture and on the nature of the afterglow flares and plateaus indicating (assuming isotropic radiation) LX > 10 erg s , [362-365], as different models make different predictions and often > 1040 erg−1. As these luminosities exceed the on the evolution of the polarization during these phases Eddington limit (at which released radiation drives away in- [366, 367]. However, this will strongly depend on space- flowing mass) for typical stellar black hole masses, they indi- craft re-pointing capabilities and is likely limited to the 20% cate the presence of either much larger-mass black holes, of brightest events. For instance, by using the mean parame- super-Eddington accretion rates, and/or beaming of radiation ters of the X-ray afterglow light curves quoted by ref. [368] into limited angles. The recent discovery of X-ray pulsations (i.e., a flux normalization at the beginning of the plateau of by XMM-Newton in three ULXs [373-376] proves that some 6 × 10−10 erg s−1 cm−2 and 3.96, −0.16 and 1.59 for the de- ULXs contain neutron stars, and the sinusoidal pulse shape J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-32 indicates low beaming, so these systems clearly accrete at However, there are models predicting FRBs associated with super-Eddington rates (likely from high-mass donors, e.g., some GRBs and, hence, a faint, broadband afterglow may refs. [377, 378]). These systems are now being intensively be expected (e.g., ref. [390]). Coordinated searches of studied, to try to understand the magnetic fields of their neu- FRBs in the repeating source revealed no detection in X- tron stars, their long-term spin behavior, and the nature of the rays and gamma-rays with 0.5-10 keV flux limits of order accretion flows. 10−7 erg s−1 cm−2 [391]. This limit can be vastly improved eXTP can provide a substantial contribution to ULX sci- with LAD and SFA to values of 10−10 to 10−11 erg s−1 cm−2. ence, by finding and timing ULX pulsars. eXTP’s SFA has Because of the WFM’s large field of view, the possibility of an effective area roughly 5.5 times that of XMM-Newton’s pn a FRB being detected inside that field of view is enhanced. camera at 6 keV (the relevant comparison, since ULX pulsa- Detection of the prompt few millisecond duration emission tions are stronger at high energies), and an angular resolu- will be difficult though given the flux limit of approximately tion of 1′, enabling high count rates for relatively faint ob- 10−6.5 erg s−1 cm−2. If there is a longer duration X-ray coun- jects. As pulsation detection is generally a function of count terpart, chances will become better. rate and pulse fraction, the higher SFA effective area means Conversely, eXTP could instead provide candidates for tar- that eXTP should be able to detect pulsations over twice as geted searches for FRBs. Many of the progenitor theories re- 40 far away (e.g., ∼10 Mpc vs. 5 Mpc for 10 erg/s ULXs quire a young neutron star that is rapidly rotating and/or has a with 20% pulsed fractions), and thus within an effective vol- high magnetic field [385]. Neutron stars of this type are pro- ume 8-10 times larger. A systematic eXTP program to sur- posed to be the central engines of some GRBs and superlumi- vey bright ULXs within 10 Mpc (identified by, for example, nous supernovae (see sect. 11). Additionally, the host galaxy eROSITA on Spectrum-X-Gamma, along with Chandra and of the repeating FRB is consistent with the host galaxies of XMM-Newton surveys), and the brightest ULXs beyond that, long GRBs and superluminous supernovae [392]. However, may discover ∼30 new ULX pulsars, allowing significant sta- for long GRBs and superluminous supernovae progenitors, it tistical conclusions about the ULX population. Repeated tim- will take a few decades to clear the surrounding medium to ing of these ULXs will measure their spin-up rates. eXTP’s allow the radio emission to escape [392]. The short GRB pro- high effective area will also allow studies of QPOs [379], time genitors do not have this issue and bursts may escape much lags [380], and eclipse searches (e.g., ref. [381]), opening up sooner. eXTP will be able to identifying the plateau phase substantial discovery space for these extreme objects. (the signature of the formation of a magnetar [336]) following GRBs and these sources can be targeted with rapid response 12.2 Fast radio bursts and late-time FRB searches [393].

In recent years, there has been much excitement about the We thank the external referees for their advise prior to submission and A. enigmatic Fast Radio Bursts (FRBs [382]). These are bright, Patruno for a revision of Figure 7. This work was supported by the Royal So- millisecond duration, radio bursts with large dispersion mea- ciety, ERC Starting (Grant No. 639217), the European Union Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie sures pointing at extra-galactic distances and large luminosi- Global Fellowship (Grant No. 703916), the National Natural Science Foun- ties. A few tens have been detected since 2001 and estimates dation of China (Grant Nos. 11233001, 11773014, 11633007, 11403074, about their all-sky rate range into the thousands per day [383]. 11333005, 11503008, and 11590781), the National Basic Research Program of China (Grant No. 2015CB857100), NASA (Grant No. NNX13AD28A), Radio telescopes are being adapted to detect many more and an ARC Future Fellowship (Grant No. FT120100363), the National aiming to distribute alerts in close to real time [384]. The ori- Science Foundation (Grant No. PHY-1430152), the Spanish MINECO gin of these bursts is undetermined and the models range from (Grant No. AYA2016-76012-C3-1-P), the ICCUB (Unidad de Excelencia ’Mar´ıa de Maeztu’) (Grant No. MDM-2014-0369), EU’s Horizon Pro- neutron-star collapse to black holes, neutron star or white gramme through a Marie Sklodowska-Curie Fellowship (Grant No. 702638), dwarf mergers and non-cataclysmic pulsar events (for a re- the Polish National Science Center (Grant Nos. 2015/17/B/ST9/03422, view, see ref. [385]). There is one FRB source which has 2015/18/M/ST9/00541, 2013/10/M/ST9/00729, and 2015/18/A/ST9/00746), the Strategic Priority Research Program of the Chinese Academy of Sciences repeated tens of times [386], while all other observed FRBs (Grant No. XDA15020100), and the NWO Veni Fellowship (Grant No. have not been observed to repeat (although it is possible that 639.041.647). repeats have been missed due to instrument sensitivity limi- tations and survey strategies, [387]). The case of the repeater Author contributions This paper is an initiative of eXTP’s Science Work- ing Group 4 on Observatory Science, whose members are representatives of gives rise to a models where the energy is from magnetic the astronomical community at large with a scientific interest in pursuing the reconnection events from a young magnetar [388] or giant successful implementation of eXTP. The paper was primarily written by S. pulses from a rapidly rotating neutron star [389]. However, Drake (stellar flares), Y. Chen and P. Zhou (supernova remnants), M. Her- nanz, D. de Martino (accreting white dwarfs), T. Maccarone (binary evolu- there is no consensus yet [385]. tion), J. in ’t Zand (thermonuclear X-ray bursts), E. Bozzo (high-mass X-ray To date FRBs have no counterparts outside the radio band. binaries), A. de Rosa (radio-quiet active galactic nuclei), I. Donnarumma J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-33

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