Detection of the second eclipsing high-mass X-ray binary in M 33

Chandra data of the X-ray source [PMH2004] 47 were obtained in the ACIS Survey of M 33 (ChASeM33) in 2006. During one of the observations, the source varied from a high state to a low state and back, in two other observations it varied from a low state to respectively intermediate states. These transitions are interpreted as eclipse ingresses and egresses of a compact object in a high-mass X-ray binary (HMXB) system. The phase of mideclipse is given by HJD 2453997.476 0.006, the eclipse half angle is 306 12. Adding XMM-Newton observations of [PMH2004] 47 in 2001 we determine the binary period to be 1.732479 0.000027 days. This period is also consistent with ROSAT HRI observations of the source in 1994. No short-term periodicity compatible with a rotation period of the compact object is detected. There are indications for a long-term variability similar to that detected for Her X-1. During the high state the spectrum of the source is hard (power-law spectrum with photon index 0.85) with an unabsorbed luminosity of 2 ×10³⁷ erg s ^-1 (0.2-4.5 keV). We identify as an optical counterpart a V 21.0 mag star with T _eff>19000 K, log(g)>2.5. The Canada-France-Hawaii Telescope optical light curves for this star show an ellipsoidal variation with the same period as the X-ray light curve. The optical light curve together with the X-ray eclipse can be modeled by a compact object with a mass consistent with a neutron star or a black hole in an HMXB. However, the hard power-law X-ray spectrum favors a neutron star as the compact object in this second eclipsing X-ray binary in M 33. Assuming a neutron star with a canonical mass of 1.4M _⊙ and the best-fit companion temperature of 33,000 K, a system inclination i = 72° and a companion mass of 10.9M _⊙ are implied.