A&A 647, A1 (2021) Astronomy https://doi.org/10.1051/0004-6361/202039313 & © P. Predehl et al. 2021 Astrophysics First science highlights from SRG/eROSITA Special issue The eROSITA X-ray telescope on SRG P. Predehl1, R. Andritschke1, V. Arefiev9, V. Babyshkin11, O. Batanov9, W. Becker1, H. Böhringer1, A. Bogomolov9, T. Boller1, K. Borm8,12, W. Bornemann1, H. Bräuninger1, M. Brüggen2, H. Brunner1, M. Brusa15,16, E. Bulbul1, M. Buntov9, V. Burwitz1, W. Burkert1,?, N. Clerc14, E. Churazov9,10, D. Coutinho1, T. Dauser4, K. Dennerl1, V. Doroshenko3, J. Eder1, V. Emberger1, T. Eraerds1, A. Finoguenov1, M. Freyberg1, P. Friedrich1, S. Friedrich1, M. Fürmetz1,?, A. Georgakakis13, M. Gilfanov9,10, S. Granato1,?, C. Grossberger1,?, A. Gueguen1, P. Gureev11, F. Haberl1, O. Hälker1, G. Hartner1, G. Hasinger5, H. Huber1, L. Ji3, A. v. Kienlin1, W. Kink1, F. Korotkov9, I. Kreykenbohm4, G. Lamer7, I. Lomakin11, I. Lapshov9, T. Liu1, C. Maitra1, N. Meidinger1, B. Menz1,?, A. Merloni1, T. Mernik12, B. Mican1, J. Mohr6, S. Müller1, K. Nandra1, V. Nazarov9, F. Pacaud8, M. Pavlinsky9, E. Perinati3, E. Pfeffermann1, D. Pietschner1, M. E. Ramos-Ceja1, A. Rau1, J. Reiffers1, T. H. Reiprich8, J. Robrade2, M. Salvato1, J. Sanders1, A. Santangelo3, M. Sasaki4, H. Scheuerle12,?, C. Schmid4,?, J. Schmitt2, A. Schwope7, A. Shirshakov11, M. Steinmetz7, I. Stewart1, L. Strüder1,?, R. Sunyaev9,10, C. Tenzer3, L. Tiedemann1,?, J. Trümper1, V. Voron 17, P. Weber 4, J. Wilms4, and V. Yaroshenko1 1 Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany e-mail: [email protected] 2 Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany 3 Institut für Astronomie und Astrophysik, Abteilung Astronomie, Universität Tübingen, Sand 1, 72076 Tübingen, Germany 4 Universität Erlangen/Nürnberg, Dr.-Remeis-Sternwarte, Sternwartstraße 7, 96049 Bamberg, Germany 5 ESAC Camino bajo de Casillo s/n Villanueva de la Canada, Madrid, Spain 6 Ludwig-Maximilians-Universität München, Universitätssternwarte, Scheinerstraße 1, 81679 Munich, Germany 7 Leibniz Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany 8 Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany 9 IKI, Space Research Institute, 84/32 Profsouznaya ulitsa, Moscow 117997, Russian Federation 10 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße, 85741 Garching, Germany 11 Lavochkin Association, 24 Leningradskaya ulitsa, Khimki 141400, Moscow Region, Russian Federation 12 Deutsches Zentrum für Luft- und Raumfahrt, Königswinterer Straße. 522-524, 53227 Bonn, Germany 13 National Observatory of Athens,V. Paulou & I. Metaxa, Athens, Greece 14 Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse, CNRS, UPS, CNES, 31028 Toulouse, France 15 Dipartimento di Fisica e Astronomia, Alma Mater Studiuorum Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy 16 INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy 17 State Space Corporation Roscosmos, 42 Schepkina ulitsa 42, Moscow 107996, Russian Federation Received 1 September 2020 / Accepted 23 September 2020 ABSTRACT eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the primary instrument on the Spectrum-Roentgen-Gamma (SRG) mission, which was successfully launched on July 13, 2019, from the Baikonour cosmodrome. After the commissioning of the instrument and a subsequent calibration and performance verification phase, eROSITA started a survey of the entire sky on December 13, 2019. By the end of 2023, eight complete scans of the celestial sphere will have been performed, each lasting six months. At the end of this program, the eROSITA all-sky survey in the soft X-ray band (0.2–2.3 keV) will be about 25 times more sensitive than the ROSAT All-Sky Survey, while in the hard band (2.3–8 keV) it will provide the first ever true imaging survey of the sky. The eROSITA design driving science is the detection of large samples of galaxy clusters up to redshifts z > 1 in order to study the large-scale structure of the universe and test cosmological models including Dark Energy. In addition, eROSITA is expected to yield a sample of a few million AGNs, including obscured objects, revolutionizing our view of the evolution of supermassive black holes. The survey will also provide new insights into a wide range of astrophysical phenomena, including X-ray binaries, active stars, and diffuse emission within the Galaxy. Results from early observations, some of which are presented here, confirm that the performance of the instrument is able to fulfil its scientific promise. With this paper, we aim to give a concise description of the instrument, its performance as measured on ground, its operation in space, and also the first results from in-orbit measurements. Key words. space vehicles: instruments – X-rays: general – surveys – dark energy ? Left