Extragalactic gamma-ray absorption and the intrinsic spectrum of Markarian 501 during the 1997 flare

Using the recent models of Malkan & Stecker 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 Mrk 501 during its 1997 high state. We find that the time-averaged spectral energy distribution of Mrk 501 while flaring had a broad, flat peak in the similar to5-10 TeV range that corresponds to the broad, flat, time-averaged X-ray peak in the similar to50-100 keV range observed during the flare. The spectral index of our derived intrinsic differential photon spectrum for Mrk 501 at energies below similar to2 TeV was found to be similar to1.6-1.7. 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 apparently favor a synchrotron self-Compton (SSC) origin for the TeV emission, together with jet parameters that are consistent with time variability constraints within the context of a simple SSC model.