Discovery and analysis of a ULX nebula in NGC 3521
  We present Very Large Telescope/X-shooter and Chandra X-ray observatory/ACIS
observations of the ULX [SST2011] J110545.62+000016.2 in the galaxy NGC 3521.
The source identified as a candidate near-infrared counterpart to the ULX in
our previous study shows an emission line spectrum of numerous recombination
and forbidden lines in the visible and near-infrared spectral regime. The
emission from the candidate counterpart is spatially extended ($\sim$ 34 pc)
and appears to be connected with an adjacent H II region, located $\sim$ 138 pc
to the NE. The measured velocities of the emission lines confirm that both the
candidate counterpart and H II region reside in NGC 3521. The intensity ratios
of the emission lines from the ULX counterpart show that the line emission
originates from the combined effect of shock and photoionisation of low
metallicity (12 + log (O/H) = 8.19 $\pm$ 0.11) gas. Unfortunately, there is no
identifiable spectral signature directly related to the photosphere of the
mass-donor star in our spectrum. From the archival Chandra data, we derive the
X-ray luminosity of the source in the 0.3-7 keV range to be (1.9 $\pm$ 0.8)
$\times$ 10$^{40}$ erg cm$^{-2}$ s$^{-1}$, almost a factor of four higher than
what is previously reported.
