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

TL;DR

This paper reports the discovery of a variable ultraluminous X-ray source in a globular cluster of NGC 4649, showing significant luminosity and spectral variability indicative of super-Eddington accretion onto a stellar-mass black hole or neutron star.

Contribution

It presents the first detailed observation of a ULX in a globular cluster with evidence of variability and spectral changes, suggesting super-Eddington accretion in such environments.

Findings

Luminosity increased from below 5 x 10^38 erg/s to 2-3 x 10^39 erg/s over years.

Significant spectral variability and short-term flux variability observed.

Possible evidence of a clumpy wind crossing the line-of-sight.

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