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-temperaturecorrected accretion disc around a black hole, with a central Comptonising corona. We find that the ULXs require reprocessing fractions of ∼ 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.

Thermal stability of winds driven by radiation pressure in super-Eddington accretion discs

“…In ULXs, of course, the scenario is complex as the bulk of the UV (and possibly the optical) emission may come from the outer disc itself but some contamination can be expected from the companion star (e.g., Grisé et al 2012;Ambrosi & Zampieri 2018). Sutton et al (2014) have shown that most objects at ∼ 10 39 erg/s exhibit disc reprocessing fractions similar to sub-Eddington objects, but at higher luminosities X-ray reprocessing seems to increase, possibly due to scattering of wind X-rays back to the outer disc.…”

A Sparkler in the Fireworks Galaxy: Discovery of an Ultraluminous X-Ray Transient with a Strong Oxygen Line in NGC 6946

“…For example, in the case of GRS 1915+105, the estimated disk size is ∼10 12 cm and the mass in the disk is ∼10 28 g, which suffices to keep the X-ray source at or above 10 39 erg s −1 for decades (Done et al 2004). As for the other requirement (i.e., a strong irradiation of the outer disk), broadband spectral modelling of observational data suggests (Sutton et al 2014) reprocessing fractions ∼10 −3 for sub-Eddington stellar-mass X-ray binaries and some ULXs, and ∼10 −2 for other ULXs, specifically those with soft X-ray spectra. The reason for the enhanced irradiation factor in some ULXs may be that their strong disk wind intercepts and scatters a fraction of photons emitted in the polar funnel and re-directs them onto the outer disk (Sutton et al 2014).…”

“…As for the other requirement (i.e., a strong irradiation of the outer disk), broadband spectral modelling of observational data suggests (Sutton et al 2014) reprocessing fractions ∼10 −3 for sub-Eddington stellar-mass X-ray binaries and some ULXs, and ∼10 −2 for other ULXs, specifically those with soft X-ray spectra. The reason for the enhanced irradiation factor in some ULXs may be that their strong disk wind intercepts and scatters a fraction of photons emitted in the polar funnel and re-directs them onto the outer disk (Sutton et al 2014). This is not the only viable explanation for transient behaviour in ULXs.…”

Spitzer Irac Observations of Ir Excess in Holmberg Ix X-1: A Circumbinary Disk or a Variable Jet?

“…Typical values of 2%-4% have been found for previous fits of ULX X-ray/optical data sets with the DISKIR model (Berghea & Dudik 2012;Grisé et al 2012;Tao et al 2012). Sutton et al (2014) used an improved model for irratiated disks (one that accounts for the color-temperature correction) to fit a sample of ULXs with disk-like spectra. In this study, Sutton et al (2014) compared the new DISKIR model results with a more traditional DISKIR model fit.…”