ACHANDRASURVEY OF FLUORESCENCE Fe LINES IN X-RAY BINARIES AT HIGH RESOLUTION

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 K line, at the highest resolution possible. We find that (1) the Fe K line is always centered at = 1.9387 0.0016 , 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). ( (3) Fe K 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. ( ( 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. ( Specifically, we present the first evidence of a Compton shoulder in the HMXB X1908+075. Also, the Fe K lines of 4U1700-37 and LMC X-4 present asymmetric wings, suggesting the presence of highly structured stellar winds in these systems.