The ARGO-YBJ Experiment Progresses and Future Extension

Gamma ray source detection above 30TeV is an encouraging approach for finding galactic cosmic ray origins. All sky survey for gamma ray sources using wide field of view detector is essential for population accumulation for various types of sources above 100GeV. To target the goals, the ARGO-YBJ experiment has been established. Significant progresses have been made in the experiment. A large air shower detector array in an area of 1km2 is proposed to boost the sensitivity. Hybrid detection with multi-techniques will allow a good discrimination between different types of primary particles, including photons and protons, thus enable an energy spectrum measurement for individual specie. Fluorescence light detector array will extend the spectrum measurement above 100PeV where the second knee is located. An energy scale determined by balloon experiments at 10TeV will be propagated to ultra high energy cosmic ray experiments

Background radioactivity in the scaler mode technique of the Argo-YBJ detector

ARGO-YBJ is an extensive air shower detector located at the Yangbajing Cosmic Ray Laboratory (4300 m a.s.l., 606 g cm−2 atmospheric depth, Tibet, China). It is made by a single layer of Resistive Plate Chambers (RPCs, total surface 6700 m2) grouped into 153 units called “clusters”. The low energy threshold of the experiment is obtained using the ”scaler operation mode”, counting all the particles hitting the detector without reconstruction of the shower size and arrival direction. For each cluster the signals generated by these particles are put in coincidence in a narrow time window (150 ns) and read by four independent scaler channels, giving the counting rates of channel 1, 2, 3 and 4 hits. The study of these counting rates pointed out a different behavior of channel 1 respect to the higher multiplicity channels: while the MC simulations can account fairly well for the coincident counting rates, the expectation for channel 1 is sensibly less than the measured value. Moreover, the regression coefficient with the atmospheric pressure for channel 1 is also about half of the value measured for the coincident counting rates: seemingly half of these counts did not cross the atmosphere. Measurements of the natural radioactivity background in the air of the detector hall and a MC simulation to estimate its contribution on our counting rates are presented and discussed

Gamma ray sources observation with the ARGO-YBJ detector

In this paper we report on the observations of TeV gamma ray sources performed by the air shower detector ARGO-YBJ. The objects studied in this work are the blazar Markarian 421 and the extended galactic source MGROJ1908+06, monitored during 2 years of operation. Mrk421 has been detected by ARGO-YBJ with a statistical significance of 11 standard deviations. The observed TeV emission was highly variable, showing large enhancements of the flux during active periods. The study of the spectral behaviour during flares revealed a positive correlation of the hardness with the flux, as already reported in the past by the Whipple telescope, suggesting that this is a long term property of the source. ARGO-YBJ observed a strong correlation between TeV gamma rays and the X-ray flux measured by RXTM/ASM and SWIFT/BAT during the whole period, with a time lag compatible with zero, supporting the one-zone SSC model to describe the emission mechanism. MGROJ1908+06 has been detected by ARGO-YBJ with 5 standard deviation of significance. From our data the source appears extended and the measured extension is ext=0.48+0.26−0.28, in agreement with a previous HESS observation. The average flux is in marginal agreement with that reported by MILAGRO, but significantly higher than that obtained by HESS, suggesting a possible flux variability

Gamma ray bursts monitoring with the ARGO-YBJ experiment in scaler mode

We report on the search for Gamma Ray Bursts (GRBs) in the energy range 1−100 GeV in coincidence with the prompt emission detected by satellites, using the Astrophysical Radiation Ground-based Observatory at YangBa-Jing ARGO-YBJ). With its big active surface (6700 m 2) and large field of view (2 sr) the ARGO-YBJ air shower detector is particularly suitable to detect unpredictable and short duration events such as GRBs. The search has been performed using the single particle technique in time coincidence with satellite detections both for single events and for the piling up of all the GRBs in time and in phase. Between November 2004 and June 2010 115 GRBs, detected by different satellites (mainly Swift and Fermi), occurred within the field of view of ARGO-YBJ. For 94 of these we searched for a counterpart in the ARGO-YBJ data finding no statistically significant emission. Search methods and results are discussed

THE ARGO-YBJ EXPERIMENT: A FULL COVERAGE ARRAY FOR γ-RAY ASTRONOMY

The ARGO-YBJ experiment is an Extensive Air Shower (EAS) array which combines high altitude location and full coverage active area in order to reach low energy threshold at a level of few hundred of GeV. The large field of view (≈ 2 sr) and the high duty cycle (≥ 90%) allow the continuous monitoring of the sky searching for unknown sources and unpredictable events, such as flares in blazar emissions and high energy Gamma-Ray Bursts (GRBs). In this paper I will briefly report on the detector performance and on some preliminary results achieved in γ-ray astronomy