* Astronomy

Title: Discovery of an eclipsing X-ray binary with a 32.69 hour period in M101: an analogue of Her X-1 or LMC X-4?
Authors: Jifeng Liu, Rosanne di Stefano, Jeffrey McClintock (CfA), Albert Kong (MIT), Joel Bregman (UMich), Kip Kuntz (JHU/GSFC)

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 hours, which we interpret as an orbital period. The folded light curve exhibits an eclipse lasting about 8 hours, suggesting a compact orbit in an nearly edge-on configuration. The X-ray binary has an average luminosity of L_X(0.3-8
m keV) \approx 1.3 x 10^38 erg/sec, with only one out of the 25 observations significantly lower in flux than the average light curve.The presence of the eclipse and the $\sim$ 1.4-day orbital period suggests that this source is an analogue 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. Follow-up optical monitoring observations should be able to identify the donor and further constrain the orbital properties.

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