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^−2s^−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.
|Numero di pagine||5|
|Rivista||THE ASTROPHYSICAL JOURNAL LETTERS|
|Stato di pubblicazione||Published - 2010|
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science