PKS 1502+106: A high-redshift Fermi blazar at extreme angular resolution

Structural dynamics with VLBI imaging up to 86 GHz^⋆

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 Agenzia Spaziale Italiana (ASI) Science Data Center, 00133 Roma, Italy
⁴ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Roma, 00040 Monte Porzio Catone ( Roma), Italy
⁵ Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy

Received: 21 August 2015
Accepted: 29 October 2015

Abstract

Context. Blazars are among the most energetic objects in the Universe. In 2008 August, Fermi/LAT detected the blazar PKS 1502+106, which showed a rapid and strong γ-ray outburst followed by high and variable flux over the next months. This activity at high energies triggered an intensive multi-wavelength campaign that also covered the radio, optical, UV, and X-ray bands, indicating that the flare was accompanied by a simultaneous outburst at optical/UV/X-rays and a delayed outburst at radio bands.

Aims. We explore the phenomenology and physical conditions within the ultra-relativistic jet of the γ-ray blazar PKS 1502+106. Additionally, we address the question of the spatial localization of the MeV/GeV-emitting region of the source.

Methods. We used ultra-high angular resolution mm-VLBI observations at 43 and 86 GHz complemented by VLBI observations at 15 GHz. We also employed single-dish radio data from the F-GAMMA program at frequencies matching the VLBI monitoring.

Results. PKS 1502+106 shows a compact core-jet morphology and fast superluminal motion with apparent speeds in the range 5–22 c. Estimating Doppler factors along the jet yields values of between ~7 up to ~50. This Doppler factor gradient implies an accelerating jet. The viewing angle towards the source differs between the inner and outer jet, with the former at θ ~ 3° and the latter at θ ~ 1°, after the jet bends towards the observer beyond 1 mas. The de-projected opening angle of the ultra-fast magnetically dominated jet is found to be (3.8 ± 0.5)°. A single jet component can be associated with the pronounced flare both at high energies and in radio bands. Finally, the γ-ray emission region is localized at ≤ 5.9 pc away from the jet base.