RX J004717.4-251811 : The first eclipsing X-ray binary outside the Local Group

The X-ray source RX J004717.4-251811 in the nearby starburst galaxy NGC 253 was found to undergo changes from a low to a high state twice, during an XMM-Newton EPIC observation in December 2000 and also during a Chandra observation one year earlier. These transitions are interpreted as egresses from eclipses of a compact object in a high mass X-ray binary system (HMXB). The phase of eclipse egress during the Chandra observation is given by barycenter corrected MJD 51539.276 ′ 0. 006 and the binary period determined to p = (352.870 ′ 0.012)d/n by the time difference between the two egresses and number n of periods in-between. Allowed periods may be further constrained by additional XMM-Newton, Chandra, ROSAT, and Einstein observations resulting in only seven acceptable periods with 1.47024 d and 3.20793 d most promising. No significant regular pulsations of the source in the range 0.3-1000 s were found. Fluctuations on time scales of 1000 s were observed together with extended intervals of low intensity. The energy spectrum during the bright state can best be described by an absorbed flat power law (N H =1.9 x 10 2 1 cm - 2 , Γ = 1.7). In the bright state the source luminosity is 4 x 10 3 8 (0.5-5 keV), just compatible with the Eddington luminosity of a 1.4 M O . neutron star. A possible optical identification is suggested. RX J004717.4-251811parameters are compared to other eclipsing X-ray binaries (XRBs).