4. The -ray photosphere
We now discuss the initial height of a plasma blob from which TeV -ray emission as observed from Mrk 421 can escape freely. Since BL Lac objects have very dilute emission line clouds, backscattering of accretion disk radiation into the trajectery of emitted -rays is negligible.
Then, the optical depth for a photon of dimensionless energy , emitted at height due to interaction with accretion disk radiation is calculated as
where is the differential number of accretion disk photons at height z, is the pair production cross section
and we assumed for simplicity that the -ray is emitted along the symmetry axis. The result for different injection heights is illustrated in Fig. 6. and shows that the -ray photosphere for -rays of energy 500 GeV - 1 TeV is located slightly above pc in the case of an accretion disk of luminosity erg s-1.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998