Gamma-Ray Flares from Mrk421 in 2008 observed with the ARGO-YBJ experiment

TL;DR

This study reports on gamma-ray flares from Mrk421 in 2008 observed with ARGO-YBJ, revealing correlated multi-wavelength activity, a second flare detection, and consistency with SSC model predictions.

Contribution

First comprehensive gamma-ray observation of Mrk421's 2008 flaring activity with ARGO-YBJ, including detection of a second flare and multi-wavelength correlation analysis.

Findings

Average emission was about twice the Crab Nebula level.

Detected a second flare with 4.2 sigma significance.

Flux consistent with Synchrotron Self-Compton model predictions.

Abstract

In 2008, the blazar Mrk421 entered in a very active phase and was one of the brightest sources in the sky at TeV energies, showing strong and frequent flaring. We searched for gamma-ray emission at energies E > 0.8 TeV during the whole 2008 with the ARGO-YBJ experiment, a full coverage air shower detector located at Yangbajing (4300 m a.s.l., Tibet, P.R. China). The observed signal is not constant and in correlation with X-ray measurements. The average emission, during the active period of the source, was about twice the Crab Nebula level, with an integral flux of (4.9$\pm 2.0$)x 10$^{-11}$ photons cm$^{-2}$ s$^{-1}$ for E$_{\gamma}$ >1 TeV. This paper concentrates on 2008 June when the Mrk421 flaring activity has been studied from optical to 100 MeV gamma rays, and only partially up to TeV energies, since the moonlight hampered the Cherenkov telescope observations after 8 June. Our data complete these observations, with the detection of a second flare of intensity of about 7 Crab units on June 11-13, with a statistical significance of 4.2 standard deviations. The observed flux is consistent with a prediction made in the framework of the Synchrotron Self-Compton model, in which the flare is caused by a rapid acceleration of leptons in the jet.