Hard X-ray tails and cyclotron features in X-ray pulsars

We review the physical processes occurring in the magnetosphere of accreting X-ray pulsars, with emphasis on those processes that give rise to observable effects in their high (E>10 keV) energy spectra. In the second part we compare the empirical spectral laws used to fit the observed spectra with theoretical models, at the light of the BeppoSAX results on the broad-band characterization of the X-ray pulsar continuum, and the discovery of new (multiple) cyclotron resonance features.Comment: Review talk given at the "X-ray Astronomy '99" Conferenc

Hard X-ray emission from the galaxy cluster A2256

After the positive detection by BeppoSAX of hard X-ray radiation up to ~80 keV in the Coma cluster spectrum, we present evidence for nonthermal emission from A2256 in excess of thermal emission at a 4.6sigma confidence level. In addition to this power law component, a second nonthermal component already detected by ASCA could be present in the X-ray spectrum of the cluster, not surprisingly given the complex radio morphology of the cluster central region. The spectral index of the hard tail detected by the PDS onboard BeppoSAX is marginally consistent with that expected by the inverse Compton model. A value of ~0.05 microG is derived for the intracluster magnetic field of the extended radio emission in the northern regions of the cluster, while a higher value of \~0.5 microG could be present in the central radio halo, likely related to the hard tail detected by ASCA.Comment: 10 pages, 2 figures. To appear in ApJ

Hard X-ray emission from the galaxy cluster A3667

We report the results of a long BeppoSAX observation of Abell 3667, one of the most spectacular galaxy cluster in the southern sky. A clear detection of hard X-ray radiation up to ~ 35 keV is reported, while a hard excess above the thermal gas emission is present at a marginal level that should be considered as an upper limit to the presence of nonthermal radiation. The strong hard excesses reported by BeppoSAX in Coma and A2256 and the only marginal detection of nonthermal emission in A3667 can be explained in the framework of the inverse Compton model. We argue that the nonthermal X-ray detections in the PDS energy range are related to the radio index structure of halos and relics present in the observed clusters of galaxie.Comment: 15 pages, 1 figure, ApJL in pres