The X-ray eclipse geometry of CAL 87

TL;DR

This study uses X-ray observations and indirect imaging to map the accretion structures of CAL 87, revealing an extended emission region, a bright spot, and evidence supporting wind-driven mass transfer.

Contribution

It applies indirect imaging techniques to X-ray light curves of CAL 87 to map accretion structures and analyze emission features, providing new insights into the system's geometry and mass transfer process.

Findings

Extended symmetric emission surface brightness

Detection of a bright spot in hardest X-rays

Evidence supporting wind-driven mass transfer scenario

Abstract

We explore XMM-{\it Newton} observations of the eclipsing super-soft X-ray source CAL~87 in order to map the accretion structures of the system. { Indirect imaging techniques were applied in X-ray light curves to provide eclipse maps}. The surface brightness distribution exhibits an extended and symmetric emission, { and from the hardest X-rays is revealed a feature that is likely due to a bright spot}. A rate of $\dot{P} = (+6\pm2) \times 10^{-10}$ for changes in the orbital period of the system was derived from the eclipses. There is no significant variation of the emission lines even during eclipses, arguing that the lines are formed in an extended region. The continuum emission dominates the decrease in flux which is observed during eclipses. The O\,{\small VIII} Ly$\alpha$ line reveals a broadening velocity { which is} estimated in 365$^{+65}_{-69}$ km\,s$^{-1}$ (at 1$\sigma$) and marginal evidence for asymmetry in its profile, and sometimes shows evidence of double-peaked emission. Together, the results support that the wind-driven mass transfer scenario is running in CAL~87.