The Orbit and Position of the X - ray Pulsar XTE J 1855 - 026 - an Eclipsing Supergiant System Robin

A pulse timing orbit has been obtained for the X-ray binary XTEJ1855-026 using observations made with the Proportional Counter Array on board the Rossi X-ray Timing Explorer. The mass function obtained of ∼16M⊙ together with the detection of an extended near-total eclipse confirm that the primary star is a supergiant as predicted. The orbital eccentricity is found to be very low with a best fit value of 0.04 ± 0.02. The orbital period is also refined to be 6.0724 ± 0.0009 days using an improved and extended light curve obtained with RXTE’s All Sky Monitor. Observations with the ASCA satellite provide an improved source location of R.A. = 18 55 31.3, decl. = -02 36 24.0 (2000) with an estimated systematic uncertainty of less than 12. A serendipitous new source, AX J1855.4-0232, was also discovered during the ASCA observations. Subject headings: stars: individual (XTEJ1855-026) — stars: neutron — X-rays: stars