The millimeter-radio emission of BL lacertae during two γ-ray outbursts

Dae Won Kim, Sascha Trippe, Sang Sung Lee, Jong Ho Park, Jae Young Kim, Juan Carlos Algaba, Jeffrey A. Hodgson, Motoki Kino, Guang Yao Zhao, Kiyoaki Wajima, Sincheol Kang, Junghwan Oh, Taeseok Lee, Do Young Byun, Soon Wook Kim, Jeong Sook Kim

Research output: Contribution to journalArticlepeer-review

4 Scopus citations

Abstract

We present a study of the inexplicit connection between radio jet activity and γ-ray emission of BL Lacertae (BL Lac; 2200+420). We analyze the long-term millimeter activity of BL Lac via interferometric observations with the Korean VLBI Network (KVN) obtained at 22, 43, 86, and 129 GHz simultaneously over three years (from January 2013 to March 2016); during this time, two γ-ray outbursts (in November 2013 and March 2015) can be seen in γ-ray light curves obtained from Fermi observations. The KVN radio core is optically thick at least up to 86 GHz; there is indication that it might be optically thin at higher frequencies. To first order, the radio light curves decay exponentially over the time span covered by our observations, with decay timescales of 411±85 days, 352±79 days, 310±57 days, and 283±55 days at 22, 43, 86, and 129 GHz, respectively. Assuming synchrotron cooling, a cooling time of around one year is consistent with magnetic field strengths B ~ 2 μT and electron Lorentz factors γ ~ 10 000. Taking into account that our formal measurement errors include intrinsic variability and thus over-estimate the statistical uncertainties, we find that the decay timescale τ scales with frequency ν like τ ∝ ν –0:2. This relation is much shallower than the one expected from opacity effects (core shift), but in agreement with the (sub-)mm radio core being a standing recollimation shock. We do not find convincing radio flux counterparts to the γ-ray outbursts. The spectral evolution is consistent with the 'generalized shock model' of Valtaoja et al. (1992). A temporary increase in the core opacity and the emergence of a knot around the time of the second γ-ray event indicate that this γ-ray outburst might be an `orphan' are powered by the `ring of fire' mechanism.

Original languageEnglish
Pages (from-to)167-178
Number of pages12
JournalJournal of the Korean Astronomical Society
Volume50
Issue number6
DOIs
StatePublished - Dec 2017

Bibliographical note

Publisher Copyright:
© 2017. The Korean Astronomical Society. All rights reserved.

Keywords

  • BL Lacertae objects: individual (BL Lacertae)
  • Galaxies: active
  • Galaxies: jets
  • Radio continuum: galaxies
  • Techniques: interferometric

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