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

TL;DR

This study analyzes Suzaku X-ray data of the eclipsing LMXB 2S 0921-630, revealing a spectrum with a blackbody component, Comptonization, and emission lines from highly ionized atoms, providing insights into the accretion disk corona and iron line origin.

Contribution

First detailed Suzaku spectral analysis of 2S 0921-630 revealing the structure of the accretion disk corona and origin of the broad iron Kα line.

Findings

Spectrum includes blackbody and Comptonized components.

Detection of highly ionized emission lines from multiple elements.

Broad semi-neutral iron Kα line suggests origin near the inner accretion disk.

Abstract

2S 0921$-$630 is an eclipsing low-mass X-ray binary (LMXB) with an orbital period of $\sim$ 9 days. 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_{\rm bb} \sim 0.3$ keV) Comptonized by optically-thick gas (``Compton cloud''; optical depth $\tau \sim 21$) with a temperature of $\sim 2$ keV, combined with thirteen emission lines. We find that most of the emission lines correspond to highly ionized atoms: O, Ne, Mg, Si, S, Ar, and Fe. A K$\alpha$ emission line and an absorption edge of semi-neutral iron (Fe I -- XVII) are also detected. The semi-neutral iron K$\alpha$ line is significantly broad with a width of $0.11 \pm 0.02$ keV in sigma, which corresponds to the Doppler broadening by the Kepler motion at a radius of $\sim 10^9$ 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 $\sim 10^9$ cm.