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Astron. Astrophys. 339, 41-51 (1998)

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1. Introduction

Mkn 421 (B2 1101+38) is a BL Lacertae (BL Lac) object extensively observed at all wavelengths and characterized by a strong variability in the optical region (Miller 1975, Liu et al. 1997). The surrounding envelope is a typical giant elliptical galaxy with redshift z = 0.03 (Ulrich et al. 1975).

Mkn 421 is very active at high energies, and it was the first extragalactic source discovered by the Whipple Observatory to emit TeV [FORMULA]-rays (Punch et al., 1992). The spectral energy distribution (SED) shows a double-peaked feature with two maxima (see, e. g. Buckley et al. 1996). The first hump has a maximum around UV-Soft X frequencies and is generally explained as the synchrotron emission of relativistic electrons in a well-collimated jet beamed toward the observer. The second hump is at [FORMULA]-ray frequencies and is generally explained as the inverse Compton scattering of low-energy photons from the relativistic electrons in the jet. The radiation field involved in this process can be the same synchrotron emission of the jet, in the so-called SSC model (see, e. g., Brodie et al. 1987, Makino et al. 1987, George et al. 1988, Mufson et al. 1990), otherwise the photons involved in the inverse Compton emission may come from external radiation field (see e. g., Dermer & Schlickeiser 1993). A different model explains the high power emitted at [FORMULA]-rays with a cascade process beginning with photomesons production by very high-energy protons accelerated by shock waves in the jet (see, e. g., Mannheim 1993).

Mkn 421 is extensively observed at high-energies, especially in the keV and TeV regions, which are the final tails of the two components in the SED. Optical and radio observations are generally used only for a comparison with the rapid high-energy flares, and there is a lack in the analysis of its variability at larger temporal scales, although Mkn 421 is a well known variable source in the optical. For this reason we collected many optical and radio observations during the last years, with the aim of analysing variability in the low-energy part of the synchrotron component. We also obtained photopolarimetric observations since the polarization of synchrotron radiation gives important informations on the source properties.

In this paper we present radio, optical and photopolarimetric observations of Mkn 421 since October 1994. In Sect. 2 we describe the observations and the analysis procedure adopted. We discuss the results in Sect. 3. Conclusions will follow in Sect. 4.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998