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

¹ Moscow State University, Sternberg Astronomical Institute, 119234 Moscow, Russia
e-mail: shakura@sai.msu.ru
² Kazan Federal University, 420008 Kazan, Russia
³ Space Research Institute of the Russian Academy of Sciences, 117997 Moscow, Russia
⁴ Max-Planck-Institut für Astrophysik (MPA), Karl-Schwarzschild-Str 1, 85741 Garching, Germany

Received: 16 December 2020
Accepted: 20 February 2021

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 ~(2−4) × 10⁶ 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.