The orbit and position of the X-ray pulsar XTE J1855-026: An eclipsing supergiant system

A pulse-timing orbit has been obtained for the X-ray binary XTE J1855-026 using observations made with the Proportional Counter Array on board the Rossi X-Ray Timing Explorer (RXTE). The mass function obtained of similar to16 M. 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 the RXTE All Sky Monitor. Observations with the ASCA satellite provide an improved source location of R.A. = 18(h)55(m)31(.)(s)3, decl. = 02degrees36'24."0 (J2000.0) with an estimated systematic uncertainty of less than 12". A serendipitous new source, AX J1855.4-0232, was also discovered during the ASCA observations.