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$_{\odot}$.
