Possible Magnetic Activity in the Low Mass X-ray Binary EXO 0748-676

TL;DR

This paper presents evidence of magnetic activity in the secondary star of the low mass X-ray binary EXO 0748-676, observed through X-ray eclipse features indicating plasma structures and possible mass ejection.

Contribution

It provides the first observational evidence of magnetic activity and plasma structures associated with the secondary star in this system.

Findings

Detection of an absorption shadow during ingress indicating plasma above the secondary star

Persistent ingress feature over multiple orbital periods suggesting stable magnetic activity

Observation of unusual post-eclipse dipping indicating possible mass ejection

Abstract

We report evidence of magnetic activity associated with the secondary star in the EXO 0748-676 low mass X-ray binary system. An analysis of a sequence of five consecutive X-ray eclipses observed during December 2003 with the RXTE satellite brings out a feature occurring during ingress we interpret as the X-ray photoelectric absorption shadow, as seen by an observer at Earth, of a plasma structure suspended above the surface of the secondary star. The light curve feature consists of an initial drop in count rate to near zero (the absorption shadow) with a very short rebound to a significant fraction of the pre-ingress count rate and then a final plunge to totality over a total time scale of ~25 s. The ingress feature persists for at least 5 consecutive orbital periods (a total of ~19 hr), and possibly up to 5 days in our data. Our data also show significant post-egress dipping during this eclipse sequence, unusual for this source, indicating possible secondary star mass ejection during this episode.