Since the gravitational collapse of matter into stars and galaxies a few hundred thousand years after the big bang, much of the visible matter in the universe has been processed through the slow but spectacular life cycle of matter—stellar formation and evolution ending in novae or supernovae, with the ejection of heavy nuclei back into the galaxy to seed a new generation of stars. Nuclear gamma-ray astrophysics is the study of emission from radioactive nuclei as tracers of this cycle of creation. In particular, ACT would map our galaxy in a broad range of nuclear line emission from radioactive decays, nuclear de-excitations, and matter-antimatter annihilations. It would measure the radioactive gamma-ray and positron emitters among the particles propagating from supernovae, novae, and stellar winds populating our galaxy. Additionally, gamma rays from accretion of matter onto galactic compact objects and massive black holes in active galactic nuclei (AGN) are used to test accretion disk and jet models and to probe relativistic plasmas. Gamma-ray polarization can be used to study the emission processes in GRBs, pulsars, AGN, and solar flares. The origins of the diffuse cosmic MeV background can also be identified.
The ACT telescope would increase detection efficiency by up to two orders of magnitude over the COMPTEL instrument (the Compton Telescope on the Compton Gamma Ray Observatory [CGRO]). The ACT instrument design is driven by its primary science goal—spectroscopy of the 56Co (0.847 MeV) line from type Ia supernovae (SNe Ia), which is expected to be Doppler-broadened to ~3 percent. ACT would allow hundreds of SNe Ia detections over its primary 5-year survey lifetime. In the process, ACT becomes an all-sky observatory for all classes of gamma-ray observations, Table B.1 provides estimates of potential ACT observations compared with those of the COMPTEL.
The ACT science would be complementary to the Gamma-ray Large Area Space Telescope (GLAST) science.5 GLAST addresses the high-energy gamma rays from ~20 MeV to 300 GeV, and ACT would cover the region from 0.2 MeV to 10 MeV. GLAST also includes a 10-keV to 30-MeV GRB detector.