Ariel 5 Hard X-Ray Observations

Results from the hard X-ray detector (8 cm² area, 8 degrees f.w.h.m. field of view, energy range 26-1200 keV) are reviewed in the context of observations by other groups. At least three different classes of galactic sources are studied by this detector. In the first, magnetospheric control of the emission, which seems to be a modified thermal spectrum is dominant. Examples are Cen X-3, Her X-1, A0535+26 and GX 301-2. So far, only direct evidence on the magnetic field strength from cyclotron line emission has come from Her X-1. In the second, the characteristics are a power law spectrum extending to over about 200 keV and fast aperiodic time variations with little evidence of the presence of strong magnetic fields. Inverse Compton production of the X-ray photons seems important here. Examples are Cyg X-1, A0620-00, Ser X-1 and perhaps Cir X-1, although this last object suffers periodic heavy low energy absorption. The third is the white dwarf emitter class, e.g. AM Herculis and possibly AM Canum Venaticorum; sources in this class surprisingly tend to exhibit a power law rather than a thermal spectral shape. Sco X-1 and Sco X-2 are perhaps a fourth class of object. Hard X-ray upper limits obtained from a survey of COS-B γ -ray source positions are discussed. Further attempts to find hard X-ray emission from the galactic centre X-ray burst sources are mentioned.