Orbital phase spectroscopy of GX 301–2 with RXTE-PCA

Department of Astronomy & Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai–400 005, India e-mail: uddipan;bpaul@tifr.res.in

Received: 26 September 2003
Accepted: 29 June 2004

Abstract

We have investigated the orbital phase dependence of the X-ray spectrum of the High Mass X-ray Binary pulsar GX 301–2. Here we present the results from a spectral analysis of two sets of observations of GX 301–2 with the Rossi X-ray Timing Explorer (RXTE). Of particular interest are the variations of the absorption column density and the iron line flux along with other parameters of the spectral model with the orbital phase. We found that the X-ray spectrum can almost always be fitted with a partial covering absorption model. We have detected enhanced absorption near the periastron. However, the column density variation with orbital phase is not smooth, as is expected in a smooth stellar wind model. We discuss the results of the column density variation in the light of the two proposed models for GX 301–2, an equatorial disk emanating from the companion star Wray 977 and a gas stream following the neutron star. We also found that the iron K and K line fluxes have peaks near the periastron and are well correlated with the continuum hard X-ray flux. The line equivalent width shows an interesting pattern with the column density, reasonably constant for low values of the column density and increasing rapidly beyond a certain value.