GAMMA-RAY FLARES FROM Mrk421 IN 2008 OBSERVED WITH THE ARGO-YBJ DETECTOR

Abstract

In 2008, the blazar Markarian 421 entered a very active phase and was one of the brightest sources in the sky at TeV energies, showing frequent flaring episodes. Using the data of ARGO-YBJ, a full coverage air shower detector located at Yangbajing (4300 m a.s.l., Tibet), we monitored the source at gamma-ray energies E > 0.3 TeV during the whole year. The observed flux was variable, with the strongest flares in March and June, in correlation with X-ray enhanced activity. While during specific episodes the TeV flux could be several times larger than the Crab Nebula one, the average emission from day 41 to 180 was almost twice the Crab level, with an integral flux of (3.6 ± 0.6) × 10 −11 photons cm −2 s −1 for energies E > 1 TeV, and decreased afterward. This Letter concentrates on the flares that occurred in the first half of June. This period has been deeply studied from optical to 100 MeV gamma rays, and partially up to TeV energies, since the moonlight hampered the Cherenkov telescope observations during the most intense part of the emission. Our data complete these observations, with the detection of a signal with a statistical significance of 3.8 standard deviations on June 11-13, corresponding to a gamma-ray flux about 6 times larger than the Crab one above 1 TeV. The reconstructed differential spectrum, corrected for the intergalactic absorption, can be represented by a power law with an index α = −2.1 +0.7 −0.5 extending up to several TeV. The spectrum slope is fully consistent with previous observations reporting a correlation between the flux and the spectral index, suggesting that this property is maintained in different epochs and characterizes the source emission processes.

Early warning for VHE gamma-ray flares with the ARGO-YBJ detector

“…The resulting spectrum is (4.2 ± 0.4 stat ) × 10 −11 (E/TeV) −2.57±0.09 stat photons TeV −1 cm −2 s −1 , which is in agreement with our previous measurement (Aielli et al 2010) and observations by other detectors, such as H.E.S.S. (Aharonian et al 2006), MAGIC (Albert et al 2008), and Tibet AS-γ (Amenomori et al 2009).…”

“…Four large flares are observed by all three detectors, and the peak times are in good agreement with one another. The fourth flare has been reported by the ARGO-YBJ experiment in Aielli et al (2010). It gives an important observation when the Cherenkov telescopes are hampered by the Moon.…”

“…To estimate the spectrum of γ -rays with a distribution of the number of events in excess as a function of N pad , we follow a widely used method that is described in detail elsewhere (Amenomori et al 2009;Aielli et al 2010). In this procedure, we assume for the spectrum of Mrk 421 a power law with a cutoff factor e −τ (E) , which takes into account the absorption of γ -rays in the extragalactic background light.…”

“…Although X-rays and γ -rays are found to be strongly correlated, neither type is evidently correlated with optical and radio emissions. The spectral index becomes harder at higher fluxes in both X-ray and γ -ray bands (Rebillot et al 2006;Krennrich et al 2002;Aielli et al 2010). An intensive multiwavelength monitoring campaign has recently been conducted with the Whipple telescope and the Rossi X-ray Timing Explorer (RXTE; Blazejowski et al 2005).…”

4.5 Years of Multi-Wavelength Observations of MRK 421 During the Argo-Ybj and Fermi Common Operation Time

“…It is a very active blazar with major outbursts, composed of many short flares, about once every two years, in both X-rays and γ-rays (Aielli et al 2010;Bartoli et al 2011a;Chen 2013). Actually, it is considered an excellent candidate to study the physical processes within the AGN jets.…”