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The ultraluminous X-ray source population of NGC 4485/4490

Gladstone, J.C.; Roberts, T.P.

The ultraluminous X-ray source population of NGC 4485/4490 Thumbnail


J.C. Gladstone


We report the results of spectral and temporal variability studies of the ultraluminous X-ray sources (ULXs) contained within the interacting pair of galaxies NGC 4485/4490, combining Chandra and XMM–Newton observations. Each of the four separate observations provide at least modest quality spectra and light curves for each of the six previously identified ULXs in this system; we also note the presence of a new transient ULX in the most recent observation. No short-term variability was observed for any ULX within our sample, but three out of five sources show correlated flux/spectral changes over longer time-scales, with two others remaining stable in spectrum and luminosity over a period of at least 5 yr. We model the spectra with simple power-law and multicolour disc blackbody models. Although the data are insufficient to statistically distinguish models in each epoch, those better modelled (in terms of their χ2 fit) by a multicolour disc blackbody appear to show a disc-like correlation between luminosity and temperature, whereas those modelled by a power-law veer sharply away from such a relationship. The ULXs with possible correlated flux/spectral changes appear to change spectral form at ∼2 × 1039 erg s−1, suggestive of a possible change in spectral state at high luminosities. If this transition is occurring between the very high state and a super-Eddington ultraluminous state, it indicates that the mass of the black holes in these ULXs is around 10–15 M⊙.


Gladstone, J., & Roberts, T. (2009). The ultraluminous X-ray source population of NGC 4485/4490. Monthly Notices of the Royal Astronomical Society, 397(1), 124-134.

Journal Article Type Article
Publication Date Jul 21, 2009
Deposit Date Feb 20, 2014
Publicly Available Date Feb 21, 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 397
Issue 1
Pages 124-134
Keywords Black hole physics, Galaxies, Individual, NGC 4485, NGC 4490, X-rays, Binaries.


Published Journal Article (3.4 Mb)

Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2009 The Authors. Journal compilation © 2009 RAS. Published by Oxford University Press on behalf of The Royal Astronomical Society. All rights reserved.

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