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 ( Metsähovi , 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 \times 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 .