Simultaneous radio-interferometric and high-energy TeV observations of the γ-ray blazar Mkn 421

¹ Observatoire de Bordeaux (OASU) – CNRS/UMR 5804, BP 89, 33270 Floirac, France e-mail: charlot@obs.u-bordeaux1.fr
² University College Cork – Physics Department, Cork, Ireland
³ Observatoire de Paris – CNRS/UMR 8631, 92195 Meudon Cedex, France
⁴ École Polytechnique – Laboratoire Leprince-Ringuet, 91128 Palaiseau, France
⁵ Université de Montpellier II, LPTA – CNRS/UMR 5207, Place Bataillon, 34095 Montpellier Cedex 5, France

Received: 20 August 2005
Accepted: 21 April 2006

Abstract

The TeV-emitting BL Lac object Mkn 421 was observed with very long baseline interferometry (VLBI) at three closely-spaced epochs one-month apart in March–April 1998. The source was also monitored at very-high γ-ray energies (TeV measurements) during the same period in an attempt to search for correlations between TeV variability and the evolution of the radio morphology on parsec scales. While the VLBI maps show no temporal changes in the Mkn 421 VLBI jet, there is strong evidence of complex variability in both the total and polarized fluxes of the VLBI core of Mkn 421 and in its spectrum over the two-month span of our data. The high-energy measurements indicate that the overall TeV activity of the source was rising during this period, with a γ-ray flare detected just three days prior to our second VLBI observing run. Although no firm correlation can be established, our data suggest that the two phenomena (TeV activity and VLBI core variability) are connected, with the VLBI core at 22 GHz being the self-absorbed radio counterpart of synchrotron self-Compton (SSC) emission at high energies. Based on the size of the VLBI core, we could derive an upper limit of 0.1 pc (3  10¹⁷ cm) for the projected size of the SSC zone. This determination is the first model-free estimate of the size of the γ-ray emitting region in a blazar.