4.2 MIDEX & SMEX

4.2.1 HARD X-RAY SURVEY

The hard X-ray sky, broadly defined as 10 keV to 600 keV, is relatively poorly explored and yet rich in promise. This is the energy range where numerous sources and source mechanisms are either already known (at 10 keV) or strongly suggested. It is the energy domain where fundamental transitions from primarily thermal to primarily nonthermal sources and phenomena are expected. Here we briefly outline the case for a broad-band sky survey mission to be followed by a focusing mission (section 4.1.2) for detailed study of individual sources over a more limited energy range. Only one truly all-sky survey has been conducted in the hard X-ray band: the pioneering HEAO-A4 survey which yielded a catalogue of some 80 sources down to flux levels of typically 50 mCrab in the 13-180 keV band. Meanwhile, the soft X-ray sky has now been explored fully to flux levels a factor of ~10³ times fainter with ROSAT but only up to energies of 2.5 keV. The need is therefore great for a hard X-ray survey mission above 10 keV that could achieve a sensitivity increase of a factor of 100 or more to close the gap. Such a mission must be imaging to avoid the source confusion limits that plagued both HEAO-A4 and follow-on hard X-ray/soft gamma-ray collimated detectors such as OSSE on CGRO, particularly in the galactic plane.  The imaging resolution needed for such a future high sensitivity survey should be at least ~15 arcmin. An all-sky survey mission for hard X-ray imaging will not only locate and study a vastly increased hard X-ray source population (e.g., hundreds of X-ray binaries and over a thousand AGN), but will provide unprecedented sensitivity and temporal coverage for time variability studies. The high energy universe is highly variable, and a future mission that not only pushes the hard X-ray boundary but the temporal limits and allows coverage of compact objects from milliseconds to months is essential. Finally, the same survey mission at hard X-ray energies must allow at least moderately high spectral resolution. This is needed for study of the variety of thermal and nonthermal emission mechanisms in a wide range of sources. It is also needed for the relatively isolated spectral line features expected: cyclotron lines in highly magnetic neutron stars in accretion powered binaries, possible 511 keV (positron annihilation) or 478 keV (⁷Li) lines in black hole binaries, or nuclear decay lines (e.g., ⁴⁴Ti lines at 68 and 78 keV) in young supernova remnants. All of these needs are described in section 2 above and could be met with a wide field-of-view coded aperture imaging survey mission operating over a broad energy band (10-600 keV) and achieving large total exposure times on any given source by virtue of its large field-of-view. Missions such as EXIST, BASIS, and BLAST which would accomplish these goals are under study as part of New Mission Concepts or MIDEX programs.