New Constraints on the Geometry and Kinematics of Matter Surrounding the Accretion Flow in X-ray Binaries from $Chandra$ HETG X-ray Spectroscopy

Abstract

The narrow, neutral Fe Kα fluorescence emission line in X-ray binaries (XRBs) is a powerful probe of the geometry, kinematics and Fe abundance of matter around the accretion flow. In a recent study it has been claimed, using $Chandra$ High-Energy Transmission Grating (HETG) spectra for a sample of XRBs, that the circumnuclear material is consistent with a solar-abundance, uniform, spherical distribution. It was also claimed that the Fe Kα line was unresolved in all cases by the HETG. However, these conclusions were based on ad hoc models that did not attempt to relate the global column density to the Fe Kα line emission. We revisit the sample and test a self-consistent model of a uniform, spherical X-ray reprocessor against HETG spectra from 56 observations of 14 Galactic XRBs. We find that the model is ruled out in 13/14 sources because a variable Fe abundance is required. In 2 sources a spherical distribution is viable but with non-solar Fe abundance. We also applied a solar-abundance Compton-thick reflection model, which can account for the spectra that are inconsistent with a spherical model, but spectra with a broader bandpass are required to better constrain model parameters. We also robustly measured the velocity width of the Fe Kα line and found full width half maximum values up to ~5000 km s${}^{-1}$. Only in some spectra was the Fe Kα line unresolved by the HETG.

Accepted by ApJS; 47 pages (introducing open source likeapj.cls, likeapj.bst AAS-style files). Main text: 22 pages, 6 tables, 2 figures. Appendix A: ~12 pages, detailed results and discussion. Appendix B: ~10 pages, detailed spectral plots. Appendix C: ~2 pages, contour plots. Some figures somewhat degraded to comply with ArXiv size limits

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