THE LUMINOSITY DEPENDENCE OF THE X-RAY-SPECTRUM OF THE TRANSIENT 42 2ND PULSAR EXO-2030+375

We report on the evolution in the X-ray spectrum of the transient X-ra y pulsar EXO 2030+375 during part of an outburst in 1985 May-August. T he overall continuum spectral shape is similar to that of other accret ing pulsars and can be represented by a power-law.spectrum modified at low energies by significant absorption and at high energies either by an exponential cutoff or by the effects of cyclotron scattering. As t he luminosity decreased by a factor of approximately 100, the X-ray sp ectrum became harder with the photon index decreasing from 1.83 +/- 0. 01 to 1.29 +/- 0.01. In addition, the high-energy cutoff energy decrea sed from 20 to 10 keV during the same interval. If the cutoff is inter preted in terms of cyclotron resonance scattering, then this variation implies a magnetic field strength that decreased from 2.6 x 10(12) G to 1.3 x 10(12) G. This variation implies that the cutoff energy does not provide a reliable measure of the surface magnetic field strength in this system.