Unveiling the X-ray/TeV engine in Mkn 421

Accepted in A&A, 16 pages, 14 figuresSimultaneous observations of Mkn 421 were taken in very high energy gamma -rays (>200 GeV, CAT experiment), X-rays (RXTE) and optical (KVA). Multi -day RXTE observations are also presented, allowing for detailed modelling of the spectral variability. Short timescale (~30')variations in VHE gamma-rays are found, correlated with X-rays but not with the optical. The X-ray spectrum hardens with flux until the photon indices saturate above a threshold flux. The fractional variability decreases from X-rays to optical as a power-law follwing E^0.24. The full spectral energy distribution is well-fitted by synchrotron self-Compton emission from cooling electrons injected with a Maxwellian distribution of characteristic energy gamma_b. Fluctuations in the injected power proportional to (gamma_b)^4 can explain the observed variability