High-energy emission from the TeV blazar Markarian 501 during multiwavelength observations in 1996

We present the results of a multiwavelength campaign for Mrk 501 performed in 1996 March with ASCA, EGRET, Whipple, and optical telescopes. The X-ray flux observed with ASCA was 5 times higher than the quiescent level and gra dually decreased by a factor of 2 during the observation in 1996 March. In the X-ray band, a spectral break was observed around 2 keV. We report here for the first time the detection of high-energy gamma-ray flux from Mrk 501 with EGRET with 3.5 sigma significance (E > 100 MeV). Higher flux was also observed in 1996 April-May, with 4.0 sigma significance for E > 100 MeV an d 5.2 sigma significance for E > 500 MeV. The gamma-ray spectrum was measur ed to be flatter than most of the gamma-ray blazars. We find that the multi band spectrum in 1996 is consistent with that calculated from a one-zone sy nchrotron self-Compton (SSC) model in which X-rays are produced via synchro tron emission and gamma-rays are produced via inverse Compton scattering of synchrotron photons in a homogeneous region. The flux of TeV gamma-rays is consistent with the predictions of the model if the decrease of the Compto n scattering cross section in the Klein-Nishina regime is considered. In th e context of this model, we investigate the values of the magnetic held str ength and the beaming factor allowed by the observational results. We compa re the 1996 March multiwavelength spectrum with that in the flare state in 1997 April. Between these two epochs, the TeV flux increase is well correla ted with that observed in keV range. The keV and TeV amplitudes during the 1997 April hare are accurately reproduced by a one-zone SSC model, assuming that the population of synchrotron photons in 1996 are scattered by newly injected relativistic electrons having maximum energies of gamma(max) simil ar to 6 x 10(6). However, the TeV spectrum observed during the 1996 March c ampaign is flatter than predicted by our models. We find that this cannot b e explained by either higher order Comptonization or the contribution of th e "seed" IR photons from the host galaxy for the first-order external radia tion Comptonization, but we cannot exclude possible effects of the IR photo ns that may arise in the parsec-size tori postulated to exist in active gal actic nuclei.