Two-phase Comptonization model of soft thermal seed photons for GRB prompt emission.

NOTE: This model is now in the Xspec release and this page is superceded by the Xspec manual

This GRBCOMP model  has been proposed  by Titarchuk et al. (2012, ApJ, 752, 116) as a possible scenario for the spectral emission of the prompt phase of Gamma Ray Bursts.

It is essentially a two-phase model: up to the peak energy Ep of the EF(E) spectrum soft thermal blackbody-like photons are comptonized by a subrelativistic bulk outflow of thermal electrons, while the high-energy tail  is obtained by a further convolution of the formerly comptonized spectrum with a Green's function.
An example of the application of the model to a sample of GRBs can be found in Frontera et al. (2013, ApJ, 779, 175).

The model parameters are the following:

par1  =  kTs, temperature of the seed blackbody spectrum (keV)

par2  =  γ, when it is set = 3 the seed soft spectrum is a blackbody, otherwise it approximates a modified blackbody

par3  =  kTe, electron temperature of the subrelativistic outflow (keV)

par4  =  τ, radial optical depth of the subrelativistic outflow

par5  =  β, bulk outflow velocity of the thermal electrons

par6  =  fbflag, if set = 0 only the first-order bulk Comptonization term is considered, if set =1 also the second-order  term 1+β2/3Θ is computed, where Θ=kTe/mec2

par7  =  log(A), geometrical covering factor which determines the relative weigths of the seed and comptonized spectra to the total flux

par8  =  z, redshift

par9  =  αboost, energy index of the Green's function with which the formerly comptonization spectrum is convolved
norm  = R29 /D2Mpc,  where R9 is the apparent blackbody radius in units of 109 cm and DMpc  is the sorce distance in Mpc

To install as an XSPEC local model download the source code and model description file.

Keith Arnaud, Lab. for High Energy Astrophysics, NASA/Goddard Space Flight Center

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