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A variable ultraluminous X-ray source in a globular cluster in NGC 4649

Roberts, T.P.; Fabbiano, G.; Luo, B.; Kim, D.-W.; Strader, J.; Middleton, M.J.; Brodie, J.P.; Fragos, T.; Gallagher, J.S.; Kalogera, V.; King, A.R.; Zezas, A.

A variable ultraluminous X-ray source in a globular cluster in NGC 4649 Thumbnail


G. Fabbiano

B. Luo

D.-W. Kim

J. Strader

M.J. Middleton

J.P. Brodie

T. Fragos

J.S. Gallagher

V. Kalogera

A.R. King

A. Zezas


We report the discovery of a new ultraluminous X-ray source associated with a globular cluster in the elliptical galaxy NGC 4649. The X-ray source was initially detected with a luminosity below 5 × 1038 erg s–1, but in subsequent observations 7 and 11 years later it had brightened substantially to 2-3 × 1039 erg s–1. Over the course of six separate observations it displayed significant spectral variability, in both continuum slope and absorption column. Short-term variability in the X-ray flux was also present in at least one observation. The properties of this object appear consistent with a stellar-mass black hole accreting at super-Eddington rates (i.e., in the ultraluminous accretion state), although a highly super-Eddington neutron star cannot be excluded. The coincidence of an increase in absorption column with a possible enhancement in short-term variability in at least one observation is suggestive of a clumpy, radiatively driven wind crossing our line of sight to the object.


Roberts, T., Fabbiano, G., Luo, B., Kim, D., Strader, J., Middleton, M., …Zezas, A. (2012). A variable ultraluminous X-ray source in a globular cluster in NGC 4649. Astrophysical Journal, 760(2), Article 135.

Journal Article Type Article
Publication Date Dec 1, 2012
Deposit Date Mar 26, 2013
Publicly Available Date Feb 12, 2014
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 760
Issue 2
Article Number 135
Keywords Globular clusters, X-rays, Binaries, Galaxies.


Published Journal Article (622 Kb)

Copyright Statement
© 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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