M. Y. Liu

TeV gamma-ray survey of the northern sky using the ARGO-YBJ detector

The Astrophysical Radiation with Ground-based Observatory at Yang Ba Jing (ARGO-YBJ) detector is an extensive air shower array that has been used to monitor the northern γ-ray sky at energies above 0.3xa0TeV from 2007 November to 2013 January. In this paper, we present the results of a sky survey in the declination band from –10° to 70°, using data recorded over the past five years. With an integrated sensitivity ranging from 0.24 to ~1 Crab units depending on the declination, six sources have been detected with a statistical significance greater than five standard deviations. Several excesses are also reported as potential γ-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 663xa0GeV γ-ray active galactic nuclei inside the ARGO-YBJ field of view are reported. The effect of the absorption of γ-rays due to the interaction with extragalactic background light is estimated.

Identification of the TeV Gamma-ray Source ARGO J2031+4157 with the Cygnus Cocoon

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 2007 November to 2013 January, 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° × 10°, finding a source extension σ{sub ext}= 1.°8 ± 0.°5. The observed differential energy spectrum is dN/dE = (2.5 ± 0.4) × 10{sup –11}(E/1 TeV){sup –2.6±0.3} photons cm{sup –2} s{sup –1} TeV{sup –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 (SNRs) indicates thatmorexa0» the particle acceleration inside a superbubble is similar to that in an 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.«xa0less

STUDY OF THE DIFFUSE GAMMA-RAY EMISSION FROM THE GALACTIC PLANE WITH ARGO-YBJ

The events recorded by ARGO-YBJ in more than fiveuf0a0years of data collection have been analyzed to determine the diffuse gamma-ray emission in the Galactic plane at Galactic longitudes 25° < l < 100° and Galactic latitudes b 5 ∣ ∣< °. The energy range covered by this analysis, from ∼350 GeV to ∼2 TeV, allows the connection of the region explored by Fermi with the multi-TeV measurements carried out by Milagro. Our analysis has been focused on two selected regions of the Galactic plane, i.e., 40° < l < 100° and 65° < l <8 5 °( the Cygnus region), where Milagro observed an excess with respect to the predictions of current models. Great care has been taken in order to mask the most intense gamma-ray sources, including the TeV counterpart of the Cygnus cocoon recently identified by ARGO-YBJ, and to remove residual contributions. The ARGO-YBJ results do not show any excess at sub-TeV energies corresponding to the excess found by Milagro, and are consistent with the predictions of the Fermi model for the diffuse Galactic emission. From the measured energy distribution we derive spectral indices and the differential flux at 1 TeV of the diffuse gamma-ray emission in the sky regions investigated.

