Accretion torques in the transient X-ray pulsar EXO2030+375.

Abstract

In 1985 May - October, EXOSAT observed two outbursts from the previously unknown accreting X-ray pulsar EXO2030+375. Using the orbital solution derived by the BATSE experiment on CGRO, the behavior of the pulse period variations can now be studied with much less ambiguity than was possible based on the EXOSAT observations alone. There is little evidence for spin-down at low luminosity during the first outburst. Three different accretion torque models are applied to the intrinsic pulse-period data, but none give formally acceptable fits. However models for the pulse period variations which allow the possibility of spin-down (magnetically-threaded disk models) provide a better description of the observed variations than the simplest accretion torque model. We confirm that the dependence of pulse period with observed X-ray luminosity is significantly steeper than that predicted by simple accretion torque theory, with -˙(P)_pulse_ {prop.to}L_X_^1.2^ rather than -˙(P)_pulse_{prop.to}L_X_^6/7^ as expected. Distance estimates using the Wang and Ghosh & Lamb models range from 4.2-5.2kpc respectively, while the magnetic dipole moment is 10-11x10^30^Gcm^3^. The distance is consistent with that derived from photometric studies of the companion star, and the magnetic moment is consistent with both the luminosity at which centrifugal inhibition of accretion would terminate the outburst, and the non-detection of a cyclotron feature in the X-ray spectrum.