2007ApJ...668..124A

Optical photometric and spectroscopic data are presented that show an association of an ultraluminous X-ray source in NGC 7331 with a young star cluster of mass M=(1.1±0.2)x10⁵ M_☉ and age t_c=4.25±0.25 Myr. If the ULX is part of the bright stellar cluster, then the mass of the progenitor of the compact accretor must have been ≳40-50 M_☉ in order to already have evolved through the supernova stage into a compact object. The companion star is also probably an evolved massive star. The emission-line spectrum of the nebula surrounding the cluster can be interpreted as being a result of photoionization by the cluster OB stars with an additional source of shock excitation producing strong [S II], [O I], and N II lines. This additional source appears to be as much as 5 times more powerful than the supernovae and stellar winds in the cluster can provide for. Additional mechanical energy input associated with the ULX itself can help explain the residual shock-excited line luminosities of the emission region.