A study of high energy emission from the TeV blazar Mrk 501 during multiwavelength observations in 1996

Abstract

We present the results of a multiwavelength campaign for Mrk 501 performed in March 1996 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 March 1996. 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 σ significance (E > 100 MeV). Higher flux was also observed in April/May 1996, with 4.0 σ significance for E > 100 MeV, and 5.2 σ 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 SSC model where X-rays are produced via synchrotron emission, and γ-rays 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 March 1996 multiwavelength spectrum with that in the flare state in April 1997. Between these two epochs, the TeV flux increase is well correlated with that observed in keV range. The keV and TeV amplitudes during the April 1997 flare are accurately reproduced by a one-zone SSC model, assuming that the population of synchrotron photons in 1996 are scattered by the newly injected relativistic electrons, having maximum energies of γ max ∼ 6 × 10 6 . However, the TeV spectrum observed during March 1996 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 torus postulated to exist in AGN.

Gamma-Ray Polarimetry with Compton Telescope

“…18 For the few blazars where the synchrotron spectra extend to a few hundred keV (such as the well-known Mkn 501, 19 or recently-discovered TeV emitter 1959+65 20 ), detection of high polarization is nearly assured by theory. A lack of hard X-ray polarization will seriously undermine current models for X-ray emission from TeV blazars.…”

Gamma-ray polarimetry with Compton Telescope

“…The other component, in hard X-rays / gamma-rays, is presumably due to the inverse Compton process by the same particle population as the synchrotron emission, with the target photons that are either internal to the jet (synchrotron) or external, due to the emission line regions or diffuse IR flux in the host galaxy. 18 For the few blazars where the synchrotron spectra extend to a few hundred keV (such as the well-known Mkn 501, 19 or recently-discovered TeV emitter 1959+65 20 ), detection of high polarization is nearly assured by theory. A lack of hard X-ray polarization will seriously undermine current models for X-ray emission from TeV blazars.…”