Optical Spectroscopy of the environment of a ULX in NGC 7331

TL;DR

This study links an ultraluminous X-ray source in NGC 7331 to a young star cluster, revealing insights into the progenitor star's mass, the surrounding nebula's excitation mechanisms, and the ULX's energetic impact.

Contribution

It provides the first optical spectroscopic analysis connecting the ULX to a specific young star cluster and explores the nebula's shock excitation sources.

Findings

ULX associated with a 4.25 Myr old star cluster

Progenitor star mass estimated >40-50 solar masses

Additional energy from ULX influences nebula shock excitation

Abstract

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 1.1e5 solar masses and age 4.25 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 about 40-50 solar masses in order to already have evolved through the supernova stage to a compact object. The companion star is also likely an evolved massive star. The emission line spectrum of the nebula surrounding the cluster can be interpreted as a result of photoionization by the cluster OB stars with an additional source of shock excitation producing strong [SII], [OI] and NII lines. This additional source appears to be as much as five times more powerful than the supernovae and stellar winds in the cluster can provide. Additional mechanical energy input associated with the ULX itself can help explain the residual shock excited line luminosities of the emission region.