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Astron. Astrophys. 324, 395-409 (1997)

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4. The [FORMULA] -ray photosphere

We now discuss the initial height of a plasma blob from which TeV [FORMULA] -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 [FORMULA] -rays is negligible.

Then, the optical depth for a photon of dimensionless energy [FORMULA], emitted at height [FORMULA] due to interaction with accretion disk radiation is calculated as

[EQUATION]

[EQUATION]

where [FORMULA] is the differential number of accretion disk photons at height z, [FORMULA] is the pair production cross section

[EQUATION]

[EQUATION]

[EQUATION]

and we assumed for simplicity that the [FORMULA] -ray is emitted along the symmetry axis. The result for different injection heights [FORMULA] is illustrated in Fig. 6. and shows that the [FORMULA] -ray photosphere for [FORMULA] -rays of energy 500 GeV - 1 TeV is located slightly above [FORMULA] pc in the case of an accretion disk of luminosity [FORMULA] erg s-1.

[FIGURE] Fig. 6. Pair production optical depth for high-energy [FORMULA] -rays, emitted at different heights ([FORMULA]), interacting with direct accretion disk radiation ([FORMULA] erg/s, [FORMULA])
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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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