4.1.9. Dbb: disk blackbody model¶

We take the model for a standard Shakura-Sunyaev accretion disk. The radiative losses from such a disk are given by where is the loss term in W m at radius , the mass of the central object, the accretion rate through the disk and the inner radius. If this energy loss is radiated as a black body, we have with the constant of Stefan-Boltzmann and the local temperature of the black body. The total spectrum of such a disk is then obtained by integration over all radii. We do this integration numerically. Note that for large , .

Warning

A popular disk model (diskbb in XSPEC) assumes this temperature dependence over the full disk. However, we correct it using the factor in which corresponds to the torque-free condition at the inner boundary of the disk.

The photon spectrum of the disk is now given by where is the inclination of the disk (0 degrees for face-on disk, 90 degrees for an edge-on disk), the photon energy, the outer edge of the disk, and is defined by

(1) and further the function is defined by where is defined by .

In addition to calculating the spectrum, the model also allows to calculate the average radius of emission at a specified energy . This is sometimes useful for time variability studies (softer photons emerging from the outer parts of the disk).

Given the fit parameters and , using (1) it is straightforward to calculate the product . Further note that if , it is possible to find both and , provided the inclination angle is known.

The parameters of the model are:

norm : Normalisation ( ), in units of  . Default value: 1.
t : The nominal temperature in keV. Default value: 1 keV.
ro : The ratio of outer to inner disk radius, ener : Energy at which the average radius of emission will be calculated
rav : Average radius of all emission at energy specified by the parameter above. Note that this is not a free parameter, it is calculated each time the model is evaluated.

Recommended citation: Shakura & Sunyaev (1973).