On the X-ray continuum and iron-line emission of the low state of Hercules X-1

Abstract

The hard X-ray continuum observed during low-state periods of Hercules X-1 is interpreted as being due to Compton scattering, by the coronal gases above and below the accretion disk, of the primary hard X-rays from the neutron star, which is hidden from our view during these periods. A similar interpretation holds for the low-state soft-X-rays. Because of photoionization, most iron ions in the corona are fully stripped and only a small fraction of them retain one electron. Because of such a high degree of iron ionization, the iron K photons from the corona, even if there are some, will make a line feature at 6.9 keV, and no line feature has been detected at this energy. The low-state iron line at 6.4 keV therefore, cannot be due to the coronal gases. The main production mechanism of this line is most likely fluorescence from the accretion disk.