The ARGO-YBJ experiment is a full-coverage air shower detector located at the Yangbajing Cosmic Ray Observatory (Tibet, Peoples Republic of China, 4300 m a.s.l.). The high altitude, combined with the full-coverage technique, allows the detection of e
xtensive air showers in a wide energy range and offer the possibility of measuring the cosmic ray proton plus helium spectrum down to the TeV region, where direct balloon/space-borne measurements are available. The detector has been in stable data taking in its full configuration from November 2007 to February 2013. In this paper the measurement of the cosmic ray proton plus helium energy spectrum is presented in the region 3-300 TeV by analyzing the full collected data sample. The resulting spectral index is $gamma = -2.64 pm 0.01$. These results demonstrate the possibility of performing an accurate measurement of the spectrum of light elements with a ground based air shower detector.
The extended TeV gamma-ray source ARGO J2031+4157 (or MGRO J2031+41) is positionally consistent with the Cygnus Cocoon discovered by $Fermi$-LAT at GeV energies in the Cygnus superbubble. Reanalyzing the ARGO-YBJ data collected from November 2007 to
January 2013, the angular extension and energy spectrum of ARGO J2031+4157 are evaluated. After subtracting the contribution of the overlapping TeV sources, the ARGO-YBJ excess map is fitted with a two-dimensional Gaussian function in a square region of $10^{circ}times 10^{circ}$, finding a source extension $sigma_{ext}$= 1$^{circ}$.8$pm$0$^{circ}$.5. The observed differential energy spectrum is $dN/dE =(2.5pm0.4) times 10^{-11}(E/1 TeV)^{-2.6pm0.3}$ photons cm$^{-2}$ s$^{-1}$ TeV$^{-1}$, in the energy range 0.2-10 TeV. The angular extension is consistent with that of the Cygnus Cocoon as measured by $Fermi$-LAT, and the spectrum also shows a good connection with the one measured in the 1-100 GeV energy range. These features suggest to identify ARGO J2031+4157 as the counterpart of the Cygnus Cocoon at TeV energies. The Cygnus Cocoon, located in the star-forming region of Cygnus X, is interpreted as a cocoon of freshly accelerated cosmic rays related to the Cygnus superbubble. The spectral similarity with Supernova Remnants indicates that the particle acceleration inside a superbubble is similar to that in a SNR. The spectral measurements from 1 GeV to 10 TeV allows for the first time to determine the possible spectrum slope of the underlying particle distribution. A hadronic model is adopted to explain the spectral energy distribution.
The ARGO-YBJ detector is an extensive air shower array that has been used to monitor the northern $gamma$-ray sky at energies above 0.3 TeV from 2007 November to 2013 January. In this paper, we present the results of a sky survey in the declination b
and from $-10^{circ}$ to $70^{circ}$, using data recorded over the past five years. With an integrated sensitivity ranging from 0.24 to $sim$1 Crab units depending on the declination, six sources have been detected with a statistical significance greater than 5 standard deviations. Several excesses are also reported as potential $gamma$-ray emitters. The features of each source are presented and discussed. Additionally, $95%$ confidence level upper limits of the flux from the investigated sky region are shown. Specific upper limits for 663 GeV $gamma$-ray AGNs inside the ARGO-YBJ field of view are reported. The effect of the absorption of $gamma$-rays due to the interaction with extragalactic background light is estimated.
We report the observation of a very high energy gamma-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 November 2007 to July 2012. 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 sigma=(0.40(+0.32,-0.22}) degree, consistent with the HESS measurement. The observed energy spectrum is dN/dE =(9.0-+1.6) x 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 gamma-ray flux above 1 TeV is 1.3-+0.4 Crab units, 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.
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 200
7. 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.
ARGO-YBJ is an air shower detector array with a fully covered layer of resistive plate chambers. It is operated with a high duty cycle and a large field of view. It continuously monitors the northern sky at energies above 0.3 TeV. In this paper, we r
eport a long-term monitoring of Mrk 421 over the period from 2007 November to 2010 February. This source was observed by the satellite-borne experiments Rossi X-ray Timing Explorer and Swift in the X-ray band. Mrk 421 was especially active in the first half of 2008. Many flares are observed in both X-ray and gamma-ray bands simultaneously. The gamma-ray flux observed by ARGO-YBJ has a clear correlation with the X-ray flux. No lag between the X-ray and gamma-ray photons longer than 1 day is found. The evolution of the spectral energy distribution is investigated by measuring spectral indices at four different flux levels. Hardening of the spectra is observed in both X-ray and gamma-ray bands. The gamma-ray flux increases quadratically with the simultaneously measured X-ray flux. All these observational results strongly favor the synchrotron self-Compton process as the underlying radiative mechanism.
