2007ApJ...668L.151W

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 2003 December 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 timescale of ∼25 s. The ingress feature persists for at least five 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.