New Hubble Space Telescope imaging of the counterparts to six ultraluminous X-ray sources

TL;DR

This study uses Hubble Space Telescope imaging to identify candidate optical counterparts to six ultraluminous X-ray sources, revealing diverse properties and potential for future dynamical mass measurements.

Contribution

First optical identifications of counterparts to six ULXs using HST imaging, providing candidates for future mass constraint studies.

Findings

Detected counterparts to four ULXs with magnitudes m ~ 22-26

Observed diverse colors and magnitudes among counterparts

Only one counterpart consistent with an OB star

Abstract

We report the results of new Hubble Space Telescope imaging of the positions of six ultraluminous X-ray sources (ULXs). Using images in three ACS filters we detect good candidate counterparts to four out of six ULXs, with one more possible detection, and observed magnitudes in the range m ~ 22 - 26 in the F606W filter. The extinction-corrected colours and absolute magnitudes vary from source to source, even after correcting for additional extinction in the host galaxy, and only one counterpart is readily explained as an OB star. Nevertheless, these counterparts are decent candidates for future follow-up in pursuit of dynamical mass constraints on the likely black holes powering these sources.