AN ASCA OBSERVATION OF THE X-RAY BINARY GX-301-2

The X-ray binary pulsar GX 301-2 was observed with the Advanced Satell ite for Cosmology and Astrophysics (ASCA) on 1994 February 13-14, at t he binary orbital phase between 0.277 and 0.308. The source was found to be in a low-intensity phase, with an intrinsic luminosity L(X) = 6. 6 x 10(35) ergs s(-1) in the 0.7-10.0 keV energy band. This is roughly 2 orders of magnitude lower than the peak luminosity generally observ ed at periastron. The spectral analysis shows the presence of a strong low-energy excess below about 4 keV and high photoelectric absorption (similar to 6 x 10(23) cm(-2)). Part of the soft excess may be caused by the scattering of X-rays around a region of increased density by t he less dense ambient stellar wind, which would explain the absence of pulsations in the low-energy excess. The spectrum also shows a strong , narrow fluorescent iron line and an absorption iron K edge. The aver age energy of the line is 6.400 +/- 0.008 keV and that of the edge is 7.183 +/- 0.013 keV (at the 90% confidence level). The iron line equiv alent width is 228 +/- 18 eV. The partial covering model does not desc ribe the low-energy excess adequately. It underestimates the continuum between 2.0 and 5.0 keV but overestimates it below 1.5 keV. This, tog ether with the absence of pulsations in the low-energy excess, rules o ut the partial covering model as a good description of the continuum s pectrum. In addition, the spectra from the solid-state imaging spectro meters (SIS) aboard ASCA reveal two new features. First, it is seen th at the fit with the scattering model is not acceptable below 2 keV and suggests an additional component as cause for this residual soft exce ss. The ultrasoft component has been observed in the spectra of GX 301 -2 for the first time and can be fitted well by thermal emission from a hot plasma with a temperature kT of similar to 0.8 keV. The ultrasof t component may arise because of emission from X-ray-induced shocks in the gas trailing the neutron star. A partial ionization model with a warm absorber in series with a neutral absorber, including scattering, does not account for the ultrasoft excess below 1.5 keV and can be ru led out. Second, it is also seen that the SIS spectrum is dominated by line features at 0.80, 1.73, 2.30, 2.98, and 3.68 keV. These can be i dentified with the fluorescent line emission from neutral or near-neut ral Ne, Si, S, Ar, and Ca, respectively. This implies that the line em ission is coming from cold (less than or equal to 10(5) K) gas. The pu lse phase spectroscopy shows that the photon index, the iron line inte nsity, and the equivalent width vary with pulse phase. The large equiv alent width variations are, however, mainly due to the changes in the underlying continuum intensity.