Rossi X-Ray Timing Explorer Observations of the X-Ray Pulsar XTE J1855–026: A Possible New Supergiant System

A new X-ray source, XTE J1855-026, was discovered during Rossi X-Ray Timing Explorer (RXTE) scans along the Galactic plane. The source shows pulsations at a period of 361 s and also modulation at a period of 6.1 days, which we interpret as the orbital period of the system. The X-ray spectrum above ~3 keV can be fitted with an absorbed power-law model with a high-energy cutoff and an iron emission line at approximately 6.4 keV. We interpret these results as indicating that XTE J1855-026 is likely to consist of a neutron star accreting from the wind of an O or B supergiant primary. A less likely interpretation is that XTE J1855-026 is instead a Be/neutron star binary, in which case it would have the shortest known orbital period for such a system.