The luminosity dependence of the pulse period and profile of the transient X-ray pulsar EXO 2030+375

Using the EXOSAT Observatory the authors 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. They have measured the dependence of the pulse period time derivative, P˙_s, on luminosity, L, for an individual pulsar. At high and intermediate luminosities they 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 ≡ L^0.86. At low luminosites the determination of this relation is complicated by the interplay between secular and orbital period changes. The authors observed marked luminosity dependent changes in pulse profile. During the second outburst, in 1985 October, the light curve was dominated by a series of flares that recurred quasi-periodically every 3.96 hr. The origin of these flares is unclear, but their discovery demonstrates that another clock can give rise to regular outbursts from OB star X-ray binary systems.