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In September 2012, the high-synchrotron-peaked (HSP) blazar Markarian 421 underwent a rapid wideband radio flare, reaching nearly twice the brightest level observed in the centimeter band in over three decades of monitoring. In response to this event we carried out a five epoch centimeter- to millimeter-band multifrequency Very Long Baseline Array (VLBA) campaign to investigate the aftermath of this emission event. Rapid radio variations are unprecedented in this object and are surprising in an HSP BL Lac object. In this flare, the 15 GHz flux density increased with an exponential doubling time of about 9 days, then faded to its prior level at a similar rate. This is comparable with the fastest large-amplitude centimeter-band radio variability observed in any blazar. Similar flux density increases were detected up to millimeter bands. This radio flare followed about two months after a similarly unprecedented GeV gamma-ray flare (reaching a daily E>100 MeV flux of (1.2 +/- 0.7)x10^(-6) ph cm^(-2) s^(-1)) reported by the Fermi Large Area Telescope (LAT) collaboration, with a simultaneous tentative TeV detection by ARGO-YBJ. A cross-correlation analysis of long-term 15 GHz and LAT gamma-ray light curves finds a statistically significant correlation with the radio lagging ~40 days behind, suggesting that the gamma-ray emission originates upstream of the radio emission. Preliminary results from our VLBA observations show brightening in the unresolved core region and no evidence for apparent superluminal motions or substantial flux variations downstream., 5 pages, 8 figures. Contributed talk at the meeting "The Innermost Regions of Relativistic Jets and Their Magnetic Fields", Granada, Spain. Updated to correct author list and references

The Fermi LAT detected an increase in γ-ray activity of the quasar 0836+710 (z=2.17) in Spring 2011 that culminated in a sharp γ-ray flare at the end of 2011 when the source reached a flux of 2.9×10−6 phot s−1cm−2 at 0.1-200 GeV. We monitor the quasar at optical wavelengths in photometric and polarimetric modes, at millimeter and centimeter wavelengths, and with the VLBA at 43 GHz. The optical brightness of the quasar increased by ~0.5 mag in R band and the degree of polarization oscillated between ~1% and ~6% during the highest γ-ray state, while the position angle of polarization rotated by ~300°. We have identified in the VLBA images a strong, highly polarized component that moves with an apparent speed of ~20 c. The component emerged from the core in the beginning of the γ-ray event and reached a flux maximum at the peak of the γ-ray outburst. We present the results of a correlative analysis of variations at different wavelengths along with the kinematic parameters of the parsec scale jet. We discuss the location of the high γ-ray emission in the relativistic jet, as well as the emission mechanisms responsible for γ-ray production.

Studying the variability of polarized AGN jet emission in the radio band is crucial for understanding the dynamics of moving shocks as well as the structure of the underlying magnetic field. The 100-m Effelsberg Telescope is a high-quality instrument for studying the long-term variability of both total and polarized intensity as well as the electric-vector position angle. Since 2007, the F-GAMMA program has been monitoring the linear polarized emission of roughly 60 blazars at 11 frequencies between 2.7 and 43 GHz. Here, we describe the calibration of the polarimetric data at 5 and 10 GHz and the resulting F-GAMMA full-Stokes light curves for the exemplary case of the radio galaxy 3C 111.