Simultaneous X-ray and TeV observations of a rapid flare from markarian 421

Mrk 421 was observed for about 2 days with BeppoSAX in 1998 April as part o f a worldwide multiwavelength campaign. A large, well-defined flare was obs erved in X-rays. The same flare was observed simultaneously at TeV energies by the Whipple Observatory gamma-ray telescope. These data provide (1) the first evidence that the X-ray and TeV intensities are well correlated on t imescales of hours and (2) the first exactly simultaneous X-ray and TeV spe ctra. The results imply that the X-ray and TeV photons derive from the same region and from the same population of relativistic electrons. 'The physic al parameters deduced from a homogeneous synchrotron self-Compton model for the spectral energy distribution yield electron cooling times close to the observed variability timescales.