High resolution X-ray spectra of the compact binary supersoft X-ray source CAL 87

TL;DR

This paper analyzes high-resolution X-ray spectra of CAL 87, revealing stable spectral features, a Thomson-scattering dominated continuum, and complex emission lines, providing insights into the nature of this supersoft X-ray binary.

Contribution

First detailed high-resolution spectral analysis of CAL 87 across two epochs, highlighting the scattering and emission regions in this supersoft X-ray binary.

Findings

X-ray spectra are consistent over two years.

Continuum flux indicates Thomson scattering around a hot WD.

Emission lines originate from extended regions beyond the central source.

Abstract

In this study, we present an analysis of the archival X-ray data of the eclipsing supersoft X-ray binary CAL 87 observed with the {\it Chandra} Advanced CCD Imaging Spectrometer (ACIS) camera and Low Energy Transmission Grating (LETG) in 2001 August and with {\it XMM-Newton} in 2003 April. The high resolution X-ray spectra are almost unchanged in the two different dates. The average unabsorbed X-ray luminosity during the exposure was 4.64$-$5.46$\times10^{36}$ ergs s$^{-1}$ in 2001 and 4.54$-$4.82 $\times10^{36}$ ergs s$^{-1}$ in 2003, with prominent and red-shifted emission lines, mostly of nitrogen, oxygen, iron and argon, contributing to at least 30\% of the X-ray flux. The continuum X-ray flux is at least an order of magnitude too small for a hot, hydrogen burning WD. However, the continuum flux is consistent with Thomson-scattering reflecting about 5\% of the light of a hydrogen burning WD with effective temperature of 800,000 K, and a mass of $\sim$ 1.2 M$_\odot$. It has been noted before that a large Thomson-scattering corona explains the X-ray eclipse of CAL 87, in which the eclipsed region is found to be of size the order of a solar radius. The emission lines originate in an even more extended region, beyond the eclipsed central X-ray source; the emission spectrum is very complex, with unusual line ratios.