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G. Pszota, W. Cui (Purdue University)
It is customary to use multi-temperature blackbody models to describe the X-ray continuum that is emitted from the accretion disk around a stellar mass black hole in a binary system. The fact that the interaction between photons and particles is dominated by scattering rather than absorption causes a deviation from the Planck-distribution. This deviation can be quantified by the ratio of the color temperature and the effective temperature, which is usually referred to as the “color correction factor” (f). The knowledge of this factor is essential if one wants to obtain fundamental information on the binary system by fitting the X-ray continuum with a multicolor disk model. Unfortunately f depends on properties of the binary system (e.g. black hole mass, mass accretion rate, viscous parameter), so it cannot be known a priori. Here we report on deriving the color correction factor observationally. The value (f = 1.53) we obtained from high quality XMM-Newton data on the black hole candidate GX 339-4 in its high-soft state seems to be reasonable considering the theoretical limits. In principle, combined with the mass of the black hole, the inclination angle and the distance of the system obtained from optical observations, we could use this value to estimate the black hole spin by fitting the X-ray continuum with a multicolor disk model. As we will show, however, that model does not seem to be entirely consistent with the data, while a model that invokes an optically thick Compton-scattering component for the disk emission gives an excellent fit. We will discuss the implications of these results.
This work was supported in part by NASA Grant NAG5-9998.
Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.