Discovery of an Eclipsing X‐Ray Binary with a 32.69 hr Period in M101: An Analog of Her X‐1 or LMC X‐4?

Abstract

We report the discovery of an eclipsing X-ray binary in M101, the first such system to be discovered outside the Local Group. Based on a sequence of 25 Chandra observations that sample a wide range of orbital phases, we find a period of 32.688+/-0.002 hr, which we interpret as an orbital period. The folded light curve exhibits an eclipse lasting about 8 hr, suggesting a compact orbit in a nearly edge-on configuration. The X-ray binary has an average luminosity of LX(0.3-8 keV)~1.3×1038 ergs s-1, with only 1 out of the 25 observations significantly lower in flux than the average light curve. The presence of the eclipse and the ~1.4 day orbital period suggests that this source is an analog of the well-studied eclipsing X-ray binary pulsars Her X-1 or LMC X-4. Combining the Chandra data and the HST ACS/WFC images, we have identified several possible optical counterparts, including an O5-O3 star with V=25.0 mag. Follow-up optical monitoring observations should be able to identify the donor and further constrain the orbital properties.