Optical and infrared signatures of ultra-luminous X-ray sources

• Published in: Monthly notices of the Royal Astronomical Society Vol. 362; no. 1; pp. 79 - 88
• Main Authors: Copperwheat, Christopher, Cropper, Mark, Soria, Roberto, Wu, Kinwah
• Format: Journal Article
• Language: English
• Published: 23 Ainslie Place , Edinburgh EH3 6AJ , UK . Telephone 226 7232 Fax 226 3803 Blackwell Science Ltd 01.09.2005; Blackwell Science; Oxford University Press
• Subjects: accretion; accretion discs; Accretion disks; accretion, accretion discs; Astrophysics; black hole physics; Black holes; Emissions; Infrared radiation; Luminosity; Stars & galaxies; X-rays; X-rays: galaxies; X-rays: stars; X-rays: stars; X-ray galaxies; Black holes; X-rays: galaxies; Cosmic x-ray sources; Luminosity; Spectral type; Optical counterpart; Accretion disks; Time variation; accretion, accretion discs; Light emission; black hole physics; Cosmology; Models; Roche lobe; Radiative equilibrium
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2005.09223.x

We have constructed a model to describe the optical emission from ultra-luminous X-ray sources (ULXs). We assume a binary model with a black hole accreting matter from a Roche lobe filling companion star. We consider the effects of radiative transport and radiative equilibrium in the irradiated surfaces of both the star and a thin accretion disc. We have developed this model as a tool with which to positively identify the optical counterparts of ULXs, and subsequently derive parameters such as the black hole mass and the luminosity class and spectral type of the counterpart. We examine the dependence of the optical emission on these and other variables. We extend our model to examine the magnitude variation at infrared wavelengths, and we find that observations at these wavelengths may have more diagnostic power than in the optical. We apply our model to existing HST observations of the candidates for the optical counterpart of ULX X-7 in NGC 4559. All candidates could be consistent with an irradiated star alone, but we find that a number of them are too faint to fit with an irradiated star and disc together. Were one of these the optical counterpart to X-7, it would display a significant temporal variation.