ACCRETION TORQUES IN THE TRANSIENT-X-RAY PULSAR EXO-2030+375

In 1985 May - October, EXOSAT observed two outbursts from the previous ly unknown accreting X-ray pulsar EXO 2030+375. Using the orbital solu tion derived by the BATSE experiment on CGRO, the behavior of the puls e period variations can now be studied with much less ambiguity than w as possible based on the EXOSAT observations alone. There is little ev idence for spin-down at low luminosity during the first outburst. Thre e different accretion torque models are applied to the intrinsic pulse -period data, but none give formally acceptable its. However models fo r the pulse period variations which allow the possibility of spin-down (magnetically-threaded disk models) provide a better description of t he observed variations than the simplest accretion torque model. We co nfirm that the dependence of pulse period with observed X-ray luminosi ty is significantly steeper than that predicted by simple accretion to rque theory, with -P-pulse proportional to L(X)(1.2) rather than -P-pu lse proportional to L(X)(6/7) as expected. Distance estimates using th e Wang and Ghosh & Lamb models range from 4.2-5.2 kpc respectively, wh ile the magnetic dipole moment is 10-11 10(30) G cm(3). The distance i s consistent with that derived from photometric studies of the compani on star, and the magnetic moment is consistent with both the luminosit y at which centrifugal inhibition of accretion would terminate the out burst, and the non-detection of a cyclotron feature in the X-ray spect rum.