A Chandra survey of fluorescence Fe lines in X-ray binaries at high resolution

In this paper we present a comprehensive survey of 41 X-ray binaries (10 HMXBs and 31 LMXBs) with Chandra with specific emphasis on the Fe K region and the narrow Fe Ka line, at the highest resolution possible. We find that: a) The Fe Ka line is always centered at λ = 1.9387 ± 0.0016 Å, compatible with Fe I up to Fe VIII; we detect no shifts to higher ionization states nor any difference between HMXBs and LMXBs. b) 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. ic) Fe Kα fluorescence is present in all the HMXB in the survey. In contrast, such emissions are astonishingly rare (∼10%) among low mass X-ray binaries (LMXB). d) The lack of Fe line emission is always accompanied by the lack of any detectable K edge. e) 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. EWKα vs NH, and show that it is consistent with a reprocessing region spherically distributed around the compact object. f) We show that fluorescence in X-ray binaries 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 and as a possible cause of the lack of narrow Fe line emission in LMXBs.