We present optical , X-ray , high energy ( \lessapprox 30 GeV ) and very high energy ( \gtrapprox 100 GeV ; VHE ) observations of the high-frequency peaked blazar Mrk 421 taken between 2008 May 24 and June 23 .
A high energy \gamma -ray signal was detected by AGILE with \sqrt { TS } = 4.5 on June 9–15 , with F ( E > 100 ~ { } \mathrm { MeV } ) = 42 ^ { +14 } _ { -12 } \times 10 ^ { -8 } photons cm ^ { -2 } s ^ { -1 } .
This flaring state is brighter than the average flux observed by EGRET by a factor of \sim 3 , but still consistent with the highest EGRET flux .
In hard X-rays ( 20-60 keV ) SuperAGILE resolved a 5-day flare ( June 9-15 ) peaking at \sim 55 mCrab .
SuperAGILE , RXTE/ASM and Swift/BAT data show a correlated flaring structure between soft and hard X-rays .
Hints of the same flaring behavior are also detected in the simultaneous optical data provided by the GASP-WEBT .
A Swift/XRT observation near the flaring maximum revealed the highest 2-10 keV flux ever observed from this source , of 2.6 \times 10 ^ { -9 } erg cm ^ { -2 } s ^ { -1 } ( i.e . > 100 mCrab ) .
A peak synchrotron energy of \sim 3 keV was derived , higher than typical values of \sim 0.5-1 keV .
VHE observations with MAGIC and VERITAS on June 6-8 show the flux peaking in a bright state , well correlated with the X-rays .
This extraordinary set of simultaneous data , covering a twelve-decade spectral range , allowed for a deep analysis of the spectral energy distribution as well as of correlated light curves .
The \gamma -ray flare can be interpreted within the framework of the synchrotron self-Compton model in terms of a rapid acceleration of leptons in the jet .