What can the 2008/10 broadband flare of PKS 1502+106 tell us?

Nuclear opacity, magnetic fields, and the location of γ rays

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: vkaramanavis@mpifr.de
² Instituto de Radio Astronomía Milimétrica, Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
³ Harvard–Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA

Received: 20 November 2015
Accepted: 11 March 2016

Abstract

Context. The origin of blazar variability, as seen from radio up to γ rays, is still a heavily debated matter, and broadband flares offer a unique testbed for developing a better understanding of these extreme objects. One of these energetic outbursts was detected by Fermi/LAT in 2008 from the blazar PKS 1502+106. The outburst was observed from γ rays down to radio frequencies.

Aims. Through the delay between flare maxima at different radio frequencies, we study the frequency-dependent position of the unit-opacity surface and infer its absolute position with respect to the jet base. This nuclear opacity profile enables the jet’s magnetic field tomography. We also localize the γ-ray emission region and explore the flare production mechanism.

Methods. The PKS 1502+106 radio flare is studied through single-dish flux density measurements at 12 frequencies in the range 2.64 to 226.5 GHz. To quantify the flare, we employ both a Gaussian process regression and a discrete cross-correlation function analysis.

Results. We find that the light curve parameters (flare amplitude and cross-band delays) show a power-law dependence on frequency. Delays decrease with frequency, and the flare amplitudes increase up to about 43 GHz, and then decay. This behavior is consistent with the propagation of a shock downstream in the jet. The self-absorbed radio cores are located between approximately ten and four pc from the jet base, and their magnetic field strengths range between 14 and 176 mG, at the frequencies 2.64 to 86.24 GHz. Finally, the γ-ray active region is located at (1.9 ± 1.1) pc away from the jet base.