Comparing Her X-1 and X Persei Employing a New Model for Spectral Formation in Accreting X-ray Pulsars

We compare the X-ray spectra of two accreting X-ray pulsars that are believed to exhibit very different accretion luminosities, Her X-1 and X Per. Her X-1 has a high luminosity of around 4.7 x 10³⁷ erg s^-1 while X Per has a significantly lower luminosity near 10³⁴ erg s^-1. Her X-1 is believed to generate its X-ray luminosity in a radiation-dominated radiative shock above the surface of the neutron star with the accretion flow coming to a halt at the neutron star surface. On the other hand, the low luminosity of X Per suggests the possibility of a strong standing shock at the top of the accretion column, which decreases the velocity of the infalling gas by a factor of 4. But after passing through this shock, the accreting matter reaccelerates and impacts the stellar surface with velocity ~0.25c. We discuss a new model for the formation of the X-ray spectrum from accreting X-ray pulsars that attempts to account for both types of accretion shock flows in accreting X-ray pulsar systems. We compare our model spectra with existing RXTE and NuSTAR X-ray data obtained for the Her X-1 and X Per systems. We show that the large difference in accretion rates implied by the luminosities in these two systems suggests significant differences in the physics of the accretion stopping processes and also differences in the radiative processes that produce the observed spectra. This work was supported by the NASA Astrophysics Explorer Program and the NuSTAR Guest Investigator Program.