Accretion Disk Wind of Hercules X-1 during the Short High State

Abstract

Hercules X-1 is a nearly edge-on X-ray binary with a warped, precessing accretion disk, which manifests through a 35-day cycle of alternating High and Low flux states. This disk precession introduces a changing line of sight toward the X-ray source, through an ionized accretion disk wind. The sightline variation allows us to uniquely determine how the wind properties vary with height above the disk. All the previous wind measurements were made in the brighter Main High state of Her X-1. Here, we analyze the only Chandra observation during the fainter “Short” High state and significantly detect blueshifted ionized absorption. We find a column density of ${2.0}_{-0.6}^{+1.1}\times {10}^{22}$ cm ^−2 , an ionization parameter $\mathrm{log}(\xi $ /erg cm s ^−1 ) = ${3.41}_{-0.12}^{+0.15}$ , and an outflow velocity of 380 ± 40 km s ^−1 . The properties of the outflow measured during the Short High state are in good agreement with those measured at equivalent precession phases during the Main High state. We conclude that we are sampling the same wind structure, seen during both the Main and Short High states, which is precessing alongside the warped accretion disk every 35 days. Finally, the high spectral resolution of Chandra gratings above 1 keV in this observation enabled us to measure the abundances of certain elements in the outflow. We find Mg/O $\,=\,{1.5}_{-0.4}^{+0.5}$ , Si/O =1.5 ± 0.4, and S/O $\,=\,{3.0}_{-1.1}^{+1.2}$ , whereas in our previous study of Her X-1 with XMM-Newton, we found an overabundance of N, Ne, and Fe compared with O. These peculiar abundance ratios were likely introduced by the pollution of the donor by the supernova that created Her X-1.

Keywords

• Accretion
• X-ray binary stars
• Neutron stars