Gamma Ray Science Interest Group
(GR SIG)

Decadal Survey White Paper Projects

List of Gamma-ray science white papers submitted by community members to the GRSIG and Astro2020.

1. Catching Element Formation In The Act — The Case for a New MeV Gamma-Ray Mission: Radionuclide Astronomy in the 2020s

Details

Lead Author	Chris L. Fryer
Lead Author email	fryer@lanl.gov
Coauthors: Fred Adams, Melina Avila, Wako Aoki, Almudena Arcones, David Arnett, Katie Auchettl, Carles Badenes, Eddie Baron, Andreas Bauswein, John Beacom, Jeff Blackmon, Stéphane Blondin, Peter Bloser, Steve Boggs, Alan Boss, Terri Brandt, Eduardo Bravo, Ed Brown, Peter Brown, Steve Bruenn, Carl Budtz-Jørgensen, Eric Burns, Alan Calder, Regina Caputo, Art Champagne, Roger Chevalier, Alessandro Chieffi, Kelly Chipps, David Cinabro, Ondrea Clarkson, Alain Coc, Devin Connolly, Benoit Côté, Sean Couch, Nicolas Dauphas et al.
Key words	Stars and Stellar Evolution; Galaxy Evolution
Link to white paper / draft	[PDF]
Abstract: Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV γ-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by γ-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted at γ-ray energies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at γ-ray energies. This science is enabled by next-generation γ-ray instruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous γ-ray instruments. This transformative capability permits: (a) the accurate identification of the γ-ray emitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new γ-ray maps of the Milky Way and other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events—nearby neutron star mergers, for example. Advances in technology push the performance of new γ-ray instruments to address: How do white dwarfs explode as Type Ia Supernovae (SNIa)? What is the distribution of 56Ni production within a large population of SNIa? How do SNIa γ-ray light curves and spectra correlate with their UV/optical/IR counterparts? How do massive stars explode as core-collapse supernovae? How are newly synthesized elements spread out within the Milky Way Galaxy? How do the masses, spins, and radii of compact stellar remnants result from stellar evolution? How do novae enrich the Galaxy in heavy elements? What is the source that drives the morphology of our Galaxy's positron annihilation γ-rays? How do neutron star mergers make most of the stable r-process isotopes? Over the next decade, multi-messenger astronomy will probe the rich astrophysics of transient phenomena in the sky, including light curves and spectra from supernovae and interacting binaries, gravitational and electromagnetic signals from the mergers of compact objects, and neutrinos from the Sun, massive stars, and the cosmos. During this new era, the terrestrial Facility for Rare Isotope Beams (FRIB) and Argonne Tandem Linac Accelerator System (ATLAS) will enable unprecedented precision measurements of reaction rates with novel direct and indirect techniques to open perspectives on transient objects such as novae, X-ray bursts, kilonovae, and the rapid neutron capture process. This ongoing explosion of activity in multi-messenger astronomy powers theoretical and computational developments, in particular the evolution of community-driven, open-knowledge software instruments. The unique information provided by MeV γ-ray astronomy to help address these frontiers makes now a compelling time for the astronomy community to strongly advocate for a new γ-ray mission to be operational in the 2020s and beyond.

2. Measurement of the Optical-IR Spectral Shape of Prompt Gamma-Ray Burst Emission: A Timely Call to Action for Gamma-Ray Burst Science

