Irradiated accretion discs in ultraluminous X-ray sources
Abstract
Observations of optical counterparts to ultraluminous X-ray sources (ULXs) could provide critical clues to the physical nature of these extraordinary objects. Recently attempts have been made to constrain the stellar types of ULX counterparts, however the detection of optical variability suggests that they may actually be dominated by reprocessed X-ray emission. Here, we report on a combined XMM-Newton and HST study of ULXs with broadened disc-like X-ray spectra. We fit the data with a new spectral model of emission from an irradiated, colour-temperature-corrected accretion disc with a Comptonising corona, finding reprocessing fractions similar to the high/soft state black hole binaries (BHBs), but less than has been reported for some ULXs with soft, wind-dominated X-ray spectra. We suggest that X-ray emission may be scattered on to the outer disc by material in a natal super-Eddington wind, then the higher reprocessing fractions in some ULXs may be associated with stronger winds. Alternatively, these disc-like ULXs may be similar in geometry to the high/soft state BHBs.
- Publication:
-
The X-ray Universe 2014
- Pub Date:
- July 2014
- Bibcode:
- 2014xru..confE.193S