FIGURE 3.6 Limiting fluxes for existing and projected x-ray and gamma-ray surveys as a function of energy. The fluxes assume an energy band equal to the energy, i.e., a broadband measurement. As in Figure 3.2, the better-performing instruments appear lower in the figure. Note the large gains in sensitivity for GLAST, EXIST, and VERITAS. COMPTEL and EGRET are instruments on the Compton Gamma Ray Observatory. Courtesy of L. Bildsten (University of California, Santa Barbara), and NASA.

CGRO, GLAST, and VERITAS. EXIST is a space station-attached telescope with a spectral resolving power of 100 and the ability to locate bright sources to about 30 arcsec. EXIST will survey the entire sky in every 90-minute orbit, which allows the study of the highly time-variable sources that characterize the x-ray sky. It will carry the study of gammaray bursts to lower energies and will be able to study the low-power gamma-ray bursts that appear to be associated with supernovae. Because energetic x rays are so penetrating, EXIST can discover supernovae embedded in molecular clouds and the luminous matter accreting onto supermassive black holes in the centers of galaxies that are obscured at lower photon energies by surrounding gas and dust. Finally, with its ability to perform high-energy-resolution, hard-x-ray spectroscopy of