Catching the radio flare in CTA 102

I. Light curve analysis

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: cfromm@mpifr.de
² Departament d’Astronomia i Astrofísica, Universitat de València, Dr. Moliner 50, 46100 Burjassot, València, Spain
³ Departament de Matemàtiques per a l’Economia i l’Empresa, Universitat de València, Av. Tarongers s/n, 46022 València, Spain
⁴ Department of Astronomy, University of Michigan, Dennison Building, Ann Arbor, MI 48109, USA
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Aalto University, Metsähovi Radio Observatory, 02540 Kylmälä, Finland

Received: 9 March 2011
Accepted: 4 May 2011

Abstract

Context. The blazar CTA 102 (z = 1.037) underwent a historical radio outburst in April 2006. This event offered a unique chance to study the physical properties of the jet.

Aims. We used multifrequency radio and mm observations to analyze the evolution of the spectral parameters during the flare as a test of the shock-in-jet model under these extreme conditions.

Methods. For the analysis of the flare we took into account that the flaring spectrum is superimposed on a quiescent spectrum. We reconstructed the latter from archival data and fitted a synchrotron self-absorbed distribution of emission. The uncertainties of the derived spectral parameters were calculated using Monte Carlo simulations. The spectral evolution is modeled by the shock-in-jet model, and the derived results are discussed in the context of a geometrical model (varying viewing angle) and shock-shock interaction

Results. The evolution of the flare in the turnover frequency-turnover flux density (ν_m − S_m) plane shows a double peak structure. The nature of this evolution is dicussed in the frame of shock-in-jet models. We discard the generation of the double peak structure in the ν_m − S_m plane purely based on geometrical changes (variation of the Doppler factor). The detailed modeling of the spectral evolution favors a shock-shock interaction as a possible physical mechanism behind the deviations from the standard shock-in-jet model.