TeV gamma ray opacity in PKS 2155-304

¹ Research School of Astronomy and Astrophysics, Australian National University, Cotter Rd. Weston, ACT 2611, Australia
e-mail: geoff@mso.anu.edu.au
² Landessternwarte Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
e-mail: S.Wagner@lsw.uni-heidelberg.de

Received: 8 September 2009
Accepted: 4 November 2010

Abstract

Context. Recently observed rapid time variability in the gamma ray emission from the blazar PKS 2155-304 indicates that either the site of gamma ray emission is close to the black hole or the jet has a very high Lorentz factor. Consideration of the opacity of gamma rays close to the black hole provides additional information related to these two possibilities.

Aims. We investigate the TeV gamma ray opacity resulting from pair production on soft photons radiated by the black hole accretion disk, considering situations where the radiation produced by the disk is close to that predicted by the Shakura-Sunyaev model and situations where the disk radiation is much less.

Methods. We utilise expressions for the pair opacity of very high energy gamma rays developed by Gould and Schréder and Donea and Protheroe and use expressions for the disk radiation field produced by an accretion disk external to a Poynting flux dominated jet. A lower level of disk emission is modelled by using the spectrum corresponding to a lower accretion rate. We also consider changes in the shape of the gamma ray sepctrum as a flare emerges from the optically thick region.

Results. If the gravitational power resulting from accretion is dissipated by radiation in the region of the disk external to the jet, then TeV gamma rays are opaque out to of order 40 gravitational radii  ≈6  ×  10¹⁵   cm from the black hole. However, the opacity is considerably reduced if the disk radiates a negligible amount of gravitational power as would be the case if there is a significant disk wind external to the jet. In either case the absence of spectral changes during observed flares excludes scenarios in which the TeV gamma rays are emitted even in regions of modest pair opacity.