HEAD Division Meeting 1999, April 1999
Session 24. Blazars
Oral, Tuesday, April 13, 1999, 4:30pm-6:02pm, Colonial Room

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[24.07] Intergalactic Extinction of High Energy Gamma-Rays

F.W. Stecker (NASA/GSFC)

We discuss the determination of the intergalactic pair-production absorption coefficient derived by Stecker & De Jager (1998) making use of an empirically based calculation of the IR background spectrum given by Malkan & Stecker (1998) which agrees well with recent data and constraints on the IR background. We then show that present air Cherenkov telescope observations of the energy spectra of Mrk 421 and Mrk 501 are consistent with the amount of absorption predicted by Stecker & De Jager and that new HEGRA observations of the flaring spectrum of Mrk 501, presented at the recent Harvard workshop, actually appear to show just the amount of intergalactic absorption which we have predicted. As a further test of our results, we give a predicted spectrum, with absorption included, for PKS2155-304. This XBL lies at a redshift of 0.12, making it the highest redshift source yet observed at energies above 0.3 TeV. This source should have its spectrum steepened by approximately 1 in its spectral index in the energy range between 0.3 and 3 TeV and should show a marked absorption turnover above about 6 TeV. We also discuss the determination of absorption cutoffs for higher redshift sources (Salamon & Stecker 1998) and their implications for GLAST.

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