Long-term Monitoring on Mrk 501 for Its VHE gamma Emission and a Flare in October 2011

As one of the brightest active blazars in both X-ray and very high energy $\gamma$-ray bands, Mrk 501 is very useful for physics associated with jets from AGNs. The ARGO-YBJ experiment is monitoring it for $\gamma$-rays above 0.3 TeV since November 2007. Starting from October 2011 the largest flare since 2005 is observed, which lasts to about April 2012. In this paper, a detailed analysis is reported. During the brightest $\gamma$-rays flaring episodes from October 17 to November 22, 2011, an excess of the event rate over 6 $\sigma$ is detected by ARGO-YBJ in the direction of Mrk 501, corresponding to an increase of the $\gamma$-ray flux above 1 TeV by a factor of 6.6$\pm$2.2 from its steady emission. In particular, the $\gamma$-ray flux above 8 TeV is detected with a significance better than 4 $\sigma$. Based on time-dependent synchrotron self-Compton (SSC) processes, the broad-band energy spectrum is interpreted as the emission from an electron energy distribution parameterized with a single power-law function with an exponential cutoff at its high energy end. The average spectral energy distribution for the steady emission is well described by this simple one-zone SSC model. However, the detection of $\gamma$-rays above 8 TeV during the flare challenges this model due to the hardness of the spectra. Correlations between X-rays and $\gamma$-rays are also investigated.Comment: have been accepted for publication at Ap

Medium scale anisotropy in the TeV cosmic ray flux observed by ARGO-YBJ

Measuring the anisotropy of the arrival direction distribution of cosmic rays provides important information on the propagation mechanisms and the identification of their sources. In fact, the flux of cosmic rays is thought to be dependent on the arrival direction only due to the presence of nearby cosmic ray sources or particular magnetic-field structures. Recently, the observation of unexpected excesses at TeV energy down to an angular scale as narrow as raised the possibility that the problem of the origin of Galactic cosmic rays may be addressed by studying the anisotropy. The ARGO-YBJ experiment is a full-coverage extensive air showers array, sensitive to cosmic rays with the energy threshold of a few hundred GeV. Searching for small-size deviations from the isotropy, the ARGO-YBJ Collaboration explored the declination region , making use of about events collected from November 2007 to May 2012. In this paper, the detection of different significant (up to 13 standard deviations) medium-scale anisotropy regions in the arrival directions of cosmic rays is reported. The observation was performed with unprecedented detail. The relative excess intensity with respect to the isotropic flux extends up to . The maximum excess occurs for proton energies of 10–20 TeV, suggesting the presence of unknown features of the magnetic fields the charged cosmic rays propagate through, or some contribution of nearby sources never considered so far. The observation of new weaker few-degree excesses throughout the sky region is reported for the first time

OBSERVATION OF TeV GAMMA RAYS FROM THE UNIDENTIFIED SOURCE HESS J1841−055 WITH THE ARGO-YBJ EXPERIMENT

We report the observation of a very high energy γ -ray source whose position is coincident with HESS J1841−055. This source has been observed for 4.5 years by the ARGO-YBJ experiment from 2007 November to 2012 July. Its emission is detected with a statistical significance of 5.3 standard deviations. Parameterizing the source shape with a two-dimensional Gaussian function, we estimate an extension σ = (0.40+0.32 −0.22)◦, which is consistent with the HESS measurement. The observed energy spectrum is dN/dE = (9.0 ± 1.6) × 10−13(E/5 TeV)−2.32±0.23 photons cm−2 s−1 TeV−1, in the energy range 0.9–50 TeV. The integral γ -ray flux above 1 TeV is 1.3 ± 0.4 Crab, which is 3.2 ± 1.0 times the flux derived by HESS. The differences in the flux determination between HESS and ARGO-YBJ and possible counterparts at other wavelengths are discussed