The Luminosity Dependence of the Pulse Period and Profile of the Transient X-ray Pulsar EXO 2030+375

Abstract

Using the EXOSAT Observatory we have discovered a new, transient, X-ray pulsar, EXO 2030+375 and followed its evolution through two outbursts. During the first outburst the 1-20 keV intensity declined by a factor ≳ 2500 from a discovery value comparable to that of the Crab Nebula. Cyclic variations in the 42 s pulse period give a likely orbital period in the range 44.3-48.6 day and an eccentricity of ~ 0.4. We have measured, for the first time, the dependence of the pulse period time derivative, Ps, on luminosity, L, for an individual pulsar. At high and intermediate luminosities we find that -P s ∝ L 1.08-1.35. This observed dependence is significantly steeper than that predicted by simple accretion torque theory where -P s ∝ L0.86. At low luminosites the determination of this relation is complicated by the interplay between secular and orbital period changes and we await an independent determination of the orbital elements that will allow these effects to be separated.