High‐Resolution X‐Ray Spectrum of the Accreting Binary X‐Ray Pulsar GX 1+4

Abstract

We present here high resolution X-ray spectrum of the accreting binary X-ray pulsar GX 1+4 obtained with the High Energy Transmission Grating (HETG) instrument of the Chandra X-ray Observatory. This was supplemented by a simultaneous observation with the Proportional Counter Array (PCA) of the Rossi X-ray Timing Explorer (RXTE). During this observation, the source was in a somewhat low intensity state and the pulse profile with both Chandra and RXTE shows a narrow dip, characteristic of GX 1+4 in medium and low intensity states. The continuum X-ray spectrum obtained with the HETG and PCA can be fitted well with a high energy cutoff power-law model with line of sight absorption. Interestingly, we find that this low state is accompanied by a relatively small absorption column density. A 6.4 keV narrow emission line with an equivalent width of 70 eV is clearly detected in the HETG spectrum. The fluorescence iron line, or at least part of it is produced in the neutral or lowly ionized iron in the circumstellar material that also causes most of the line of sight absorption. In the HETG spectrum, we have found evidence for a weak (equivalent width ~30 eV) emission line at 6.95 keV. This line is identified as Ly_alpha emission line from hydrogen-like iron and the spectrum does not show emission lines from helium-like iron. We discuss various emission regions for the hydrogen-like iron emission line, like gas diffused into the Alfven sphere or an accretion curtain flowing from the inner accretion disk to the magnetic poles.