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Soft Extragalactic X-Ray Binaries at the Eddington Threshold

Earnshaw, H.M.; Roberts, T.P.

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H.M. Earnshaw


The luminosity range at and just below the 1039 erg s−1 cut-off for defining ultraluminous X-ray sources (ULXs) is a little-explored regime. It none-the-less hosts a large number of X-ray sources, and has great potential for improving our understanding of sources with ∼Eddington accretion rates. We select a sample of four sources in this Eddington Threshold regime with good data for further study; these objects possess a variety of soft spectral shapes. We perform X-ray spectral and timing analysis on the XMM-Newton and Chandra data for these objects to gain insight into their accretion mechanisms, and also examine their optical counterparts using HST images. NGC 300 X-1 is a highly luminous and well-known example of the canonical steep power-law accretion state. M51 ULS exhibits a cool blackbody-like spectrum and is consistent with being an ultraluminous supersoft source (ULS), possibly a super-Eddington accreting object viewed at a high inclination through an optically thick outflowing wind. NGC 4395 ULX-1 and NGC 6946 ULX-1 have unusually steep power-law tails, for which we discuss a variety of possible physical mechanisms and links to similar features in Galactic microquasars, and we conclude that these sources are likely intermediate objects between the soft ultraluminous regime of ULXs and classic ULSs.


Earnshaw, H., & Roberts, T. (2017). Soft Extragalactic X-Ray Binaries at the Eddington Threshold. Monthly Notices of the Royal Astronomical Society, 467(3), 2690-2705.

Journal Article Type Article
Acceptance Date Feb 1, 2017
Online Publication Date Feb 6, 2017
Publication Date Jun 1, 2017
Deposit Date Feb 21, 2017
Publicly Available Date Feb 24, 2017
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 467
Issue 3
Pages 2690-2705
Related Public URLs


Accepted Journal Article (2.1 Mb)

Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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