Correlated Intense X‐Ray and TeV Activity of Markarian 501 in 1998 June

Abstract

We present exactly simultaneous X-ray and TeV monitoring with RXT E and HEGRA of the TeV blazar Mrk 501 during 15 days in 1998 June. After an initial period of very low ux at both wavelengths, the source underwent a remarkable are in the TeV and X-ray energy bands, lasting for about 6 days and with a larger amplitude at TeV energies than in the X-ray band. At the peak of the TeV are, rapid TeV ux variability on subhour timescales is found. Large spectral variations are observed at X-rays, with the 320 keV photon index of a pure power-law continuum attening from !\\ 2.3 to !\\ 1.8 on a timescale of 23 days. This implies that during the maximum of the TeV activity the synchrotron peak shifted to energies keV, a behavior similar to that observed during the longer lasting, Z50 more intense are in 1997 April. The TeV spectrum during the are is described by a power law with photon index !\\ 1.9 and an exponential cuto+ at D4 TeV ; an indication for spectral softening during the are decay is observed in the TeV hardness ratios. Our results generally support a scenario in which the TeV photons are emitted via inverse Compton scattering of ambient seed photons by the same electron population responsible for the synchrotron X-rays. The simultaneous spectral energy distributions can be t with a one-zone synchrotron self-Compton model assuming a substantial increase of the magnetic eld and the electron energy by factors of 3 and 10, respectively.