(back to main page)

(one page up)

Gamma-gamma absorption Tutorial

PRELIMINARY

This section is still under construction and the documentation will be improved with time.

In this tutorial we explain how to use the latest functionality added to GAMERA: the iclusion of gamma-gamma absorption effects on the photon spectra.

All the new functionalities have been implemented in the Radiation class.

When setting up the target photon fields for the IC scattering, it is possible to compute also the effect of the gamma-gamma absorption on the resulting radiation.

If the photon fiels is not isotropic but has an angular dependency, defined in the same way as done for the IC scattering (see the tutorial on Inverse Compton scattering), GAMERA can compute the absorbed flux using the full angular dependent cross section, while it will use the integrated version otherwise.

To make the calculation possible, the photon field must be given a linear size to perform the spatial integration of the absoprtion coefficient so as to obtain the actual value of the optical depth.

The size of the photon field is given with the following function:

fr.SetSizePhotonField(0,sizeph)

where the first argument is the photon field counter and the second is the size of the photon field given in parsecs.

Once this is done, you can obtain the absorbed differential spectrum or the absorbed SED through the following functions:

abs_sed = fr.GetTotalAbsorbedSED([0,1,..])
abs_spectrum = fr.GetTotalAbsorbedSpectrum([0,1,..])

where the argument is the array with the index of the photon fields we are taking into account for the absorption This can be useful in case a user would like to consider a certain photon field for the scattering, but not for the absoption and viceversa.

It is also possible to define a certain spatial dependency of the photon field through the function

fr.SetTargetFieldSpatialDep(field, dependency)

where field is the number of the photon field and dependency is a vector of tuples in which the first element is the distance from the source (given in pc) and the second element is the fraction of the energy density that was given when setting up the photon field.

If you have more complicated cases, you can always do the spatial integration directly in your code by calculating the absorption coefficient for the field via fr.ComputeAbsCoeff(energy, field) which takes the energy of the gamma-ray and the field index as arguments, returning the absorption coefficient in units of 1/cm.

CAVEAT: No effect of the secondaries on the radiation spectrum is implemented.