Observations of Her X-1 in low states during SRG/eROSITA all-sky survey

TL;DR

This paper reports on eROSITA observations of Her X-1 during low states, revealing X-ray scattering from multiple regions around the binary system, providing insights into the accretion environment during obscured phases.

Contribution

First detailed X-ray analysis of Her X-1 during low states using eROSITA, identifying scattering regions and their properties.

Findings

Detection of orbital modulation during low states.

Identification of scattering regions: corona, halo, and star atmosphere.

Insights into the accretion disk and stellar environment during obscured phases.

Abstract

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) instrument onboard the Russian-German `Spectrum-Roentgen-Gamma' (SRG) mission observed the Her X-1/HZ Her binary system in multiple scans over the source during the first and second SRG all-sky surveys. Both observations occurred during a low state of the X-ray source when the outer parts of the accretion disk blocked the neutron star from view. The orbital modulation of the X-ray flux was detected during the low states. We argue that the detected X-ray radiation results from scattering of the emission of the central source by three distinct regions: (a) an optically thin hot corona with temperature $\sim (2-4) \times 10^6$ K above the irradiated hemisphere of the optical star; (b) an optically thin hot halo above the accretion disk; and (c) the optically thick cold atmosphere of the optical star. The latter region effectively scatters photons with energies above $5-6$ keV.