Speaker
Description
Radiation-driven, optically thick outflows are predicted to launch in X-ray binaries or quasars when the accreting rates are super-Eddington. The optically thick outflows would thermalize the radiation to a blackbody-like emission with a cool temperature $kT_{\rm bb} <\sim0.4~ keV$. The bolometric luminosity of the soft thermal emission is also predicted to limit around one Eddington luminosity by supercritical wind models.Hence the luminosity of soft emission could be an indicator to the mass of the compact object. Soft emissions in excess of a single power-law spectra are commonly seen in ultraluminous X-ray sources (ULXs), which is considered to be from the supercritical winds. However, robust observational evidence is lacked. In this talk, I will show our study on the soft emission of 20 luminous ULXs, and the connection between the soft emission and the supercritical winds. We fit the high-quality XMM-Newton/EPIC spectra with a thermal blackbody component plus a physical Comptonized component. We find that the bolometric luminosities of the blackbody component ($L_{\rm bb}$) are constant among multiple epoch observations for some ULXs, such as NGC 1313 X-1 and M51 X-8, even through their total X-ray luminosities are significantly variable. More interesting, the $L_{\rm bb}$ distribution of the 20-ULX sample is bimodal, with $L_{\rm bb} <\sim 10^{39}~ {\rm erg~s}^{-1}$ for all the confirmed neutron star ULXs (except for M51 X-7), and with $L_{\rm bb} >\sim 10^{39} ~{\rm erg~s}^{-1}$ for all the other unknown compact object ULXs. While the latter are more likely to be black hole ULXs. The results shows that luminosities of the soft emissions are basically around the Eddington limit within one order of magnitude, and the model-predicted accretion rates are $\sim 100$ times of the Eddington rate. This demonstrate that the soft excess emission in our ULX sample are highly consistent with the prediction of the supercritical models, that the soft emission is produced by the optically thick outflow from super-Eddington accretion.
