Very Long Baseline Polarimetric monitoring at 15 GHz of the TeV blazar Markarian 421

¹ INAF Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
² Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Instituto de Astrofísica de Andalucia, IAA-CSIC, Apdo. 3004, 18080 Granada, Spain
⁴ National Radio Astronomy Observatory, Charlottesville, 520 Edgemont Road, VA 22903-2475, USA
⁵ CSIRO Australia Telescope National Facility, Locked Bag 194, Narrabri NSW 2390, Australia
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁷ Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
⁸ Sternberg Astronomical Inst., Moscow State University, Universitetskij prosp. 13, 119992 Moscow, Russia
⁹ Institute for Astrophysical Research, Boston University, 725 Commonwealth Av. Boston, MA 02215, USA
¹⁰ Astronomical Inst. , St. Petersburg State University, Universitetskij Pr.28, 198504 St. Petersburg, Russia
¹¹ ISAS/JAXA, 3-1-1 Yoshinodai, 229-8510 Sagamihara, Japan
¹² Max-Planck-Institut für Physik, Föhringer Ring 6, D-80805 München, Germany
¹³ Dep. of Physics and Astronomy, Whittier College, 13406 E. Philadelphia Street, Whittier, CA 90608, USA

^a e- mail: rocco.lico@studio.unibo.it

Published online: 9 December 2013

Abstract

Thanks to high resolution radio observations it is possible to obtain a direct imaging of the innermost regions of Active Galactic Nuclei; in particular, it is possible to investigate about the jet’s morphology and any proper motions, and the time evolution of physical parameters, such as flux densities and spectral index. Furthermore, with the study of the polarization properties, it is possible to obtain important information about the magnetic field structure and the emission mechanisms. In this work we present recent results about the nearby (z=0.031) TeV blazar Mrk 421. We analyzed data obtained with the Very Long Baseline Array (VLBA),both in total and polarized intensity, at twelve epochs (one observation per month from January to December 2011) at 15, 24 and 43 GHz, in the context of a broadband campaign from the radio to gamma-ray. We investigate the inner jet structure on parsec scale through the study of model-fit components for each epoch. At these frequencies the source shows a compact (about 0.13 mas, or 0.08 pc) and bright component, with a one sided jet detected out to about 10 mas. All model-fit components in the jet appear to be almost stationary during our observation period, and the spectral index is fairly flat in the core region and steepens along the jet’s length. In particular, we present a preliminary study of the polarization properties for the 15 GHz dataset: we found a degree of polarization of ∼ 1% for the core region and for the C3 component, at near 1 mas from the core, we found a value of near 14%.