VLBA monitoring of Mrk 421 at 15 GHz and 24 GHz during 2011^⋆,⋆⋆

¹ INAF Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
e-mail: rocco.lico@studio.unibo.it
² Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ National Radio Astronomy Observatory, Charlottesville, 520, Edgemont Road, VA 22903-2475, USA
⁴ CSIRO Australia Telescope National Facility, Locked Bag 194, Narrabri NSW 2390, Australia
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
⁷ Sternberg Astronomical Institute, Moscow State University, Universitetskij prosp. 13, 119992 Moscow, Russia
⁸ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁹ Astronomical Institute, St. Petersburg State University, Universitetskij Pr. 28, 198504 St. Petersburg, Russia
¹⁰ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, 181-8588 Tokyo, Japan
¹¹ Max-Planck-Institut für Physik, Föhringer Ring 6, 80805 München, Germany
¹² Instituto de Astrofísica de Andalucia, IAA-CSIC, Apdo. 3004, 18080 Granada, Spain
¹³ Department of Physics and Astronomy, Whittier College, 13406 E. Philadelphia Street, Whittier, CA 90608, USA

Received: 22 June 2012
Accepted: 27 July 2012

Abstract

Context. High-resolution radio observations are ideal for constraining the value of physical parameters in the inner regions of active-galactic-nucleus jets and complement results on multiwavelength (MWL) observations. This study is part of a wider multifrequency campaign targeting the nearby TeV blazar Markarian 421 (z = 0.031), with observations in the sub-mm (SMA), optical/IR (GASP), UV/X-ray (Swift, RXTE, MAXI), and γ rays (Fermi-LAT, MAGIC, VERITAS).

Aims. We investigate the jet’s morphology and any proper motions, and the time evolution of physical parameters such as flux densities and spectral index. The aim of our wider multifrequency campaign is to try to shed light on questions such as the nature of the radiating particles, the connection between the radio and γ-ray emission, the location of the emitting regions and the origin of the flux variability.

Methods. We consider data obtained with the Very Long Baseline Array (VLBA) over twelve epochs (one observation per month from January to December 2011) at 15 GHz and 24 GHz. We investigate the inner jet structure on parsec scales through the study of model-fit components for each epoch.

Results. The structure of Mrk 421 is dominated by a compact (~0.13 mas) and bright component, with a one-sided jet detected out to  ~10 mas. We identify 5–6 components in the jet that are consistent with being stationary during the 12-month period studied here. Measurements of the spectral index agree with those of other works: they are fairly flat in the core region and steepen along the jet length. Significant flux-density variations are detected for the core component.

Conclusions. From our results, we draw an overall scenario in which we estimate a viewing angle 2° < θ < 5° and a different jet velocity for the radio and the high-energy emission regions, such that the respective Doppler factors are δ_r ~ 3 and δ_h.e. ~ 14.