2011MNRAS.418L.124V

We investigate the spatial coincidence of ultraluminous X-ray sources (ULXs) with young massive stellar clusters. In particular, we perform astrometry on Chandra and Hubble Space Telescope (HST) data of two ULXs that are possibly associated with such clusters.

To date, M82 X-1 is the only ULX claimed to be coincident with a young massive stellar cluster. We remeasure the position of this source with a high accuracy and find that the position of the X-ray source is 0.65 arcsec away from the stellar cluster, corresponding to an offset significance of 3σ.

We also report the discovery of a new candidate, based on observations of NGC 7479. One of the ULXs observed in three X-ray observations is found to be spatially coincident (within 1σ of the position error) with a young supercluster observed in the HST images. In the brightest state, the absorbed luminosity of the ULX is a few times 10⁴⁰ erg/s, and in the faintest state below the detection limit of ∼ 4 times 10³⁹ erg/s. The luminosity in the brightest state requires an accreting black hole mass of at least 100 M_☉ assuming isotropic emission. However, it is possible that the source is contaminated by X-ray emission from the nearby supernova SN2009jf. In this case, the luminosity of the ULX is in a range where it is strongly debated whether it is a super-Eddington stellar mass black hole or an intermediate-mass black hole. The colours of the host cluster indicate a young stellar population, with an age between 10 and 100 Myr. The total stellar mass of the cluster is ∼ 5 {x} 10⁵ M_☉.