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

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).\nMethods. 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.\nResults. 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 νr, of synchrotron self-absorption (SSA) emission with a mean value of νr̅ = 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.\nConclusions. From the results, we concluded that the equipartition region is located upstream from the SSA region.