We present the results of a multiwavelength campaign for Mrk 501 performed in March 1996 with ASCA, EGRET, Whipple, and optical telescopes. We report here for the first time the detection of a GeV γ-ray flux from Mrk 501 with EGRET with 3.5σ significance (E > 100 MeV). A higher flux was also observed in April/May 1996, with 4.0σ significance for E > 100 MeV, and 5.2σ significance for E > 500 MeV. We find that the multiband spectrum in March 1996 is consistent with that calculated from a one-zone SSC model, except for the extremely 'flat' TeV spectrum. We show that the discrepancy cannot be explained by either second order Comptonization or the contribution of the 'seed' IR photons from the host galaxy.
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