Compact Optical Counterparts of Ultraluminous X-ray Sources

TL;DR

This study analyzes the optical counterparts of 13 ultraluminous X-ray sources using HST data, revealing their variability, spectral properties, and possible emission mechanisms, with implications for understanding their nature and accretion processes.

Contribution

First comprehensive multiband optical analysis of ULX counterparts, identifying emission mechanisms and spectral characteristics across multiple sources.

Findings

Optical emission mostly dominated by X-ray reprocessing on the disk.

Optical spectra generally follow a power-law with spectral index 1.0 to 2.0.

Variability detected on timescales from days to years.

Abstract

Using archival Hubble Space Telescope (HST) imaging data, we report the multiband photometric properties of 13 ultraluminous X-ray sources (ULXs) that have a unique compact optical counterpart. Both magnitude and color variation are detected at time scales of days to years. The optical color, variability, and X-ray to optical flux ratio indicate that the optical emission of most ULXs is dominated by X-ray reprocessing on the disk, similar to that of low mass X-ray binaries. For most sources, the optical spectrum is a power-law, $F_{\nu} \propto \nu^{\alpha}$ with $\alpha$ in the range 1.0 to 2.0 and the optically emitting region has a size on the order of 1e12 cm. Exceptions are NGC 2403 X-1 and M83 IXO 82, which show optical spectra consistent with direct emission from a standard thin disk, M101 ULX-1 and M81 ULS1, which have X-ray to optical flux ratios more similar to high-mass X-ray binaries, and IC 342 X-1, in which the optical light may be dominated by the companion star. Inconsistent extinction between the optical counterpart of NGC 5204 X-1 and the nearby optical nebulae suggests that they may be unrelated.