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

T. P. Roberts*, 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

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

19 Scopus citations


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.

Original languageEnglish (US)
Article number135
JournalAstrophysical Journal
Issue number2
StatePublished - Dec 1 2012


  • X-rays: binaries
  • X-rays: galaxies
  • globular clusters: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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