A variable ultraluminous X-ray source in a globular cluster in NGC 4649
Abstract
We report the discovery of a new ultraluminous X-ray source (ULX) associated with a globular cluster in the elliptical galaxy NGC 4649. The X-ray source was initially detected with a luminosity below 5 x 10^38 erg/s, but in subsequent observations 7 and 11 years later it had brightened substantially to 2 - 3 x 10^39 erg/s. 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
Cite
@article{arxiv.1210.5163,
title = {A variable ultraluminous X-ray source in a globular cluster in NGC 4649},
author = {T. P. Roberts and G. Fabbiano and B. Luo and D. -W. Kim and J. Strader and M. J. Middleton and J. P. Brodie and T. Fragos and J. S. Gallagher and V. Kalogera and A. R. King and A. Zezas},
journal= {arXiv preprint arXiv:1210.5163},
year = {2015}
}
Comments
7 pages, 4 figures, accepted for publication in ApJ