Astronomy. A1, page 1 of 16 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Open Access funding provided by Max Planck Society. A&A 647, A1 (2021) 1. Introduction 1986; Rosati et al. 2002; Voit 2005; Arnaud 2005; Norman 2005; Borgani 2008; Borgani & Kravtsov 2011; Allen et al. 2011; The eROSITA (extended ROentgen Survey with an Imaging Reiprich et al. 2013). On the other hand, X-ray emission is a Telescope Array) instrument concept is based on a long series of universal signature of accretion of matter onto the supermassive previous scientific and technological developments at the Max black holes (SMBHs) that likely seed the entire population of Planck Institute for extraterrestrial Physics (MPE), dating back galaxies and may strongly influence their formation and subse- to the very successful German/US/UK ROSAT X-ray satellite quent evolution (Hopkins et al. 2008; Hickox et al. 2009; Fabian mission (1990–1999; Trümper 1982), which was developed and 2012; Alexander & Hickox 2012; Kormendy & Ho 2013; Brandt managed under the leadership of MPE. ROSAT carried out the & Alexander 2015). Thus, a sufficiently detailed map of the Uni- first complete survey of the sky with an imaging X-ray telescope verse in X-rays highlights both the interconnected structure of in the energy range between 0.1 and 2.4 keV, and performed the dark-matter web and the role of black holes in galaxy for- tens of thousands of pointed observations. Just as ROSAT has mation. The required sensitivity of an all-sky survey that could been the reference for the past 30 yr, so will eROSITA on SRG map the large-scale structure implies moreover that data are (Spectrum-Roentgen-Gamma) be the reference in the future. accumulated for a large variety of astronomical source classes, The SRG is an astrophysical observatory, comprising two and for a plethora of possible science applications well beyond imaging X-ray telescopes: the primary payload eROSITA, devel- the main design-driving objectives. These data are therefore oped under the responsibility of MPE, Germany, and ART- endowed with tremendous legacy value. XC (Astronomical Roentgen Telescope X-ray Concentrator), an In the soft X-ray band (0.2–2.3 keV), the eROSITA sur- X-ray mirror telescope complementing the eROSITA sensitiv- vey was designed to be about 25 times more sensitive than the ity towards higher energies, developed under the lead of the ROSAT all-sky survey, while in the hard band (2.3–8 keV) it will Russian Space Research Institute IKI (Pavlinsky et al. 2018). provide the first ever true imaging survey of the sky at those The scientific payloads of SRG are mounted on the “Navigator” energies. With soft X-ray effective area and on-axis spatial res- spacecraft platform built by Lavochkin Association (“NPOL”) in olution comparable to XMM-Newton, better energy resolution, Khimky near Moscow in Russia. Navigator has been developed and a much larger field of view, eROSITA is a powerful X-ray as a universal medium-class platform for scientific missions to telescope. Together with ART-XC, which expands the energy be launched into various orbits. Since January 2011, the Naviga- range up to 30 keV, this makes SRG a highly competitive X-ray tor platform has been used in the three Elekro-L meteorological observatory. satellite missions, as well as in the scientific Spektr-R mission According to several independent analyses (Pillepich et al. (RadioAstron; Kardashev et al. 2013), which was launched in 2012, 2018; Merloni et al. 2012; Kolodzig et al. 2013; Borm 2011 and operated until 2018. et al. 2014; Clerc et al. 2018; Zandanel et al. 2018; Comparat This paper presents a concise description of the main sci- et al. 2019), eROSITA is expected to yield a sample of at least entific goals of eROSITA, of the instrument itself, of its perfor- 100 000 clusters of galaxies, a few million active galactic nuclei mance as measured on ground, and its operations in space, and (AGNs), and around 700 000 active stars among many other presents some of the first results from in-orbit measurements. X-ray-emitting objects within and outside our own Galaxy. More details about the in-flight calibration program, and the Moreover, such a deep imaging survey at medium to high performance of the various eROSITA subsystems, as well as a spectral resolution, with its scanning strategy that is sensitive to description of the ART-XC telescope and of the SRG mission as a range of variability timescales from tens of seconds to years a whole, will be published separately. (see Sect.7 below), will undoubtedly open up a vast discovery space for the study of rare, unexpected, or even yet unpre- dictable high-energy astrophysical phenomena (Merloni et al. 2. The eROSITA mission 2012; Khabibullin et al. 2014). 2.1.