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Published February 1, 2022 | Accepted Version + Published
Journal Article Open

A Spectroscopic Angle on Central Engine Size Scales in Accreting Neutron Stars

Abstract

Analyses of absorption from disk winds and atmospheres in accreting compact objects typically treat the central emitting regions in these systems as point sources relative to the absorber. This assumption breaks down if the absorbing gas is located within a few × 1000 GM/c², in which case a small component of the absorber's Keplerian motion contributes to the velocity width of absorption lines. Here, we demonstrate how this velocity-broadening effect can be used to constrain the sizes of central engines in accreting compact objects via a simple geometric relationship, and develop a method for modeling this effect. We apply this method to the Chandra/HETG spectra of three ultracompact and short-period neutron star X-ray binaries in which evidence of gravitationally redshifted absorption, owing to an inner-disk atmosphere, has recently been reported. The significance of the redshift is above 5σ for XTE J1710−281 (this work) and 4U 1916−053, and is inconsistent with various estimates of the relative radial velocity of each binary. For our most sensitive spectrum (XTE J1710−281), we obtain a 1σ upper bound of 310 km s⁻¹ on the magnitude of this geometric effect and a central engine of size R_(CE)

Additional Information

© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 June 7; revised 2021 November 3; accepted 2021 November 6; published 2022 January 31. The scientific results reported in this work are based on observations made by the Chandra X-ray Observatory and data obtained from the Chandra Data Archive. N.T. acknowledges helpful discussions with Kevin Whitley and Nuria Calvet. R.M.L. acknowledges the support of NASA through Hubble Fellowship Program grant HST-HF2-51440.001. We thank the anonymous referee for comments and suggestions that improved this work. N.T. acknowledges the support of the Horace H. Rackham School of Graduate Studies through the Rackham Predoctoral Fellowship.

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Published - Nicolas_Trueba_et_al_2022_ApJ_925_113.pdf

Accepted Version - 2111.04764.pdf

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Additional details

Created:
August 22, 2023
Modified:
October 23, 2023