Irradiated, colour-temperature-corrected accretion discs in ultraluminous X-ray sources

Abstract

Although attempts have been made to constrain the stellar types of optical counterparts to ULXs, the detection of optical variability instead suggests that they may be dominated by reprocessed emission from X-rays which irradiate the outer accretion disc. Here, we report results from a combined X-ray and optical spectral study of a sample of ULXs, which were selected for having broadened disc-like X-ray spectra, and known optical counterparts. We simultaneously fit optical and X-ray data from ULXs with a new spectral model of emission from an irradiated, colour-temperature-corrected accretion disc around a black hole, with a central Comptonising corona. We find that the ULXs require reprocessing fractions of $\sim 10^{-3}$, which is similar to sub-Eddington thermal dominant state BHBs, but less than has been reported for ULXs with soft ultraluminous X-ray spectra. We suggest that the reprocessing fraction may be due to the opposing effects of self-shielding in a geometrically thick super-critical accretion disc, and reflection from far above the central black hole by optically thin material ejected in a natal super-Eddington wind. Then, the higher reprocessing fractions reported for ULXs with wind-dominated X-ray spectra may be due to enhanced scattering onto the outer disc via the stronger wind in these objects. Alternatively, the accretion discs in these ULXs may not be particularly geometrically thick, rather they may be similar in this regard to the thermal dominant state BHBs.

Accepted for publication in MNRAS; 16 pages, 4 figures, 5 tables

References in corpus (15)

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• XMM-Newton observations of the brightest Ultraluminous X-ray sources
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• New Hubble Space Telescope imaging of the counterparts to six ultraluminous X-ray sources
• Cross-Calibration of the XMM-Newton EPIC pn & MOS On-Axis Effective Areas Using 2XMM Sources
• XMM-Newton and INTEGRAL observations of the black hole candidate XTE J1817-330
• Evolution of the spectral curvature in the ULX Holmberg II X-1
• Broad Components in Optical Emission Lines from the Ultra-Luminous X-ray Source NGC 5408 X-1
• Detection of X-ray elongated emission from a ultraluminous X-ray source in the interacting pair of galaxies NGC 5953/5954

Cited by in corpus (12)

• Ultraluminous X-ray sources
• Nature of the soft ULX in NGC 247: super-Eddington outflow and transition between the supersoft and soft ultraluminous regimes
• Multiband counterparts of two eclipsing ultraluminous X-ray sources in M 51
• The ultraluminous X-ray source bubble in NGC 5585
• The slim-disk state of the ultraluminous X-ray source in M83
• Thermally driven winds in ULXs
• Crossing the Eddington limit: examining disk spectra at high accretion rates
• A multi-wavelength view of distinct accretion regimes in the pulsating ultraluminous X-ray source NGC 1313 X-2
• Spitzer IRAC observations of IR excess in Holmberg IX X-1: A circumbinary disk or a variable jet?
• Optical Counterparts of ULXs in NGC 1672
• X-ray reverberation models of the disc wind in ultraluminous X-ray source NGC 5408 X-1
• Comprehensive X-ray and Multiwavelength Study of ULXs in NGC 1566