Optical spectroscopy of the environment of a ULX in NGC 7331

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); 10(5) M-circle dot 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 greater than or similar to 40Y50M(circle dot) 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.