1 Introduction

Among Active Galactic Nuclei (AGNs), blazars are those radio-loud objects having a jet pointing towards the observer, which has a relatively high bulk Lorentz factor, giving rise to a strong Doppler boosting of the observed fluxes. Blazar emission is dominated by their jet power output, which is mainly non-thermal, extending over more than fifteen energy decades. At low energies, their featureless optical continuum as well as their strong radio and optical polarization are due to synchrotron radiation of the magnetized plasma jet. At high energies, blazar jets also show remarkable properties. During the last ten years, the EGRET detector, operating above $30\:{\rm MeV}$ on board the Compton Gamma-Ray Observatory, definitively opened the field of high-energy astrophysics by revealing that most extragalactic strong $\gamma$-ray emitters were blazars (von Montigny et al. 1995; Hartman et al. 1999). Their $\gamma$-radiation power often dominates their entire spectrum, and it must be produced in a small enough region to account for rapid variability, already observed on time-scales less than one hour at ${\rm TeV}$ energies (Gaidos et al. 1996). Although the origin of jets is still uncertain, the study of their $\gamma$-ray emission in blazars can shed light on the nature and content of their plasma (e⁺e^- pairs or e^-p), and give new insight into the high-energy particle acceleration and cooling processes occuring at the sub-parsec scale. Ultimately this could give useful information on how jets take form and lead to a better understanding of the energy extraction mechanisms in the surroundings of the central supermassive black hole.

Markarian 421 (Mkn 421) is the closest known BL Lac blazar (at a redshift of 0.031), and the first discovered in the high and very-high energy (VHE) ranges. It was first detected as a weak source by the EGRET instrument up to a few ${\rm GeV}$ during summer 1991 (Lin et al. 1992). Eight months later, the Whipple Observatory detected a clear signal from this object between 0.5 and $1.5\:{\rm TeV}$ (Punch et al. 1992). Since then, Mkn 421 has been confirmed many times as a VHE source by various atmospheric Cherenkov imaging telescopes, e.g. again by the Whipple Observatory (Krennrich et al. 1997), by the stereoscopic system of HEGRA (Petry et al. 1996) and by the CAT (Cherenkov Array at Thémis) experiment (Piron et al. 1998,1999b). Along with Markarian 501 (Mkn 501), Mkn 421 has thus become one of the two extragalactic sources of the Northern hemisphere which has been firmly established in the VHE range. It has been also one of the most studied blazars and the target of several multi-wavelength observation campaigns from the radio band to the $\gamma$-ray range (see, e.g., Macomb et al. 1995; Buckley et al. 1996; Takahashi et al. 1996; Takahashi et al. 1999, 2000; Charlot et al. 1998; Maraschi et al. 1999). Recently, a major step was achieved during the 1999-2000 winter by the CELESTE atmospheric Cherenkov sampling experiment, which detected Mkn 421 for the first time around $50~{\rm GeV}$, filling the last energy gap still remaining on this source between satellites and ground-based detectors. These observations, made in part simultaneously with the CAT telescope, have been presented by Holder et al. (2001).

In this paper we concentrate on the temporal variability and the spectral properties of Mkn 421 above $250~{\rm GeV}$, as seen by the CAT experiment since it began operation in autumn 1996. Section 2 describes the detector and the analysis methods used to extract the signal and the spectra. The Mkn 421 data sample, the light curves up to June 2000 and the corresponding spectra are presented in Sect. 3. We discuss our results in Sect. 4, comparing them with those from other ground-based atmospheric Cherenkov telescopes, and with those obtained on Mkn 501. The conclusions are given in Sect. 5.

Figure 1: The Mkn 421 $\gamma$-ray signal seen with the CAT imaging telescope, for the most significant observations (signal significance greater than $3\:\sigma$ in $\sim$ $30\:{\rm min}$) taken in 1997-98 and 1999-2000 at zenith angle $\theta _{\rm z}<45\hbox {$^\circ $ }$ (left panel), and for the highest flare, recorded between 4 and 5 February, 2000 (right panel). In each panel, the main plot shows the pointing angle, $\alpha$, distribution for ON (solid line) and OFF data (points with error bars, same duration as ON data), and the bottom-left inset gives the "ON-OFF'' distribution; events are selected by the $Q_{\rm 4}$, $Q_{{\rm tot}}$, and $\mathcal{P}(\chi^2)$ cuts (see text), and the values quoted in legend come from the additional cut $\alpha <6\hbox {$^\circ $ }$(first two bins). The bottom-right inset shows the significance map of event excess per bin of $0.05\hbox {$^\circ $ }$, obtained from the "ON-OFF'' distribution of reconstructed angular origins (no cut on $\alpha$): the source lies at the centre, and the axes denote the relative right ascension and declination with respect to the source coordinates ( $RA_{\rm S}$ and $\delta _{\rm S}$).