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Chandra observations of five ultraluminous X-ray sources in nearby galaxies

Roberts, T.P.; Warwick, R.S.; Ward, M.J.; Goad, M.R.

Chandra observations of five ultraluminous X-ray sources in nearby galaxies Thumbnail


R.S. Warwick

M.R. Goad


We report the results of a programme of dual-epoch Chandra ACIS-S observations of five ultraluminous X-ray sources (ULXs) in nearby spiral galaxies. All five ULXs are detected as unresolved, point-like X-ray sources by Chandra, though two have faded below the 10^39 erg/s luminosity threshold used to first designate these sources as ULXs. Using this same criterion, we detect three further ULXs within the imaged regions of the galaxies. The ULXs appear to be related to the star-forming regions of the galaxies, indicating that even in normal spiral galaxies the ULX population is predominantly associated with young stellar populations. A detailed study of the Chandra ACIS-S spectra of six of the ULXs shows that five are better described by a power-law continuum than a multicolour disc blackbody model, though there is evidence for additional very soft components to two of the power-law continua. The measured photon indices in four out of five cases are consistent with the low/hard state in black hole binaries, contrary to the suggestion that power-law-dominated spectra of ULXs originate in the very high state. A simple interpretation of this is that we are observing accretion on to intermediate-mass black holes, though we might also be observing a spectral state unique to very high mass accretion rates in stellar-mass black hole systems. Short-term flux variability is only detected in one of two epochs for two of the ULXs, with the lack of this characteristic arguing that the X-ray emission of this sample of ULXs is not dominated by relativistically beamed jets. The observational characteristics of this small sample suggest that ULXs are a distinctly heterogeneous source class.


Roberts, T., Warwick, R., Ward, M., & Goad, M. (2004). Chandra observations of five ultraluminous X-ray sources in nearby galaxies. Monthly Notices of the Royal Astronomical Society, 349(4), 1193-1210.

Journal Article Type Article
Publication Date 2004-04
Deposit Date Apr 16, 2008
Publicly Available Date Aug 11, 2014
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 349
Issue 4
Pages 1193-1210
Keywords Black hole physics, X-rays, Binaries, Galaxies.


Published Journal Article (1.7 Mb)

Copyright Statement
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society © 2004 The Authors Published on behalf of Royal Astronomical Society. All rights reserved.

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