EXIST Science

The hard x-ray (HX) band, from ~5-600 keV reveals an unusually rich range of astrophysical processes in both compact and diffuse sources. The HX band is key for study of the obscured universe, particularly the heavily absorbed active galactic nuclei (AGN) of galaxies. A deep HX survey, can reveal the accretion history of the universe.

The HX band includes the transition from primarily thermal objects -- either optically thin, like supernova remnants or galaxy clusters, or optically thick, like the blackbody emission components of high luminosity x-ray binaries -- to objects which are primarily non-thermal, or at least display significantly Comptonized spectra.

Examples of the very hottest thermal plasmas directly measurable in astronomical objects, the ~108-9K coronae around or above accretion disks in compact binaries and active galactic nuclei (AGN), are best studied in the HX band. The rapid pace of discovery of hard x-ray emission from black hole binaries (the x-ray novae) as well as AGN have shown that most of their radiated power comes out at ~100 keV. Black holes are thus both identified and studied most directly in the hard x-ray band.

Gamma-ray bursts (GRBs) emit most of their prodigious luminosity in the HX band and can be studied at the highest redshifts (z ~ 10-30) with a very high sensitivity (large area) HX imaging mission. GRBs may be the most direct probe of the earliest generation of massive stars (Pop III) from which the required compact objects formed.

Key Scientific Objectives for a Deep HX Survey:
(see also discussion in Evolution & Development of EXIST Concepts document)