Unveiling the X-ray/TeV engine in Mkn 421

¹ Laboratoire Leprince-Ringuet, UMR 7638 CNRS, École Polytechnique, 91128 Palaiseau, France e-mail: berrie@poly.in2p3.fr
² Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, Paris 6, 98 bis Bd. Arago, 75014 Paris, France

Received: 28 July 2006
Accepted: 18 September 2006

Abstract

Aims.Our goal was to find important clues concerning particle acceleration in relativistic jets through the analysis of multi-wavelength observations of TeV blazars.

Methods.Simultaneous observations of Mkn 421 were taken in very high energy γ-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.

Results.Short timescale (≈30 mn) 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^-1 cm^-2. The fractional variability decreases from X-rays to optical as a power law with . The full spectral energy distribution is well-fitted by synchrotron self-Compton emission from cooling electrons injected with a Maxwellian distribution of characteristic energy . Fluctuations in the injected power with explain the observed variability.

Conclusions.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.