A new candidate Wolf-Rayet X-ray binary in NGC 253

We have discovered a persistent, but highly variable X-ray source in the nearby starburst galaxy NGC 253. The source varies at the level of a factor of about 5 in count rate on time-scales of a few hours. Two long observations of the source with Chandra and XMM-Newton show suggestive evidence for the source having a period of about 14-15 hours, but the time sampling in existing data is insufficient to allow a firm determination that the source is periodic. Given the amplitude of variation and the location in a nuclear starburst, the source is likely to be a Wolf-Rayet X-ray binary, with the tentative period being the orbital period of the system. In light of the fact that we have demonstrated that careful examination of the variability of moderately bright X-ray sources in nearby galaxies can turn up candidate Wolf-Rayet X-ray binaries, we discuss the implications of Wolf-Rayet X-ray binaries for predictions of the gravitational wave source event rate, and, potentially, interpretations of the events.