Multiwavelength observations of the TeV blazar Mrk 501 in March 1996. The first report of the detection by EGRET

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 gamma-ray flux from Mrk 501 with EGRET with 3.5 sigma significance (E > 100 MeV). A higher flux was als o observed in April/May 1996, with 4.0 sigma significance for E > 100 MeV, and 5.2 sigma significance for E > 500 MeV. We find that the multiband spec trum in March 1996 is consistent with that calculated from a one-zone SSC m odel, except for the extremely 'flat' TeV spectrum. We show that the discre pancy cannot be explained by either second order Comptonization or the cont ribution of the 'seed' IR photons from the host galaxy. (C) 1999 Elsevier S cience B.V. All rights reserved.