Unusual flaring activity in the blazar PKS 1424−418 during 2008−2011

¹ Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
e-mail: sara.buson@pd.infn.it
² Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, 35131 Padova, Italy
³ Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, 34127 Trieste, Italy
e-mail: francesco.longo@trieste.infn.it
⁴ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁵ Department of Physics, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
⁶ The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, 106 91 Stockholm, Sweden
⁷ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
⁸ Agenzia Spaziale Italiana (ASI) Science Data Center, via del Politecnico, 00133 Roma, Italy
⁹ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Roma, 00040 Monte Porzio Catone, Roma, Italy
¹⁰ Space Science Division, Naval Research Laboratory, Washington DC 20375-5352, USA
¹¹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹² INAF Istituto di Radioastronomia, 40129 Bologna, Italy
¹³ Center for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁴ Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁵ Landessternwarte, Universitat Heidelberg, Konigstuhl, 69117 Heidelberg, Germany
¹⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁷ The Catholic University of America, 620 Michigan Ave., N.E. Washington, DC 20064, USA
¹⁸ Institut für Theoretische Physik and Astrophysik, Universität Würzburg, 97074 Würzburg, Germany
¹⁹ Dr. Remeis-Sternwarte Bamberg, Sternwartstrasse 7, 96049 Bamberg, Germany
²⁰ Erlangen Centre for Astroparticle Physics, 91058 Erlangen, Germany
²¹ Universities Space Research Association (USRA), Columbia, MD 21044, USA
²² CSIRO Astronomy and Space Science, ATNF, Locked Bag 194, Narrabri NSW 2390, Australia
²³ School of Mathematics & Physics, Private Bag 37, University of Tasmania, Hobart TAS 7001, Australia
²⁴ CSIRO Astronomy and Space Science, ATNF, PO Box 76, Epping NSW 1710, Australia
²⁵ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
²⁶ Space Science Institute, Boulder, CO, USA
²⁷ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Received: 1 January 2014
Accepted: 28 June 2014

Abstract

Context. Blazars are a subset of active galactic nuclei (AGN) with jets that are oriented along our line of sight. Variability and spectral energy distribution (SED) studies are crucial tools for understanding the physical processes responsible for observed AGN emission.

Aims. We report peculiar behavior in the bright γ-ray blazar PKS 1424−418 and use its strong variability to reveal information about the particle acceleration and interactions in the jet.

Methods. Correlation analysis of the extensive optical coverage by the ATOM telescope and nearly continuous γ-ray coverage by the Fermi Large Area Telescope is combined with broadband, time-dependent modeling of the SED incorporating supplemental information from radio and X-ray observations of this blazar.

Results. We analyse in detail four bright phases at optical-GeV energies. These flares of PKS 1424−418 show high correlation between these energy ranges, with the exception of one large optical flare that coincides with relatively low γ-ray activity. Although the optical/γ-ray behavior of PKS 1424−418 shows variety, the multiwavelength modeling indicates that these differences can largely be explained by changes in the flux and energy spectrum of the electrons in the jet that are radiating. We find that for all flares the SED is adequately represented by a leptonic model that includes inverse Compton emission from external radiation fields with similar parameters.

Conclusions. Detailed studies of individual blazars like PKS 1424−418 during periods of enhanced activity in different wavebands are helping us identify underlying patterns in the physical parameters in this class of AGN.