Details

Lead Author	Bruce Grossan
Lead Author email	Bruce_Grossan@lbl.gov
Coauthors	Pawan Kumar, Kevin Hurley, Bing Zhang
Key words	gamma-ray burst prompt optical emilsison; emission mechanism; synchrotron emission; spectrum
Link to white paper / draft	[PDF]
Abstract: There is still no consensus on the emission mechanism of any type of gamma-ray burst (GRB). This is because a given gamma-band spectrum can come from different mechanisms. Measurement of the prompt optical-IR (OIR) spectral shape (POSS) breaks this degeneracy, however. There are also no direct measurements of the physical conditions in the relativistic jets where burst emission originates. If the synchrotron self-absorption frequency, νa, expected in the OIR region, can be measured, the radius of emission, the electron Lorentz factor, and the B field can be determined. Solving the 50+ year mystery of the emission mechanism is critical for understanding the light we observe from GRBs, and therefore the general problem of the physics of the jet. Although single band prompt optical fluxes have been measured, no POSS measurement has been made, because telescopes with multi-channel cameras cannot point within the ≲ 10 s required. For a good chance of measuring νa, near-IR (NIR)-optical coverage is also required. NIR measurements require ~ 2-m aperture ground-based telescopes, in turn requiring major engineering efforts for fast pointing. Space telescopes co-located with a gamma instrument have orders of magnitude lower NIR background, always point near the GRB, and never suffer clouds or weather, yielding a much higher detection rate. A ~30 cm space telescope with ≥ 4 OIR imaging channels would measure POSS from most GRBs. Widely available, digitally-controlled, sub-arc sec resolution motors make this fast-pointing space telescope feasible. Sub-second time resolution data from electron multiplied CCDs will allow cross-correlation of optical and gamma-band data, a new tool allowing separation of multi-component spectra. Any GRB mission with ≲ several arc min localization would then be missing essential science without a POSS instrument; i.e., trading the capability of a POSS instrument, even for a much larger aperture, slow-pointing, single channel + filter wheel is not good scientific judgment, because of the loss of such fundamental emission mechanism science. We also note additional capabilities enabled by a POSS-capable instrument, including dust evaporation identification and photo-z.

3. Energetic Particles of Cosmic Accelerators I

Details

Lead Authors	Tonia Venters & Kenji Hamaguchi
Lead Author email	tonia.m.venters@nasa.gov
Coauthors: Terri J. Brandt, Marco Ajello, Harsha Blumer, Michael Briggs, Paolo Coppi, Filippo D'Ammando, Michaël De Becker, Brian Fields, Sylvain Guiriec, John W. Hewitt, Brian Humensky, Stanley D. Hunter, Hui Li, Amy Y. Lien, Francesco Longo, Julie McEnery, Roopesh Ojha, Vasiliki Pavlidou, Chanda Prescod-Weinstein, Marcos Santander, John A. Tomsick, Zorawar Wadiasingh, Roland Walter
Key words	Multi-Messenger Astronomy and Astrophysics; Stars and Stellar Evolution
Link to white paper/draft	[PDF]
Executive Summary: The high-energy universe has revealed that energetic particles are ubiquitous in the cosmos and play a vital role in the cultivation of cosmic environments on all scales. Our pursuit of more than a century to uncover the origins and fate of these cosmic energetic particles has given rise to some of the most interesting and challenging questions in astrophysics. Energetic particles in our own galaxy, galactic cosmic rays (GCRs), engage in a complex interplay with the galaxy's interstellar medium and magnetic fields, giving rise to many of its key characteristics. For instance, GCRs act in concert with the galaxy's magnetic fields to support its disk against its own weight. GCR ionization and heating are essential ingredients in promoting and regulating the formation of stars and protostellar disks. GCR ionization also drives astrochemistry, leading to the build up of complex molecules in the interstellar medium. GCR transport throughout the galaxy generates and maintains turbulence in the interstellar medium, alters its multi-phase structure, and amplifies magnetic fields. GCRs could even launch galactic winds that enrich the circumgalactic medium and alter the structure and evolution of galactic disks. As crucial as they are for many of the varied phenomena in our galaxy, there's still much we don't understand about GCRs. While they have been linked to supernova remnants (SNRs), it remains unclear whether these objects can fully account for their entire population, particularly at the lower (≲ GeV per nucleon) and higher (~PeV) ends of the spectrum. In fact, it is entirely possible that the SNRs that have been found to accelerate CRs merely re-accelerate them, leaving the origins of the original GCRs a mystery. The conditions for particle acceleration that make SNRs compelling source candidates are also likely to be present in sources such as protostellar jets, superbubbles, and colliding wind binaries (CWBs), but we have yet to ascertain their roles in producing GCRs. For that matter, key details of diffusive shock acceleration (DSA) have yet to be revealed, and it remains to be seen whether DSA can adequately explain particle acceleration in the cosmos. This White Paper is the first of a two-part series highlighting the most well-known high-energy cosmic accelerators and contributions that MeV γ-ray astronomy will bring to understanding their energetic particle phenomena. For the case of GCRs, MeV astronomy will: Search for fresh acceleration of GCRs in SNRs; Test the DSA process, particularly in SNRs and CWBs; Search for signs of CR acceleration in protostellar jets and superbubbles.

