Abstract
The standard spectral model for analyzing the soft component of thermal emission from a thin accretion disk around a black hole is the multitemperature blackbody model. The widely used implementation of this model, which is known as diskbb, assumes nonzero torque at the inner edge of the accretion disk. This assumption is contrary to the classic and current literature on thin-disk accretion, which advocates the use of a zero-torque boundary condition. Consequently, we have written code for a zero-torque model, ezdiskbb, which we compare to the nonzero-torque model diskbb by fitting Rossi X-Ray Timing Explorer spectra of three well-known black hole binaries: 4U 1543-47, XTE J1550-564, and GRO J1655-40. The chief difference we find is that the zero-torque model gives a value for the inner disk radius that is ≈2.2 times smaller than the value given by diskbb. This result has important implications, especially for the determination of black hole angular momentum and mass accretion rate.
Highlights
One of the most widely used models in studies of the spectra of accretion disks in black hole X-ray binaries is the multi-temperature blackbody (MTB) model, which has been standard in the literature for over 30 years (Pringle & Rees 1972; Shakura & Sunyaev 1973; Pringle 1981; Frank, King & Raine 1992)
Fitting spectra with the MTB model allows us to determine important properties of accretion disks, including accretion rate, inner radius, and temperature. This information about the accretion disk can in principle be used to deduce the angular momentum of the black hole (e.g., Zhang, Cui, & Chen 1997), and the radius at which an accretion disk might be truncated in low accretion rate phases (e.g., Esin et al 2001)
As we stated in the introduction, Gierlinski et al (1999) have pointed out that diskbb assumes a nonzero torque at the inner boundary of the accretion disk
Summary
One of the most widely used models in studies of the spectra of accretion disks in black hole X-ray binaries is the multi-temperature blackbody (MTB) model, which has been standard in the literature for over 30 years (Pringle & Rees 1972; Shakura & Sunyaev 1973; Pringle 1981; Frank, King & Raine 1992). Fitting spectra with the MTB model allows us to determine important properties of accretion disks, including accretion rate, inner radius, and temperature. This information about the accretion disk can in principle be used to deduce the angular momentum of the black hole (e.g., Zhang, Cui, & Chen 1997), and the radius at which an accretion disk might be truncated in low accretion rate phases (e.g., Esin et al 2001). Gierlinski et al (1999) have shown, that the widely used MTB model known as “diskbb” (see, e.g., Mitsuda et al 1984) that is found in various X-ray spectral fitting packages assumes a nonzero torque at the inner boundary of the disk
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