X-ray spectroscopy of the accretion disk corona source 2S 0921-630 with Suzaku archival data

Abstract

2S 0921-630 is an eclipsing low-mass X-ray binary (LMXB) with an orbital period of ~9 d. Past X-ray observations have revealed that 2S 0921-630 has an extended accretion disk corona (ADC), from which most of the X-rays from the system are emitted. We report the result of our Suzaku archival data analysis of 2S 0921-630. The average X-ray spectrum is reproduced with a blackbody emission (kT_bb ~ 0.3 keV) Comptonized by optically thick gas ("Compton cloud"; optical depth τ ~ 21) with a temperature of ~2 keV, combined with 13 emission lines. We find that most of the emission lines correspond to highly ionized atoms: O, Ne, Mg, Si, S, Ar, and Fe. A Kα emission line and an absorption edge of semi-neutral iron (Fe textsci- textscxvii) are also detected. The semi-neutral iron Kα line is significantly broad, with a width of 0.11 ± 0.02 keV in sigma, which corresponds to the Doppler broadening by the Kepler motion at a radius of ~10⁹ cm. We suggest that the observed semi-neutral iron line originates at the inner part of the accretion disk in the immediate outside of the Compton cloud, i.e., the Compton cloud may have a radius of ~10⁹ cm.