2007A&A...462...29G

Our goal was to find important clues concerning particle acceleration in relativistic jets through the analysis of multi-wavelength observations of TeV blazars. Simultaneous observations of Mkn 421 were taken in very high energy γ-rays (>200GeV, 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 (≃30mn) variations in VHE γ-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 ≃10^–9erg/s/cm2. The fractional variability decreases from X-rays to optical as a power law with F_var∝E^0.24±0.01. The full spectral energy distribution is well-fitted by synchrotron self-Compton emission from cooling electrons injected with a Maxwellian distribution of characteristic energy γ_b. Fluctuations in the injected power with P_inj∝γ_b⁴ explain the observed variability. The spectral saturation and the power-law dependence of the fractional variability are novel results that may extend to other TeV blazars. The ability of Maxwellian injections to reproduce the observed features suggest second-order Fermi acceleration or magnetic reconnection may play the dominant role in particle acceleration.