Complex spectral variability from intensive multiwavelength monitoring of Markarian 421 in 1998

We conducted a multifrequency campaign for the TeV blazar Markarian 421 in 1998 April. The campaign started from a pronounced high-amplitude flare rec orded by BeppoSAX and Whipple; the ASCA observation started 3 days later. I n the X-ray data, we detected multiple flares, occurring on timescales of a bout 1 day. ASCA data clearly reveal spectral variability. The comparison o f the data from ASCA, the Extreme Ultraviolet Explorer, and the Rossi X-Ray Timing Explorer indicates that the variability amplitudes in the low-energ y synchrotron component are larger at higher photon energies. In TeV gamma -rays, large intraday variations-which were correlated with the X-ray flux- were observed when results from three Cerenkov telescopes were combined. Th e rms variability of TeV gamma -rays was similar to that observed in hard X -rays, above 10 keV. The X-ray light curve reveals flares that are almost s ymmetric for most cases, implying that the dominant timescale is the light crossing time through the emitting region. The structure function analysis based on the continuous X-ray light curve of 7 days indicates that the char acteristic timescale is similar to0.5 days. The analysis of ASCA light curv es in various energy bands appears to show both soft (positive) and hard (n egative) lags. These may not be real, as systematic effects could also prod uce these lags, which are all much smaller than an orbit. If the lags of bo th signs are red, these imply that the particle acceleration and X-ray cool ing timescales are similar.