Monitoring of GAmma-ray Bright AGN : The Multi-frequency Polarization of the Flaring Blazar 3C 279

Abstract

We present results of long-term multi-wavelength polarization observations of the powerful blazar 3C~279 after its $\gamma$-ray flare on 2013~December 20. We followed up this flare with single-dish polarization observations using two 21-m telescopes of the Korean VLBI Network. Observations carried out weekly from 2013~December~25 to 2015~January~11, at 22~GHz, 43~GHz, 86~GHz simultaneously, as part of the Monitoring Of GAmma-ray Bright AGN (MOGABA) program. We measured 3C~279 total flux densities of 22--34~Jy at 22~GHz, 15--28~Jy (43~GHz), and 10--21~Jy (86~GHz), showing mild variability of $\le 50\%$ over the period of our observations. The spectral index between 22~GHz and 86~GHz ranged from $-0.13$ to $-0.36$. Linear polarization angles were 27${}^{\circ }$--38${}^{\circ }$, 30${}^{\circ }$--42${}^{\circ }$, and 33${}^{\circ }$--50${}^{\circ }$ at 22~GHz, 43~GHz, and 86~GHz, respectively. The degree of linear polarization was in the range of 6--12\,\%, and slightly decreased with time at all frequencies. We investigated Faraday rotation and depolarization of the polarized emission at 22--86~GHz, and found Faraday rotation measures (RM) of $-300$ to $-1200$~rad~m${}^{-2}$ between 22~GHz and 43~GHz, and $-800$ to $-5100$~rad~m${}^{-2}$ between 43~GHz and 86~GHz. The RM values follow a power law with a mean power law index $a$ of $2.2$, implying that the polarized emission at these frequencies travels through a Faraday screen in or near the jet. We conclude that the regions emitting polarized radio emission may be different from the region responsible for the 2013 December $\gamma$-ray flare and are maintained by the dominant magnetic field perpendicular to the direction of the radio jet at milliarcsecond scales.

9 pages, 6 figures, 2 tables, accepted for publication in JKAS

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• Interferometric Monitoring of Gamma-ray Bright AGNs I: Results of Single-epoch Multifrequency Observations
• Revealing the Nature of Blazar Radio Cores through Multi-Frequency Polarization Observations with the Korean VLBI Network
• Probing the gamma-ray variability in 3C279 using broadband observations
• Linear Polarization of Class I Methanol Masers in Massive Star-Forming Regions
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• Constraining Einstein's Equivalence Principle With Multi-Wavelength Observations of Polarized Blazars
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