Extragalactic Gamma‐Ray Absorption and the Intrinsic Spectrum of Markarian 501 during the 1997 Flare

Abstract

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 ?-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 ?-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 ~5-10 TeV range that corresponds to the broad, flat, time-averaged X-ray peak in the ~50-100 keV range observed during the flare. The spectral index of our derived intrinsic differential photon spectrum for Mrk 501 at energies below ~2 TeV was found to be ~1.6-1.7. 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.