Detection of the second eclipsing XRB in M33 by Chandra

ATel #905; W. Pietsch (MPE), P. P. Plucinsky (CfA), F. Haberl (MPE), A. Shporer (Wise Obs.), T. Mazeh (Wise Obs.) for the CHASeM33 team
on 2 Oct 2006; 14:19 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

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The variable XMM-Newton source [PMH2004] 47 was identified as an XRB candidate by Misanovic et al. (2006, A&A 448, 1247). It coincides with the ROSAT source [HP2001] 19 and was in the field of view of several Chandra CHASeM33 very large project observations of M33 obtained from 2005 November 23 to 2006 September 26. It varied strongly in brightness as during the XMM-Newton observations from 2000 to 2003. On 2006 September 18/19 the source was bright and during the observation we monitored a drop to an intensity consistent with zero for about 25 ks. We interpret this as an eclipse of the X-ray source. Ingress and egress transitions take no longer than 500 s each. During the bright phase the source shows a hard X-ray spectrum with an absorption column density of 2.2x10^21 cm^-2 and a photon index of 0.97 when modeled with an absorbed power-law. The observed (0.5-10.0 keV) flux of 6.1x10^-13 erg cm^-2 s^-1 corresponds to an intrinsic source luminosity of 5x10^37 erg s^-1 at a distance of 795 kpc. During an observation 84 days earlier another fast transition from low to high intensity was detected. On 2006 September 26 the source showed another transition from low to intermediate intensity. Interpreting these transitions as eclipse egresses we derive a unique orbital period of 1.73245 d which also is consistent with the other Chandra and also the much earlier XMM-Newton detections of the source. We derive a mid-eclipse epoch of JD2453997.470 and an eclipse half angle of 30.7 degrees. The position of the source coincides with star #4571 of Massey et al. (2006, AJ 131, 2478) which shows V=21.011 mag and colours consistent with a B0V to B2III star after extinction and colour correction. [PMH2004] 47 is therefore most likely the second eclipsing HMXB in M33 after the eclipsing black hole X-ray binary M33 X-7 (see e.g. Atel #633 or Pietsch et al. 2006, ApJ 646, 420). [PMH2004] 47 shows long term X-ray variability similar to the high-mass system LMC X-4 or the low-mass system Her X-1, respectively.