Unveiling the X-ray/TeV engine in Mkn 421

Issue		A&A Volume 462, Number 1, January IV 2007


Page(s)		29 - 41
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20066134

A&A 462, 29-41 (2007)
DOI: 10.1051/0004-6361:20066134

Unveiling the X-ray/TeV engine in Mkn 421

B. Giebels¹, G. Dubus^{1, 2}, and B. Khélifi¹

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 $\gamma$ -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.
Results.Short timescale ( $\approx$ 30 mn) 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 $\approx$ 10^-9erg s^-1 cm^-2. The fractional variability decreases from X-rays to optical as a power law with $F_{\rm var}\propto E^{0.24\pm0.01}$ . The full spectral energy distribution is well-fitted by synchrotron self-Compton emission from cooling electrons injected with a Maxwellian distribution of characteristic energy $\gamma_b$ . Fluctuations in the injected power with $P_{\rm inj}\propto \gamma_b^4$ 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.

Key words: acceleration of particles -- radiation mechanisms: non-thermal -- galaxies: active -- BL Lacertae objects: individual: Mkn 421 -- gamma rays: observations -- X-rays: galaxies

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