A New Characterization of the Compton Process in the ULX Spectra
Abstract
Ultra Luminous X-ray sources (ULXs) are unusually luminous point sources located at arms of spiral galaxies, and are candidates for the intermediate mass black holes (Makishima+2000). Their spectra make transition betweens power-law shapes (PL state) and convex shapes (disk-like state). The latter state can be explained with either the multi-color disk (MCD)+thermal Comptonization (THC) model or a Slim disk model (Watari+2000). We adopt the former modeling, because it generally gives physically more reasonable parameters (Miyawaki+2009). To characterize the ULXs spectra with a unified way, we applied the MCD+THC model to several datasets of ULXs obtained by Suzaku, XMM-Newton, and Nu-Star. The model well explains all the spectra, in terms of cool disk (T_{in}∼0.2 keV), and a cool thick (T_{e}∼2 keV, τ ∼10) corona. The derived parameters can be characterized by two new parameters. One is Q≡ T_{e}/T_{in} which describes balance between the Compton cooling and gravitational heating of the corona, while the other is f≡ L_{raw}/L_{tot}, namely, the directly-visible (without Comptonization) MCD luminosity. Then, the PL state spectra have been found to show Q∼10 and f∼0.7, while those of the disk-like state Q∼ 3 and f≤0.01. Thus, the two states are clearly separated in terms of Q and f.
- Publication:
-
The Extremes of Black Hole Accretion
- Pub Date:
- July 2015
- Bibcode:
- 2015ebha.confE..41K