Hard X-ray Emission from Her X-1

FROM the 2 to 20 keV X-ray observations of Uhuru1, Her X-l has been identified as 1.7 d eclipsing binary pulsating with a periodicity of 1.25 s. More recent observations by Giacconi et al. (ASE Preprint 3166, 1973) show that this source exhibits regular 'on' states of duration 11 to 12 d followed by about 24 d 'off' states and that there are dips in intensity corresponding to the 1.7 d binary period, the occurrence of which is related to the orbital phase. The same data have also provided evidence for the day-to-day variations in the intensity and spectral features. Extension of these measurements to 40 keV by Ulmer et al.2,3 with the UCSD telescope on OSO-7 has substantially confirmed these features, even though there is an indication that the spectral characteristics at higher energies are different from those at lower energies. We therefore intuitively feel that the extension of these observations to still higher energies is extremely important for obtaining a better understanding of the mechanism of X-ray emission from this complex source.