4.1.2 HARD X-RAY FOCUSING

Dramatic increases in flux sensitivity have always yielded major progress in astrophysics. For the traditional X-ray band (, 0.1 to 10 keV), this came in the late 1970's and early 1980's with the flight of the first focusing X-ray telescopes, Einstein and EXOSAT. The upcoming launches of AXAF, XMM, and ASTRO-E will extend this field further, thus bringing soft X-ray astronomy to maturity. The situation is far less satisfying at higher energies. In particular, the decade in energy from 10 to 100 keV has barely been probed. Current experiments in this energy range (e.g., the HEXTE instrument on XTE) are more than two orders of magnitude less sensitive than even the early focusing X-ray telescopes. Compelling motivation for extending observational capabilities in the hard X-ray band (10 - 100 keV) is provided both by the existence of sources whose energy output peaks in this range, and by astrophysical processes which are uniquely observable there. The separation of thermal and nonthermal processes becomes distinct above 10's of keV and the diagnostics provided by observing these nonthermal processes are astrophysically important, and often are unique and fundamental. Unfortunately, in the hard X-ray band, the internal detector background count rates dominate the source fluxes by several orders of magnitude for most sources. Focusing - or concentration of the signal onto a small region of the detector - is the only approach to achieving detection thresholds comparable to what has been obtained at lower energies. The use of focusing optics for hard X-ray experiments has been limited by the fact that the maximum incidence, or graze angle, allowed for significant X-ray reflection decreases approximately linearly with X-ray energy, making it difficult to obtain substantial effective area for telescopes of moderate focal length. The recent development of graded multilayer coatings, which are capable of substantially increasing the graze angles of traditional focusing optics over a broad energy band, allow for the design of a mission which would greatly extend the flux sensitivity of grazing incidence telescopes to 1 100 keV. A mission incorporating such focusing optics with graded multilayer coatings is a priority of the gamma-ray astrophysics program. This mission would incorporate an array of Wolter type I or conical approximation telescopes coated with graded multilayers capable of reflecting hard X-rays in the 10 to ~ 100 keV band. Table 4.2 lists typical instrument parameters for such a mission. The typical flux sensitivity achievable at 50 keV is approximately two and a half orders of magnitude lower than that achieved by HEXTE.

The most important science objectives for a hard X-ray focusing mission include:

• Active Galactic Nuclei. A principal goal of this mission will be sensitive studies of AGN, obtaining for the first time high-quality hard X-ray/soft gamma-ray spectra for a significant number of quasars and Seyfert galaxies. Observations in the energy band from 10 - 100 keV provide a measure of the intrinsic luminosity of the central source and address many basic questions, including the relationship among the various classes of AGN, in particular Seyfert Is, IIs, and quasars (QSOs).

• Population Studies in the Local Group. Studying the compact objects, such as weakly magnetic (B < 10⁹ G) neutron stars in low-mass binaries, and black hole candidates with both high-and low-mass companions outside of our own galaxy at hard X-ray energies is an important goal possible only with a focusing instrument.

• Measurement of the Intracluster Magnetic Field in Clusters of Galaxies. As described in Section 2.3.2, hard X-ray observations can provide a direct measurement of the magnetic field strength in galaxy clusters.

• Nucleosynthesis and Dynamics in Type II Supernovae. The high sensitivity and imaging capability of a hard X-ray focusing telescope make it ideally suited for mapping the at 68 and 78 keV emission from 44Ti in young supernova remnants (see 3.2.1).

• Investigation of Shock Acceleration in Young Supernova Remnants. In addition to the soft thermal X-radiation produced in the shocked ejecta, young supernova remnants like Cas A exhibit hard X-ray "tails" extending out to ~50 keV.