The Orbit of the Eclipsing X-Ray Pulsar EXO 1722–363

With recent and archival Rossi X-Ray Timing Explorer (RXTE) X-ray measurements of the heavily obscured X-ray pulsar EXO 1722-363 (IGR J17252-3616), we carried out a pulse-timing analysis to significantly improve the accuracy of the orbital parameters. The binary system is characterized by a_x sin i = 101 ± 3 lt-s and P_orb = 9.7403 ± 0.0004 days (90% confidence), with the precision of the orbital period obtained by connecting data sets separated by more than 7 yr (272 orbital cycles). The orbit is consistent with circular, and e < 0.19 at the 90% confidence level. The orbital solution also shows that a torque reversal occurred sometime between 1998 November and 2006 February. The mass function is 11.7 ± 1.2 M_☉ and confirms that this source is a high-mass X-ray binary (HMXB) system. Using previous eclipse time measurements by Corbet et al. and our orbital solution, combined with the assumption that the primary underfills its Roche lobe, we find that i > 61° at the 99% confidence level, the radius of the primary is between 21 and 37 R_☉, and its mass is less than about 22 M_☉. The acceptable range of radius and mass shows that the primary is probably a supergiant of spectral type B0 I-B5 I. Photometric measurements of its likely counterpart are consistent with the spectral type and luminosity if the distance to the system is between 5.3 and 8.7 kpc. Spectral analysis of the pulsar as a function of orbital phase reveals an evolution of the hydrogen column density suggestive of dense filaments of gas in the downstream wake of the pulsar, with higher levels of absorption seen at orbital phases 0.5-1.0, as well as a variable Fe Kα line.