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 gradually 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 γ-ray flux from Mrk 501 with EGRET with 3.5 δ significance (E > 100 MeV). Higher flux was also observed in 1996 April-May, with 4.0 δ significance for E > 100 MeV and 5.2 δ significance for E > 500 MeV. The γ-ray spectrum was measured to be flatter than most of the γ-ray blazars. We find that the multiband spectrum in 1996 is consistent with that calculated from a one-zone synchrotron self-Compton (SSC) model in which X-rays are produced via synchrotron emission and γ-rays are produced via inverse Compton scattering of synchrotron photons in a homogeneous region. The flux of TeV γ-rays is consistent with the predictions of the model if the decrease of the Compton scattering cross section in the Klein-Nishina regime is considered. In the context of this model, we investigate the values of the magnetic field strength and the beaming factor allowed by the observational results. We compare the 1996 March multiwavelength spectrum with that in the flare state in 1997 April. Between these two epochs, the TeV flux increase is well correlated with that observed in keV range. The keV and TeV amplitudes during the 1997 April flare 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 γ_max ∼ 6 × 10⁶. However, the TeV spectrum observed during the 1996 March campaign is flatter than predicted by our models. We find that this cannot be explained by either higher order Comptonization or the contribution of the "seed" IR photons from the host galaxy for the first-order external radiation Comptonization, but we cannot exclude possible effects of the IR photons that may arise in the parsec-size tori postulated to exist in active galactic nuclei.