The June 2008 Flare of Markarian 421 from Optical to Tev Energies

We present optical, X-ray, high energy (/ 30 GeV) and very high energy (' 100 GeV; VHE) observations of the high-frequency peaked blazar Mrk 421 taken between 2008 May 24 and June 23. A high energy γ-ray signal was detected by AGILE with √ TS = 4.5 on June 9–15, with F (E > 100 MeV) = 42 −12 × 10 photons cm s. This flaring state is brighter than the average flux observed by EGRET by a factor of ∼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 ∼ 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 ×10 erg cm s (i.e. > 100 mCrab). A peak synchrotron energy of ∼3 keV was derived, higher than typical values of ∼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 γ-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. 2 I. Donnarumma et al. Subject headings: BL Lacertae objects: individual (Mrk 421); radiation mechanisms: non-thermal; X-rays: galaxies; gamma-rays: observations; galaxies: jets