The local environment of ultraluminous X-ray sources viewed by Xmm-newton's optical monitor

Abstract

We have used XMM-Newton's Optical Monitor (OM) images to study the local environment of a sample of 27 ultraluminous X-ray sources (ULXs) in nearby galaxies. UVW1 fluxes were extracted from 100 pc regions centered on the ULX positions. We find that at least 4 ULXs (out of 10 published) have spectral types that are consistent with previous literature values. In addition, the colors are similar to those of young stars. For the highest-luminosity ULXs, the UVW1 fluxes may have an important contribution from the accretion disk. We find that the majority of ULXs are associated with recent star formation. Many of the ULXs in our sample are located inside young OB associations or star-forming regions (SFRs). Based on their colors, we estimated ages and masses for SFRs located within 1 kpc from the ULXs in our sample. The resolution of the OM was insufficient to detect young dense superclusters, but some of these SFRs are massive enough to contain such clusters. Only three ULXs have no associated SFRs younger than ∼50 Myr. The age and mass estimates for clusters were used to test runaway scenarios. The data are, in general, compatible with stellar-mass binaries accreting at super-Eddington rates and ejected by natal kicks. We also tested the hypothesis that ULXs are sub-Eddington accreting intermediate mass black holes ejected by three-body interactions; however, this is not supported well by the data. © 2013. The American Astronomical Society. All rights reserved.