(No) dynamical constraints on the mass of the black hole in two ULXs

TL;DR

This study uses optical spectroscopy to investigate the black hole mass in two ULXs, finding variable broad emission lines that suggest a face-on accretion disc but no definitive mass constraints.

Contribution

First optical spectroscopic campaigns targeting ULX counterparts, revealing variable broad emission lines and providing insights into disc orientation and black hole properties.

Findings

Radial velocity variations observed but not sinusoidal.

Broad He II line highly variable on short timescales.

Disc likely close to face-on based on line breadth.

Abstract

We present the preliminary results of two Gemini campaigns to constrain the mass of the black hole in an ultraluminous X-ray source (ULX) via optical spectroscopy. Pilot studies of the optical counterparts of a number of ULXs revealed two candidates for further detailed study, based on the presence of a broad He II 4686 Angstrom emission line. A sequence of 10 long-slit spectra were obtained for each object, and the velocity shift of the ULX counterpart measured. Although radial velocity variations are observed, they are not sinusoidal, and no mass function is obtained. However, the broad He II line is highly variable on timescales shorter than a day. If associated with the reprocessing of X-rays in the accretion disc, its breadth implies that the disc must be close to face-on.