Dense, multi-phase accretion disk atmosphere in the low-luminosity state of black hole transient V4641 Sgr

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ID: 319315
2026
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Abstract
Abstract We present soft X-ray spectroscopy of the black-hole X-ray binary V4641 Sgr with the XMM-Newton Reflection Grating Spectrometer (RGS). The RGS spectrum shows narrow emission features from N vi–vii and O vii–viii superimposed on a partially covered disk blackbody continuum. A blind Gaussian search confirms the presence of significant lines at the expected rest wavelengths. He-like triplet ratios (high G, low R) and full photoionization modelling both indicate a dense, photoionized plasma. Small redshifted velocities of ~540–720 km s−1 are suggested, which are consistent with quasi-static or slowly flowing gas away from the observer after accounting for systematics. Photoionization modelling requires two xstar components with an intermediate ionization parameter (log ξ ≃ 3.1) and a low ionization parameter (log ξ ≃ 0.36), respectively. The simultaneous EPIC-pn spectrum suggests highly ionized Fe emission structures, hinting at an additional, more highly ionized component. These results imply the existence of a radially extended, multiphase, and dense disk atmosphere in the source. We compare the source with other X-ray binaries showing similar emission lines. V4641 Sgr shares a similarly high inclination with other sources; however, the presence of low ionization emission lines distinguishes it from the rest.
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Authors Zuobin Zhang, Rob Fender, James Matthews, Jiachen Jiang, Honghui Liu, A. Ambrifi, T. Muñoz‐Darias, Maxime Parra, M. Shidatsu, Menglei Zhou, Yuexin Zhang, Abdurakhmon Nosirov, Cosimo Bambi, Justine Crook-Mansour
Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag1242
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