CRAB NEBULA: FIVE-YEAR OBSERVATION WITH ARGO-YBJ

The ARGO-YBJ air shower detector monitored the Crab Nebula g amma ray emission from 2007 November to 2013 February. The integrated signal, consisting of ∼3.3× 105 events, reached the statistical significance of 21.1 standard deviations. The obtained energy spectrum in t he energy range 0.3-20 TeV can be described by a power law function dN/dE = I 0 (E / 2 TeV)−α, with a flux normalization I 0 = (5.2± 0.2)× 10−12 photons cm−2 s−1 TeV−1 andα = 2.63± 0.05, corresponding to an integrated flux above 1 TeV of 1.97 × 10−11 photons cm−2 s−1. The systematic error is estimated to be less that 30% for the flux normalization and 0.06 for the spectral index. Assuming a power law spectrum with an exponential cut off dN/dE = I0 (E / 2 TeV)−α exp (-E / Ecut), the lower limit of the cutoff energy Ecut is 12 TeV, at 90% confidence level. Our extended dataset allow s the study of the TeV emission over long timescales. Over five years, the lig t curve of the Crab Nebula in 200-day bins is compatible with a steady emission with a probability of 7.3 × 10−2. A correlated analysis with Fermi-LAT data over∼4.5 years using the light curves of the two experiments gives a Pearson correlation coefficient r = 0.56± 0.22. Concerning flux variations on timescales of days, a “bl ind” search for flares with a duration of 1-15 days gives no excess with a significance higher than four standard eviations. The average rate measured by ARGOYBJ during the three most powerful flares detected by Fermi-L AT is 205± 91 photons day −1, consistent with the average value of 137 ± 10 day−1.The ARGO-YBJ air shower detector monitored the Crab Nebula gamma-ray emission from 2007 November to 2013 February. The integrated signal, consisting of ~3.3 × 105 events, reached the statistical significance of 21.1 standard deviations. The obtained energy spectrum in the energy range 0.3-20 TeV can be described by a power law function dN/dExa0= I 0 (E/2 TeV)–α, with a flux normalization I 0xa0= (5.2 ± 0.2) × 10–12 photons cm–2 s–1 TeV–1 and αxa0= 2.63 ± 0.05, corresponding to an integrated flux above 1 TeV of 1.97 × 10–11 photons cm–2 s–1. The systematic error is estimated to be less than 30% for the flux normalization and 0.06 for the spectral index. Assuming a power law spectrum with an exponential cutoff dN/dExa0= I 0 (E/2 TeV)–α exp (–E/E cut), the lower limit of the cutoff energy E cut is 12 TeV, at 90% confidence level. Our extended data set allows the study of the TeV emission over long timescales. Over five years, the light curve of the Crab Nebula in 200-day bins is compatible with a steady emission with a probability of 7.3 × 10–2. A correlated analysis with Fermi-LAT data over ~4.5xa0yr using the light curves of the two experiments gives a Pearson correlation coefficient rxa0= 0.56 ± 0.22. Concerning flux variations on timescales of days, a blind search for flares with a duration of 1-15xa0days gives no excess with a significance higher than four standard deviations. The average rate measured by ARGO-YBJ during the three most powerful flares detected by Fermi-LAT is 205 ± 91 photons day–1, consistent with the average value of 137 ±xa010xa0day–1.

4.5 Years of Multi-Wavelength Observations of Mrk 421 During the Argo-Ybj and Fermi Common Operation Time

We report on the extensive multi-wavelength observations of the blazar Markarian 421 (Mrk 421) covering radio to gamma-rays, during the 4.5 year period of ARGO-YBJ and Fermi common operation time, from August 2008 to February 2013. In particular, thanks to the ARGO-YBJ and Fermi data, the whole energy range from 100 MeV to 10 TeV is covered without any gap. In the observation period, Mrk 421 showed both low and high activity states at all wavebands. The correlations among flux variations in different wavebands were analyzed. Seven large flares, including five X-ray flares and two GeV gamma-ray flares with variable durations (3-58 days), and one X-ray outburst phase were identified and used to investigate the variation of the spectral energy distribution with respect to a relative quiescent phase. During the outburst phase and the seven flaring episodes, the peak energy in X-rays is observed to increase from sub-keV to few keV. The TeV gamma-ray flux increases up to 0.9-7.2 times the flux of the Crab Nebula. The behavior of GeV gamma-rays is found to vary depending on the flare, a feature that leads us to classify flares into three groups according to the GeV flux variation. Finally, the one-zone synchrotron self-Compton model was adopted to describe the emission spectra. Two out of three groups can be satisfactorily described using injected electrons with a power-law spectral index around 2.2, as expected from relativistic diffuse shock acceleration, whereas the remaining group requires a harder injected spectrum. The underlying physical mechanisms responsible for different groups may be related to the acceleration process or to the environment properties.

Detection of thermal neutrons with the PRISMA-YBJ array in Extensive Air Showers selected by the ARGO-YBJ experiment

Abstract We report on a measurement of thermal neutrons, generated by the hadronic component of extensive air showers (EAS), by means of a small array of EN-detectors developed for the PRISMA project (PRImary Spectrum Measurement Array), novel devices based on a compound alloy of ZnS(Ag) and 6 LiF. This array has been operated within the ARGO-YBJ experiment at the high altitude Cosmic Ray Observatory in Yangbajing (Tibet, 4300xa0m a.s.l.). Due to the tight correlation between the air shower hadrons and thermal neutrons, this technique can be envisaged as a simple way to estimate the number of high energy hadrons in EAS. Coincident events generated by primary cosmic rays of energies greater than 100xa0TeV have been selected and analyzed. The EN-detectors have been used to record simultaneously thermal neutrons and the air shower electromagnetic component. The density distributions of both components and the total number of thermal neutrons have been measured. The correlation of these data with the measurements carried out by ARGO-YBJ confirms the excellent performance of the EN-detector.

