Extragalactic Gamma-ray Absorption and the Intrinsic Spectrum of Mkn 501 During the 1997 Flare

Abstract

Using the recent models of Malkan & Stecker (2001) for the infrared background radiation and extrapolating them into the optical and UV range using recent galaxy count data, we rederive the optical depth of the Universe to high energy gamma-rays as a function of energy and redshift for energies between 50 GeV and 100 TeV and redshifts between 0.03 and 0.3. We then use these results to derive the intrinsic gamma-ray spectrum of Mkn 501 during its 1997 high state. We find that the time averaged spectral energy distribution of Mkn 501 while flaring had a broad, flat peak in the 5 to 10 TeV range which corresponds to a broad, flat time averaged X-ray peak in the 50 to 100 keV range observed during the flare. The spectral index of our derived intrinsic differential photon spectrum for Mkn 501 at energies below about 2 TeV was found to be apx. 1.6 to 1.7. This corresponds to a time averaged spectral index of 1.76 found in soft X-rays at energies below the X-ray (synchrotron) peak. These results appear to favor a synchrotron-self Compton origin for the TeV emission together wuth jet parameters which are consistent with time variability constraints within the context of a simple SSC model.