4. Gamma-Ray Science in the 2020s

Details

Lead Authors	Sylvain Guiriec, John Tomsick, and Dieter Hartmann
Lead Author email	sylvain.guiriec@gmail.com
Coauthors: Terri Brandt, Marco Ajello, Alessandro De Angelis, Elisabetta Bissaldi, Bindu Rani, Zorawar Wadiasingh, Frank Timmes, Alexander van der Horst, Filippo D'Ammando, Maria Petropoulou, Vincent Tatischeff, Tonia Venters, Roland Walter, Bing Zhang, Roopesh Ojha, Christopher Fryer, Xilu Wang, Paolo Coppi, Fabian Kislat, Julie McEnery, Regina Caputo
Key words	Multi-Messenger Astronomy and Astrophysics
Link to white paper / draft	[PDF]
Conclusion: Through observations of compact objects, their formation, and their environments, we obtain information that allows us to test the laws of physics in the most extreme conditions, to track the evolution of the chemical composition of the Universe, to study the origin of the heaviest elements, and to probe the geometry of space-time fabric at small and large scales. Although compact objects in various stages of their lives, radiate at all frequencies and emit all types of messengers, high-energy astronomy is the cornerstone to decipher these cosmological encrypted messages. With very large fields of view, high-cadence sampling, high angular and spectral resolutions, and polarization capabilities, γ-ray instruments are mature and ready to take the challenge, and they are essential for the time-domain multi-messenger era. In particular, space γ-ray missions are needed for catching the GRBs that come with gravitational waves or the blazar flares that come with high-energy neutrinos. They will also complement the scientific results that will be collected with the ground-based Cherenkov Telescope Array and the large optical and radio surveys such as the Large Synoptic Survey Telescope and Square Kilometre Array.

5. A Unique Messenger to Probe Active Galactic Nuclei: High-Energy Neutrinos

Details

Lead Author	Marcos Santander
Lead Author email	jmsantander@ua.edu
Coauthors: Sara Buson, Ke Fang, Azadeh Keivani, Thomas Maccarone, Kohta Murase, Maria Petropoulou, Ignacio Taboada, Nathan Whitehorn
Key words	AGN; neutrinos; multimessenger; multiwavelength
Link to white paper / draft	[PDF]
Abstract: The detection of astrophysical neutrinos by IceCube and the evidence for neutrino emission from a blazar offer exciting opportunities for the study of high-energy neutrinos and photons from AGN in the coming decade. We advocate for a multi-messenger approach that combines high-energy neutrino observations performed by telescopes that will come online in the next decade, and multi-wavelength EM observations by existing and future instruments, with an emphasis on soft X-ray to VHE gamma-ray coverage. The unique capabilities of these instruments combined promise to solve several long-standing issues in our understanding of AGN, the most powerful and persistent cosmic accelerators..

