Interferometric monitoring of gamma-ray bright AGNs: Measuring the magnetic field strength of 4C +29.45^⋆

¹ Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon 34055, Korea
e-mail: sslee@kasi.re.kr
² University of Science and Technology, Korea, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea
³ Department of Physics and Astronomy, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul, South Korea
⁴ Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.
⁷ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁸ Kogakuin University of Technology & Engineering, Academic Support Center, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
⁹ Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 08826, Korea
¹⁰ SNU Astronomy Research Center, Seoul National University, Gwanak-gu, Seoul 08826, Korea

Received: 22 December 2020
Accepted: 30 April 2021

Abstract

Aims. We present the results of multi-epoch, multifrequency monitoring of blazar 4C +29.45, which was regularly monitored as part of the Interferometric Monitoring of GAmma-ray Bright Active Galactic Nuclei (iMOGABA) program – a key science program of the Korean Very long baseline interferometry Network (KVN).

Methods. Observations were conducted simultaneously at 22, 43, 86, and 129 GHz over the 4 years from 5 December 2012 to 28 December 2016. We also used additional data from the 15 GHz Owens Valley Radio Observatory (OVRO) monitoring program.

Results. From the 15 GHz light curve, we estimated the variability timescales of the source during several radio flux enhancements. We found that the source experienced six radio flux enhancements with variability timescales of 9–187 days during the observing period, yielding corresponding variability Doppler factors of 9–27. From the simultaneous multifrequency KVN observations, we were able to obtain accurate radio spectra of the source and hence to more precisely measure the turnover frequencies ν_r, of synchrotron self-absorption (SSA) emission with a mean value of ν_r̅ = 28.9 GHz. Using jet geometry assumptions, we estimated the size of the emitting region at the turnover frequency. We found that the equipartition magnetic field strength is up to two orders of magnitude higher than the SSA magnetic field strength (0.001–0.1 G). This is consistent with the source being particle dominated. We performed a careful analysis of the systematic errors related to the making of these estimations.

Conclusions. From the results, we concluded that the equipartition region is located upstream from the SSA region.