Recent results inγ-ray astronomy with the ARGO-YBJ detector

Abstract

Abstract. The ARGO-YBJ air shower detector has been in stable data taking for five years at the YangBaJing Cosmic Ray Laboratory (Tibet, P.R. China, 4300m a.s.l.) with a duty cycle > 86% and an energy threshold of a few hundreds of GeV. With the scaler mode technique, the minimum threshold of 1 GeV can be reached. In this paper recent results in γ-ray astronomy will be presented, including those from 4.5 years of observations of the blazar Mrk 421 in common with the Fermi satellite. The detectorThe ARGO-YBJ experiment is located at Yangbajing (Tibet, P.R. China, 4300 m a.s.l.) and consists of a single layer of Resistive Plate Counters (RPCs) on a total area of about 110 × 100 m 2 . The detector has a modular structure, the basic module being a cluster (5.7 × 7.6 m 2 ), made of 12 RPCs. Each RPC is read by 80 strips (6.75×61.8 cm 2 ) which are the space pixels, logically organized in 10 independent pads (55.6 × 61.8 cm 2 ) which are individually acquired and represent the time pixels of the detector [1]. The detector carpet is connected to two different DAQ systems, which work independently: in shower mode, for each event the location and timing of each detected particle is recorded, allowing the reconstruction of the lateral distribution and of the arrival direction; in scaler mode, the counting rate of each cluster is measured every 0.5 s, with little information on the space distribution and arrival direction of the detected particles. The trigger of the shower mode was N pad ≥ 20 in a time window of 420 ns, with a rate of 3.5 kHz. In the scaler mode, for each cluster four scalers recorded the rate of counts ≥ 1, ≥ 2, ≥ 3 and ≥ 4 in a time window of 150 ns. The corresponding measured rates are, respectively, ∼40 kHz, ∼2 kHz, ∼300 Hz and ∼120 Hz [2]. The experiment has been taking data with its full layout from November 2007 to February 2013. The detector pointing accuracy, angular resolution and absolute energy calibration have been determined studying the deficit in the cosmic ray flux due to the Moon [3]. Sky surveyThe ARGO-YBJ detector surveyed the northern hemisphere, in the declination band from -10 • to 70 • , at energies above 0.3 TeV. With an integrated sensitivity down to 0.24 Crab unit (depending on the declination) after five years of data taking, six sources were detected with a statistical significance S>5 standard deviations (s.d.), and five excesses are reported as potential (S>4 s.d.) γ-ray emitters. The list of excess regions, with their corresponding significances and TeV associations, is in table 1 [4]. In the rest of this section, a selection of results concerning the detected sources will be presented. a

The LHAASO PeVatron Bright Sky: What We Learned

“…Its complex morphology could be a hint of the presence of overlapping sources, merged in the Galactic Plane Survey. In TevCat, this is associated with ARGO J1841-0332 [161] and 2HWC J1844-32 [21], and the Tibet Asγ array recently detected a TeV emission above 25 TeV near the position of HESS J1843-033, TASG J1844-038 [162]. The Tibet-ASγ emission has a measured spectrum above 100 TeV, in perfect agreement with the LHAASO results [162] (see Figure 9, right).…”