2.3.3 CLUSTERS

The detection of hard X-ray emission from inverse Compton scattering of 2.7 K background photons from high-energy intracluster electrons would provide a very direct way of investigating intergalactic magnetic fields. The presence of electrons is inferred from the observation of cluster radio halos. The Coma cluster is the best known example, but others include A2255, A2256, A2319, and A1367. The origin of the magnetic fields in galaxy clusters is not understood, nor is the mechanism by which the electrons are accelerated. Since we cannot be confident of equipartition, the magnetic field cannot be estimated from the radio observations alone. Observation of the inverse Compton X-rays together with the radio observations would provide a much more direct method of establishing the magnetic fields, but requires X-ray observations above about 20 keV to avoid confusion from thermal emission. An estimated flux sensitivity of ~3 x 10^-6 photons cm^-2 s^-1 keV^-1 in the ~20-60 keV range is needed to make the crucial measurement. While HEXTE will attempt this, imaging instruments are needed to avoid confusion with cluster AGNs. High spectral resolution in the hard X-ray band is also needed for a clean separation of the thermal cluster gas component. Focusing hard X-ray telescopes, with high resolution detectors, are particularly well suited to this important problem.

2.3.4 THE DIFFUSE GAMMA-RAY BACKGROUND

Data on the cosmic diffuse gamma-ray background has been obtained by over 20 balloon- and satellite-borne instruments over the past 30 years, but only recently have good spectral measurements been made. Many questions remain unanswered though, such as the spectral shape in the MeV region, the presence or absence of nucleosynthetic lines, the angular distribution, and the origin of the radiation. The low-energy portion of the cosmic diffuse spectrum (10 keV to 60 keV) is characterized by a bremsstrahlung spectral form that can be approximated by a power-law segment of energy index ~0.4. The energy spectrum transitions to a power law of index ~1.6 above 60 keV. At an MeV, there is still uncertainty as to the shape. Prior to 1995, the spectrum was thought to have a hump at ~2 MeV as detected by instruments on balloons, HEAO-1, and Apollo 16/17. However, recent measurements by the COMPTEL instrument on Compton in the 0.8 - 30 MeV range, and a careful reanalysis of data obtained with SMM, have not detected the hump. Above several MeV the spectrum has an energy index of ~1.0 as seen by recent measurements by EGRET.

Various theoretical attempts have been made to model the source of the diffuse background as unresolved AGN. It is generally possible to fit the spectrum with dominant contributions from absorbed Seyfert 2's between 10 and 400 keV and blazars between 3 MeV and 10 GeV. There may be an excess emission at ~1 MeV above the AGN models, although the data quality is not good in this range. The origin of such an excess could be gamma-ray line and continuum emission from unresolved Type 1a supernovae.

Objectives:

• Detection of an expanded sample of Seyferts and blazars at much higher S/N than with CGRO OSSE. Coordinated observations of Seyferts and blazars with good sensitivity into the MeV band.

Requirements:

• Flux sensitivity at high energies at least a factor of 10 better than EGRET with comparable angular resolution. Hard X-ray telescope.

2.4 THE PUZZLE OF THE UNIDENTIFIED HIGH-ENERGY GAMMA-RAY SOURCES

Only 45% of the gamma-ray sources detected by EGRET have been identified with sources known at longer wavelengths. The nature of the remaining sources is an important question. The distribution of these unidentified sources suggests a largely galactic population; however, there are some at high galactic latitudes.