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

We present here the 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 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 Lyα 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, such as gas diffused into the Alfvén sphere or an accretion curtain flowing from the inner accretion disk to the magnetic poles.