The Orbit of the Binary X-Ray Pulsar 4U 1538–52 from Rossi X-Ray Timing Explorer Observations

Improved orbital parameters of the high-mass binary X-ray pulsar 4U 1538-52 have been derived from high count rate data obtained from the Rossi X-Ray Timing Explorer. Pulse-timing analysis yields an eccentricity of 0.174 ± 0.015, a periastron at 64° ± 9°, and evidence of orbital decay with orb/Porb = (-2.9 ± 2.1) × 10-6 yr-1.