Multi-frequency study of the gamma-ray flaring BL Lac object PKS 2233-148 in 2009-2012

TL;DR

This study combines radio and gamma-ray observations to analyze the jet physics of BL Lac object PKS 2233-148 during a flare, revealing jet structure, opacity, and gamma-ray emission zones, and suggesting dominant emission mechanisms.

Contribution

It provides the first multi-frequency VLBA analysis of PKS 2233-148 during a gamma-ray flare, identifying the gamma-ray production zone and jet properties with high precision.

Findings

Jet is conical on parsec scales.

Gamma-ray emission zone is 10-20 pc from the core.

Propagation speed of flare-related features is about 1.2 mas/yr.

Abstract

We study the jet physics of the BL Lac object PKS 2233-148 making use of synergy of observational data sets in the radio and gamma-ray energy domains. The four-epoch multi-frequency (4-43 GHz) VLBA observations focused on the parsec-scale jet were triggered by a flare in gamma-rays registered by the Fermi-LAT on April 23, 2010. We also used 15 GHz data from the OVRO 40-m telescope and MOJAVE VLBA monitoring programs. Jet shape of the source is found to be conical on scales probed by the VLBA observations setting a lower limit of about 0.1 on its unknown redshift. Nuclear opacity is dominated by synchrotron self-absorption, with a wavelength-dependent core shift $r_{\text{core[mas]}}\approx0.1\lambda_{[\text{cm}]}$ co-aligned with the innermost jet direction. The turnover frequency of the synchrotron spectrum of the VLBI core shifts towards lower frequencies as the flare propagates down the jet, and the speed of this propagation is significantly higher, about 1.2 mas/yr, comparing to results from traditional kinematics based on tracking bright jet features. We have found indications that the gamma-ray production zone in the source is located at large distances, 10-20 pc, from a central engine, and could be associated with the stationary jet features. These findings favour synchrotron self-Compton, possibly in a combination with external Compton scattering by infrared seed photons from a slow sheath of the jet, as a dominant high-energy emission mechanism of the source.