| Abstract: | We report on variability and correlation studies using multiwavelength
observations of the blazar Mrk 421 during the month of February, 2010 when an
extraordinary flare reaching a level of $sim$27~Crab Units above 1~TeV was
measured in very-high-energy (VHE) $gamma$-rays with the VERITAS observatory.
This is the highest flux state for Mrk 421 ever observed in VHE $gamma$-rays.
Data are analyzed from a coordinated campaign across multiple instruments
including VHE $gamma$-ray (VERITAS, MAGIC), high-energy (HE) $gamma$-ray
(Fermi-LAT), X-ray (Swift}, RXTE, MAXI), optical (including the GASP-WEBT
collaboration and polarization data) and radio (Mets"ahovi, OVRO, UMRAO).
Light curves are produced spanning multiple days before and after the peak of
the VHE flare, including over several flare ’decline’ epochs. The main flare
statistics allow 2-minute time bins to be constructed in both the VHE and
optical bands enabling a cross-correlation analysis that shows evidence for an
optical lag of $sim$25-55 minutes, the first time-lagged correlation between
these bands reported on such short timescales. Limits on the Doppler factor
($delta gtrsim 33$) and the size of the emission region ($ delta^{-1}R_B
lesssim 3.8 imes 10^{13},,mbox{cm}$) are obtained from the fast
variability observed by VERITAS during the main flare. Analysis of
10-minute-binned VHE and X-ray data over the decline epochs shows an
extraordinary range of behavior in the flux-flux relationship: from linear to
quadratic to lack of correlation to anti-correlation. Taken together, these
detailed observations of an unprecedented flare seen in Mrk 421 are difficult
to explain by the classic single-zone synchrotron self-Compton model. |
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