0320076

Mrk 421 one of the brightest TeV gamma-ray sources in the sky is one of five BL Lac objects which have two broad emission maxima at higher energies than normal BL Lac objects show one at X-ray energies and one at TeV energies. Both maxima are separated by a deep and sharp minimum which lies exactly in the energy range of INTEGRAL and which is nearly unexplored. Both emission components are believed to be connected via a common particle population which creates both emission components by different physical emission processes (Synchrotron and inverse Compton). This conjecture can be tested by simultaneous observations at different wavelengths looking for time correlations of the measured intensities. In addition with INTEGRAL the transition region can be explored for the first time. It is the aim of this proposal to observe Mrk 421 with INTEGRAL when it is in an active state together with radiotelescopes IR and optical telescopes and with Cherenkov telescopes.In its quiescent state Mrk 421 is too weak to be observable by the two main instruments (IBIS and SPI) of INTEGRAL in the r^ant energy range. Therefore an observation of this source with INTEGRAL makes only sense when it is in an active state. So we ask for a ToO observation of this source when it is active. [truncated! Please see actual data for full text]