Ultraviolet Spectra of ULX Systems

TL;DR

This study presents ultraviolet spectra of ULX optical counterparts, revealing accretion disks and companion star types, which enhances understanding of their nature and mass transfer mechanisms.

Contribution

First ultraviolet spectral analysis of ULX counterparts, identifying accretion disks and companion star characteristics, suggesting Roche Lobe overflow as the mass transfer process.

Findings

Presence of emission lines indicating accretion disks.

Spectral similarities to intermediate mass X-ray binaries.

Evidence of late-type Wolf-Rayet star in M81 ULX.

Abstract

To further understand the nature of the optical counterparts associated with Ultraluminous X-Ray sources (ULXs), we obtained far ultraviolet spectra of the reported counterparts to the ULXs in NGC 1313, Holmberg II, NGC 5204, and M81. The spectral resolution of the ACS prism spectra degrades from 300 at 1300 A to 40 at 1850 A, so longer wavelength features have lower S/N. The spectra of the ULXs in NGC 1313, Ho II, and NGC 5204 are quite similar, showing the N V 1240 A line at about the same equivalent width strength. The presence of this and other emission lines confirms the presence of an accretion disk, probably diluted by the light of an early B star companion. The spectra differ strongly from high mass X-ray binaries dominated by O star winds (e.g., Cyg X-1) and are most similar to intermediate mass X-ray binary systems in the Milky Way and LMC. This indicates that the mass transfer is due to Roche Lobe overflow. The spectrum of the ULX in M81 is quite weak but suggestive of a late-type Wolf-Rayet star.