Astrophysics Colloquium: The Future of Gamma-ray Astronomy in Space

Abstract: Fermi-LAT is nearly twenty years old, and Swift is no more. To be compelling, the next generation of instruments with large collecting areas must enable transformative science. Gamma rays can reveal how stars explode, what radiates in relativistic jets, and how energetic particles shape galaxies. Many decisive signatures lie in the poorly explored MeV band. I will discuss how opening that window, alongside a large new GeV observatory, could answer important questions over the next few decades. Radioactive decay lines identify newly forged nuclei and their velocities, allowing us to reconstruct supernova explosions and seek r-process products in neutron star mergers with less dependence on reprocessed optical light. At larger scales, MeV polarization would distinguish competing explanations for the emission and composition of black hole jets, while maps of radio lobes, galactic bubbles, and clusters would constrain the cosmic ray energy deposited by jets and winds, testing how black holes reshape their environments. Magnetar spectropolarimetry would probe untested QED processes in the regime of ultrastrong fields. Sufficiently sensitive all-sky MeV surveys could discover hundreds of extragalactic magnetar giant flares each year, testing models of magnetar formation/evolution and constraining connections to FRBs. Meanwhile, at GeV energies, an improved PSF and a larger collecting area could test the dark matter interpretation of the GeV Galactic center excess, while timing thousands of millisecond pulsars in the disk over a decade would map the nanohertz gravitational wave sky from supermassive black hole binaries, including possible sky anisotropies.