An Exceptional Radio Flare in Markarian 421
  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.