6. Prompt Emission Polarimetry of Gamma-Ray Bursts

Details

Lead Author	Mark McConnell
Lead Author email	mark.mcconnell@unh.edu
Coauthors: Marco Ajello, Matthew Baring, Peter Bloser, Tanmoy Chattopadhyay, R.M. Curado da Silva, Sylvain Guirec, Dieter Hartmann, Hui Li, Alex Lowell, Chanda Prescod-Weinstein, Bindu Rani, Vincent Tatischeff, John Tomsick, Alexander van der Horst, Tom Vestrand, Zorawar Wadiasingh, Silvia Zane, Bing Zhang, Haocheng Zhang
Key words	GRB; polarimetry
Link to white paper / draft	[PDF]
Abstract: The nature of astrophysical jets is a central theme in many areas of astrophysical research. Jets form and propagate under conditions covering a wide range of size and mass scales in a variety of astrophysical systems, including proto-stars, white dwarfs, neutron stars, stellar-mass black holes, and supermassive black holes. Jets are typically observed as an outflow along the rotation axis of an accretion disk and result from the conversion of inflowing energy into axially directed energy, by mechanisms not yet thoroughly understood. Many aspects of astrophysical jets (launching, energization, propagation, composition, and emission) can be studied by measuring the polarization of the prompt emission from γ-ray bursts (GRBs). Theoretically associated with the formation of stellar-mass black holes, GRBs are among the most distant objects observed. The instantaneous electromagnetic intensity released during a typical GRB is eclipsed only by the Big Bang. The intense prompt emission is short-lived, typically lasting < 100 s and is believed to be associated with the formation of an ultra-relativistic jet. Extensive observational and theoretical studies in recent years have largely focused on time histories, spectra, and spatial distributions. Theoretical models show that a more complete understanding of the inner structure of GRBs, including the geometry and physical processes close to the central engine, can only be achieved by γ-ray polarimetry. Studies of ultra-relativistic GRB jets could broaden our understanding of one of the most ubiquitous phenomena in the Universe. After years of investigating time variability and spectra, now is the time to study GRBs in a new and revolutionary way using γ-ray polarimetry. NASA's 2014 Astrophysics Roadmap ("Enduring Quests, Daring Visions") echoed the sentiment that X-ray and γ-ray polarimetry should be a goal of future missions.

7. Pulsars in a Bubble? Following electron diffusion in the Galaxy with TeV gamma rays

Details

Lead Author	Henrike Fleischhack
Lead Author email	hfleisch@mtu.edu
Coauthors: A. Albert, C. Alvarez, R. Arceo, H. A. Ayala Solares, J. F. Beacom, R. Bird, C. A. Brisbois, K. S. Caballero-Mora, A. Carramiñana, S. Casanova, P. Cristofari, P. Coppi, B. L. Dingus, M. A. DuVernois, K. L. Engel, J. A. Goodman, T. Greenshaw, J. P. Harding, B. Hona, P. H. Huentemeyer, H. Li, T. Linden, K. Malone, J. Martìnez-Castro, M. A. Mostafá, M. U. Nisa, C. Rivière, F. Salesa Greus, A. Sandoval, A. J. Smith, W. Springer, T. Sudoh, K. Tollefson, A. Zepeda, H. Zhou
Key words	TeV Halos, TeV gamma-ray astromy, pulsars, electrons
Link to white paper / draft	[PDF]
Abstract: TeV Halos, extended regions of TeV gamma-ray emission around middle-aged pulsars, have recently been established as a new source class in gamma-ray astronomy. These halos have been attributed to relativistic electrons and positrons that are have left the acceleration region close to the pulsar and are diffusing freely in the surrounding medium. Measuring the morphology of TeV Halos enables for the first time a direct measurement of the electron diffusion on scales of tens of parsecs. There are hints that the presence of relativistic particles affects the diffusion rate in the pulsars' surroundings. Understanding electron diffusion is necessary to constrain the origins of the apparent "excess" of cosmic-ray positrons at tens of GeV. TeV Halos can also be used to find mis-aligned pulsars, as well as study certain properties of the Galaxy's pulsar population . Future VHE gamma-ray instruments will detect more of those TeV Halos and determine how much pulsars contribute to the observed cosmic-ray electron and positron fluxes, and how they affect diffusion in their environments.

8. Energetic Particles of Cosmic Accelerators II