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.

Abstract

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.

Citation

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. https://doi.org/10.1088/0004-637x/760/2/135

• 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
• DOI: https://doi.org/10.1088/0004-637x/760/2/135
• Keywords: Globular clusters, X-rays, Binaries, Galaxies.