An ASCA observation of the x-ray binary GX 301-2

The X-ray binary pulsar GX 301-2 was observed with the Advanced Satellite for Cosmology and Astrophysics (ASCA) on 1994 February 13-14, at the 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 observed 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 the 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 average 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 equivalent width is 228 +/- 18 eV. The partial covering model does not describe the low-energy excess adequately. It underestimates the continuum between 2.0 and 5.0 keV but overestimates it below 1.5 keV. This, together with the absence of pulsations in the low-energy excess, rules out the partial covering model as a good description of the continuum spectrum. In addition, the spectra from the solid-state imaging spectrometers (SIS) aboard ASCA reveal two new features. First, it is seen that the fit with the scattering model is not acceptable below 2 keV and suggests an additional component as cause for this residual soft excess. 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 ultrasoft 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 ruled 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 identified with the fluorescent line emission from neutral or near-neutral Ne, Si, S, Ar, and Ca, respectively. This implies that the line emission is coming from cold (less than or equal to 10(5) K) gas. The pulse phase spectroscopy shows that the photon index, the iron line intensity, and the equivalent width vary with pulse phase. The large equivalent width variations are, however, mainly due to the changes in the underlying continuum intensity.