The June 2008 flare of Markarian 421 from optical to TeV energies

TL;DR

This paper reports multi-wavelength observations of the 2008 flare of Mrk 421, revealing correlated optical to TeV emissions and providing insights into rapid particle acceleration in the jet.

Contribution

First simultaneous optical to TeV observations of Mrk 421 during a flare, demonstrating correlated multi-band variability and supporting the SSC model of jet emission.

Findings

Detected a high-energy gamma-ray flare brighter than previous observations.

Observed the highest 2-10 keV flux ever recorded for Mrk 421.

Found correlated variability across X-ray, optical, and gamma-ray bands.

Abstract

We present optical to very-high energy (VHE) gamma-ray observations of Mrk 421 between 2008 May 24 and June 23. A high-energy (HE) gamma-ray signal was detected by AGILE-GRID during June 9-15, brighter than the average flux observed by EGRET in Mrk 421 by a factor of approx. 1.5. In 20-60 keV X-rays, a large-amplitude 5-day flare (June 9-15) was resolved with a maximum flux of approx. 55 mCrab. SuperAGILE, RXTE/ASM and Swift/BAT data show a clearly correlated flaring structure between soft and hard X-rays, with a high flux/amplitude variability in hard X-rays. Hints of the same flaring behavior is also detected in the simultaneously recorded GASP-WEBT optical data. A target of opportunity observation by Swift near the flare maximum on June 12-13 revealed the highest 2-10 keV flux ever observed (>100 mCrab) and a peak synchrotron energy of approx. 3 keV, a large shift from typical values of 0.5-1 keV. Observations at VHE (E>200 GeV) gamma-rays during June 6-8 show the source flux peaking in a bright state, well correlated with the simultaneous peak in the X-rays. 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.