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 Marka rian 421 at X-ray and TeV gamma-ray energies with the Rossi X-Ray Timing Ex plorer and the stereoscopic Cerenkov telescope system of the High Energy Ga mma-Ray Astronomy (HEGRA) experiment, respectively. The source was monitore d 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-ra y band, the flux increased and decreased with e-folding times as short as a bout 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 spe ctral curvature. The photon index varied substantially on very short timesc ales: on 2000 February 11 it hardened within 1.6 hr by Delta Gamma = 0.18, and on February 14 it softened within 1.6 hr by Delta Gamma = 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 s pectrum averaged over all the observations of the campaign shows a similar steep slope as in earlier HEGRA observations: dN/dE = N-o(E/1 TeV)(-Gamma) with N-o = (25 +/- 1(stat)) x 10(-12) photons cm(-2) s(-1) TeV-1 and Gamma = 2.94 +/- 0.06(stat). Within statistical errors no evidence for a curvatur e 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 ap proaching 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 sever al emission zones.