The Status and Future of Ground-Based Tev Gamma-Ray
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The Status and future of ground-based TeV gamma-ray astronomy A White Paper prepared for the Division of Astrophysics of the American Physical Society Organizers and working-group chairs: J. Buckley, Washington University, St. Louis, K. Byrum, Argonne National Laboratory, B. Dingus, Los Alamos National Laboratory, A. Falcone, Pennsylvania State University, P. Kaaret, University of Iowa, H. Krawzcynski, Washington University, St. Louis, M. Pohl, Iowa State University, V. Vassiliev, University of California, Los Angeles, D.A. Williams, University of California, Santa Cruz Contents 1 Summary and Overview 1 1.1 ExecutiveSummary ................................ .... 1 1.1.1 Summaryoffindings.............................. 1 1.1.2 Recommendations ............................... 3 1.2 Ground based γ-ray astronomy - historical milestones . 4 1.3 Scientificoverview ............................... ...... 6 1.3.1 Unveiling an important component of our Universe: high-energy particles . 6 1.3.2 Radiation processes and the sky in high-energy γ-rays ............. 7 arXiv:0810.0444v1 [astro-ph] 2 Oct 2008 1.3.3 Diffuse emission and the nature and distribution of dark matter . 9 1.3.4 Powerful particle accelerators in our Milky Way Galaxy: supernova remnants, pulsars,andstellarmassblackholes . .... 9 1.3.5 Extragalactic sources of TeV γ-rays ....................... 10 1.4 Technology and the path toward a future observatory . ............. 11 1.5 Synergies with other wavebands and particle astronomy missions ........... 13 2 Galactic diffuse emission, supernova remnants, and the origin of cosmic rays 15 2.1 Whyaretheyimportant? ............................ ..... 15 2.2 Whatdoweknowalready? ............................ 16 2.2.1 Supernovaremnants .. .. .. .. .. .. .. .. .. 16 2.2.2 Diffuse galactic emission . ..... 21 2.3 Whatmeasurementsareneeded? . ....... 23 2.3.1 Supernovaremnants .. .. .. .. .. .. .. .. .. 23 2.3.2 Diffuse galactic emission . ..... 25 2.4 What is the required instrument performance? . ............ 26 1 3 Galactic compact objects 28 3.1 Introduction.................................... ..... 28 3.2 Pulsarwindnebulae ............................... ..... 28 3.2.1 Measurementsneeded . 29 3.3 Pulsedemissionfromneutronstars . .......... 31 3.3.1 Measurementsneeded . 32 3.4 Relativistic jets from binaries . ........... 32 3.4.1 Currentstatus ................................. 33 3.4.2 Measurementsneeded . 34 3.5 Requiredinstrumentperformance. .......... 36 4 Dark matter searches with a future VHE gamma-ray observatory 37 4.1 Introduction.................................... ..... 37 4.2 Dark Matter Annihilation into γ-rays, and the uncertainties in the predicted flux . 42 4.3 Targets for Gamma-Ray Detection . ........ 43 4.3.1 DwarfSpheroidals .............................. 46 4.3.2 Localgroupgalaxies .. .. .. .. .. .. .. .. 48 4.3.3 Detecting the Milky Way Substructure .................................... 49 4.3.4 Detecting Microhalos . ..... 49 4.3.5 Spikes around Supermassive and Intermediate-Mass Black Holes . 49 4.3.6 Globularclusters .............................. 51 4.4 Complementarity of γ-Ray Searches with Other Methods for Dark Matter Searches . 51 4.5 Conclusions ..................................... 53 5 Extragalactic VHE astrophysics 55 5.1 Introduction.................................... ..... 55 5.2 Gamma-ray observations of supermassive black holes . ............... 55 5.3 Cosmic rays from star-forming galaxies . ............ 58 5.4 The largest particle accelerators in the universe: radio galaxies, galaxy clusters, and largescalestructureformationshocks . ......... 60 5.5 Extragalactic radiation fields and extragalactic magneticfields ............ 62 6 Gamma-ray bursts 66 6.1 Introduction.................................... ..... 66 6.1.1 Status of theory on emission models........................................ 66 6.1.2 GRBProgenitors................................ 69 6.2 High-energy observations of gamma-ray bursts . ............. 70 6.3 High Energy Emission Predictions for Long Bursts . ............. 71 6.3.1 Promptemission ................................ 71 6.3.2 Decelerationphase . .. .. .. .. .. .. .. .. 72 6.3.3 Steepdecay.................................... 73 6.3.4 Shallowdecay.................................. 73 6.3.5 High-energy photons associated with X-ray flares . ............ 74 6.3.6 High-energy photons from externalreprocessing.. .. .. .. .. .. .. .. .. 75 6.4 High Energy Emission Predictions for Short Bursts . .............. 75 2 6.5 Supernova-associated gamma-ray bursts . ............ 76 6.6 UltraHighEnergyCosmicRaysandGRBs . ....... 76 6.7 Tests of Lorentz Invariance with Bursts . ........... 78 6.8 Detection Strategies for VHE Gamma-Ray Burst Emission . .............. 