A CHANDRA SURVEY OF FLUORESCENCE Fe LINES IN X-RAY BINARIES AT HIGH RESOLUTION

Abstract

Fe K line fluorescence is commonly observed in the X-ray spectra of many X-ray binaries (XRBs) and represents a fundamental tool to investigate the material surrounding the X-ray source. In this paper, we present a comprehensive survey of 41 XRBs (10 HMXBs and 31 LMXBs) with Chandra with specific emphasis on the Fe K region and the narrow Fe Kα line, at the highest resolution possible. We find that (1) the Fe Kα line is always centered at λ = 1.9387 ± 0.0016 Å, compatible with Fe I up to Fe X; we detect no shifts to higher ionization states nor any difference between high mass X-ray binaries (HMXBs) and low mass X-ray binaries (LMXBs). (2) The line is very narrow, with FWHM ≤ 5 mÅ, normally not resolved by Chandra which means that the reprocessing material is not rotating at high speeds. (3) Fe Kα fluorescence is present in all the HMXBs in the survey. In contrast, such emissions are astonishingly rare (~10%) among LMXBs where only a few out of a large number showed Fe K fluorescence. However, the line and edge properties of these few are very similar to their high mass cousins. (4) The lack of Fe line emission is always accompanied by the lack of any detectable K edge. (5) We obtain the empirical curve of growth of the equivalent width of the Fe Kα line versus the density column of the reprocessing material, i.e., EW[subscript Kα] versus N [subscript H], and show that it is consistent with a reprocessing region spherically distributed around the compact object. (6) We show that fluorescence in XRBs follows the X-ray Baldwin effect as previously only found in the X-ray spectra of active galactic nuclei. We interpret this finding as evidence of decreasing neutral Fe abundance with increasing X-ray illumination and use it to explain some spectral states of Cyg X-1 as a possible cause of the lack of narrow Fe line emission in LMXBs. (7) Finally, we study anomalous morphologies such as Compton shoulders and asymmetric line profiles associated with the line fluorescence. Specifically, we present the first evidence of a Compton shoulder in the HMXB X1908+075. Also, the Fe Kα lines of 4U1700–37 and LMC X-4 present asymmetric wings, suggesting the presence of highly structured stellar winds in these systems.