The luminosity dependence of the X-ray spectrum of the transient 42 second pulsar EXO 2030 + 375

We report on the evolution in the X-ray spectrum of the transient X-ray pulsar EXO 2030 + 375 during part of an outburst in 1985 May-August. The overall continuum spectral shape is similar to that of other accreting 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 the luminosity decreased by a factor of about 100, the X-ray spectrum became harder with the photon index decreasing from 1.83 +/- 0.01 to 1.29 +/- 0.01. In addition, the high-energy cutoff energy decreased from 20 to 10 keV during the same interval. If the cutoff is interpreted in terms of cyclotron resonance scattering, then this variation implies a magnetic field strength that decreased from 2.6 x 10 exp 12 to 1.3 x 10 exp 12. This variation implies that the cutoff energy does not provide a reliable measure of the surface magnetic field strength in this system.