On the association of ULXs with young superclusters: M82 X-1 and a new candidate in NGC 7479

We investigate the spatial coincidence of ultra-luminous X-ray sources (ULXs) with young massive stellar clusters. In particular we perform astrometry on Chandra and 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 3 sigma. 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 1 sigma of the position error) with a young super-cluster observed in the HST images. In the brightest state, the absorbed luminosity of the ULX is a few times $10^{40}$ erg s$^{-1}$, and in the faintest state below the detection limit of $\sim4$ times $10^{39}$ erg s$^{-1}$. The luminosity in the brightest state requires an accreting black hole mass of at least 100 M$_{\odot}$ 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 $\sim5\cdot10^{5}$M$_{\odot}$.