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

We present exactly simultaneous X-ray and TeV monitoring with RXTE and HEGRA of the TeV blazar Mrk 501 during 15 days in 1998 June. After an initial period of very low flux at both wavelengths, the source underwent a remarkable flare 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 flare, rapid TeV flux variability on subhour timescales is found. Large spectral variations are observed at X-rays, with the 3-20 keV photon index of a pure power-law continuum flattening from ? = 2.3 to ? = 1.8 on a timescale of 2-3 days. This implies that during the maximum of the TeV activity the synchrotron peak shifted to energies 50 keV, a behavior similar to that observed during the longer lasting, more intense flare in 1997 April. The TeV spectrum during the flare is described by a power law with photon index ? = 1.9 and an exponential cutoff at ~4 TeV; an indication for spectral softening during the flare 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 fit with a one-zone synchrotron self-Compton model assuming a substantial increase of the magnetic field and the electron energy by factors of 3 and 10, respectively.