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Astron. Astrophys. 334, L85-L87 (1998)

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Letter to the Editor

Absorption of very high energy gamma-rays by intergalactic infrared radiation: A new determination

F.W. Stecker 1 and O.C. de Jager 2

1 Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
2 Space Research Unit, Department of Physics, Potchefstroom University for CHE, Potchefstroom 2520, South Africa

Received 28 January 1998 / Accepted 20 April 1998


We present a new calculation of the intergalactic [FORMULA] -ray pair-production absorption coefficient as a function of both energy and redshift. In reexamining this problem, we make use of a new empirially based calculation (as opposed to previous model calculations) of the intergalactic infrared radiation field (IIRF). We find smaller opacities than those given previously (Stecker & De Jager 1997). We apply our results to the new observations of the flaring [FORMULA] -ray spectra of Mrk 421 and Mrk 501, both at a redshift of [FORMULA] 0.03. Our new calculations indicate that there should be no significant curvature in the spectra of these sources for energies below 10 TeV, as indicated by recent observations. However, the intrinsic spectra of these sources should be harder by amounts of [FORMULA] 0.25 to 0.45 in the spectral index (in the 1-10 TeV range), with an intergalactic absorption cutoff above [FORMULA] TeV.

Key words: fl-rays: theory – infrared: general – quasars: general – quasars: individual (Markarian 421, Markarian 501)

Send offprint requests to: F.W. Stecker

© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998