Simultaneous X-Ray and TeV Gamma-Ray Observation of the TeV Blazar Markarian 421 during 2000 February and May

We present the results of simultaneous observations of the TeV blazar Markarian 421 at X-ray and TeV gamma-ray energies with the Rossi X-Ray Timing Explorer and the stereoscopic Cerenkov telescope system of the High Energy Gamma-Ray Astronomy (HEGRA) experiment, respectively. The source was monitored from 2000 February 2 to 16 and from 2000 May 3 to 8. In both energy bands several flares with very rapid flux variability were observed. In the X-ray band, the flux increased and decreased with e-folding times as short as about 5 hr. The 3-20 keV photon index varied between values of 2.2 and 2.9. For five pointings the data show statistically significant evidence for spectral curvature. The photon index varied substantially on very short timescales: on 2000 February 11 it hardened within 1.6 hr by ΔΓ=0.18, and on February 14 it softened within 1.6 hr by ΔΓ=0.2. The TeV observations of February 7/8 showed statistically significant evidence for substantial TeV flux variability on a 30 minute timescale. The TeV energy spectrum averaged over all the observations of the campaign shows a similar steep slope as in earlier HEGRA observations: dN/dE=N₀(E/1 TeV)^-Γ with N₀=(25+/-1_stat)×10^-12 photons cm^-2 s^-1 TeV^-1 and Γ=2.94+/-0.06_stat. Within statistical errors no evidence for a curvature of the TeV energy spectrum is found. We show the results of modeling the data with a time-dependent homogeneous synchrotron self-Compton model. The X-ray and TeV gamma-ray emission strengths and energy spectra together with the rapid flux variability strongly suggest that the emission volume is approaching the observer with a Doppler factor of 50 or higher. The different flux variability timescales observed at X-rays and TeV gamma rays indicate that a more detailed analysis will require inhomogeneous models with several emission zones.