78 6.9 Synergywithotherinstruments . ......... 80 6.10Conclusions .................................... ..... 80 7 Technology working group 82 7.1 Introductionandoverview . ........ 82 7.2 Status of ground-based gamma-ray observatories . .............. 83 7.3 Design considerations for a next-generation gamma-ray detector . 85 7.4 FutureIACTarrays ................................ 85 7.5 FutureEASobservatory . ...... 90 7.6 Technologyroadmap ............................... ..... 93 References 96 Appendices 108 A Glossary 108 A.1 Astronomicalandphysicsterms. .........108 A.2 Abbreviationsandacronyms. ........111 B Charge from APS 113 C Letter to community (via e-mail) 115 D Agenda for Malibu meeting 117 E Agenda for Santa Fe meeting 121 F Agenda for Chicago meeting 122 G Agenda for SLAC meeting 124 H Agenda and record of white paper teleconference meetings 126 I Membership 128 3 1 Summary and Overview tronomy by Hinton1, and by Aharonian et al.2. Appendix A lists the authors of the White Pa- 1.1 Executive Summary per, the make-up of the working groups, and the meetings organized by the White Paper team. Appendix B reproduces the APS charge. High-energy γ-ray astrophysics studies the most energetic processes in the Universe. It explores cosmic objects such as supermassive black holes 1.1.1 Summary of Findings and exploding stars which produce extreme con- (F1) The current generation of instruments has ditions that cannot be created in experiments demonstrated that TeV γ-ray astronomy is a on Earth. The latest generation of γ-ray instru- rich field of research. TeV γ-ray astronomy saw ments has discovered objects that emit the bulk its first major success in the year 1989 with the of their power in the form of high-energy γ-rays. firm detection of a cosmic source of TeV γ-ray High-speed imaging technology, with gigahertz emission, the Crab Nebula, with the Whipple frame rates allow us to detect individual cosmic 10-m Cherenkov telescope. To date, advances γ-ray photons produced under the most violent in instrumentation and analysis techniques have and extreme conditions. Gamma-ray astronomy established TeV γ-ray astronomy as one of the has unique capabilities to reveal the nature of most exciting new windows into the Universe. the elusive dark matter that dominates the mat- The H.E.S.S., MAGIC, and VERITAS experi- ter contents of the Universe. ments have shown us a glimpse of the discovery Motivated by the recent advances of TeV γ- potential of this new type of astrophysics. The ray astronomy, the Division of Astrophysics of Milagro and Tibet experiments have explored an the American Physical Society (APS) charged alternative experimental technique that permits the editorial board of this White Paper to sum- a full survey of the sky. Due to the increased marize the status and future of ground-based sensitivity of these instruments, the number of γ-ray astronomy. The APS requested a review known TeV γ-ray sources has increased by an of the science accomplishments and potential of order of magnitude (from ∼10 to ∼100) in the the field. Furthermore, the charge called for a past 3 years. Known source classes include the description of a clear path beyond the immedi- remnants of supernova explosions, neutron stars, ate future to assure the continued success of this supermassive black holes, and possibly groups field. The editorial board solicited input from of massive stars. Many new TeV sources and all sectors of the astroparticle physics commu- source classes have been discovered. Several of nity through six open working groups, targeted them have heretofore unobserved substructures international meetings, and emails distributed resolved by TeV γ-ray telescopes, indicating through the APS and the High-Energy Astro- isolated regions in which energy is transferred physics Division of the American Astronomical to high-energy particles. Several sources show Society. The board also enlisted senior advisers brightness variations clearly resolved in the that represent ground-based and satellite-based TeV data; the previously known sources Mrk γ-ray astronomy, particle physics, and the inter- 421 and PKS 2155-304, both of which involve national community of astroparticle physicists. a supermassive black hole, have been observed This section summarizes the findings and rec- to vary on timescales as short as 2 minutes, ommendations of the White Paper team. It also indicating that the emission regions may be gives a brief introduction to the science topics comparable in size to the event horizon of that can be addressed with TeV γ-ray astron- the parent black hole, and revealing the inner omy. The interested reader is referred to the 1 detailed discussions in the reports of the work- Hinton, J., 2007, http://arxiv.org/abs/0712.3352 2Aharonian, F., Buckley,