The analog Resistive Plate Chamber detector of the ARGO-YBJ experiment

Abstract The ARGO-YBJ experiment has been in stable data taking from November 2007 till February 2013 at the YangBaJing Cosmic Ray Observatory (4300xa0m a.s.l.). The detector consists of a single layer of Resistive Plate Chambers (RPCs) (6700xa0m 2 ) operated in streamer mode. The signal pick-up is obtained by means of strips facing one side of the gas volume. The digital readout of the signals, while allows a high space–time resolution in the shower front reconstruction, limits the measurable energy to a few hundred TeV. In order to fully investigate the 1–10xa0PeV region, an analog readout has been implemented by instrumenting each RPC with two large size electrodes facing the other side of the gas volume. Since December 2009 the RPC charge readout has been in operation on the entire central carpet (∼5800xa0m 2 ). In this configuration the detector is able to measure the particle density at the core position where it ranges from tens to many thousands of particles per m 2 . Thus ARGO-YBJ provides a highly detailed image of the charge component at the core of air showers. In this paper we describe the analog readout of RPCs in ARGO-YBJ and discuss both the performance of the system and the physical impact on the EAS measurements.

Northern Sky Galactic Cosmic Ray Anisotropy between 10 and 1000 TeV with the Tibet Air Shower Array

We report on the analysis of the 10−1000 TeV large-scale sidereal anisotropy of Galactic cosmic rays (GCRs) with the data collected by the Tibet Air Shower Array from 1995 October to 2010 February. In this analysis, we improve the energy estimate and extend the decl. range down to −30◦. We find that the anisotropy maps above 100 TeV are distinct from that at a multi-TeV band. The so-called tail-in and loss-cone features identified at low energies get less significant, and a new component appears at ∼ 100 TeV. The spatial distribution of the GCR intensity with an excess (7.2σ pre-trial, 5.2σ post-trial) and a deficit (−5.8σ pre-trial) are observed in the 300 TeV anisotropy map, in close agreement with IceCube’s results at 400 TeV. Combining the Tibet results in the northern sky with IceCube’s results in the southern sky, we establish a full-sky picture of the anisotropy in hundreds of TeV band. We further find that the amplitude of the first order anisotropy increases sharply above ∼ 100 TeV, indicating a new component of the anisotropy. All these results may shed new light on understanding the origin and propagation of GCRs.

New prototype scintillator detector for the Tibet ASγ experiment

The hybrid Tibet AS array was successfully constructed in 2014. It has 4500 m2 underground water Cherenkov pools used as the muon detector (MD) and 789 scintillator detectors covering 36900 m2 as the surface array. At 100 TeV, cosmic-ray background events can be rejected by approximately 99.99%, according to the full Monte Carlo (MC) simulation for γ-ray observations. In order to use the muon detector efficiently, we propose to extend the surface array area to 72900 m2 by adding 120 scintillator detectors around the current array to increase the effective detection area. A new prototype scintillator detector is developed via optimizing the detector geometry and its optical surface, by selecting the reflective material and adopting dynode readout. {This detector can meet our physics requirements with a positional non-uniformity of the output charge within 10% (with reference to the center of the scintillator), time resolution FWHM of ~2.2 ns, and dynamic range from 1 to 500 minimum ionization particles}.

Wide field-of-view atmospheric Cherenkov telescope based on refractive lens

A wide field of view (FOV) is an important feature of a detector in the gamma ray observation of sporadic, extended, and transient sources. In this work, we discuss an atmospheric Cherenkov telescope (ACT) with a refractive water convex lens as its light collector, and we test the feasibility of this new approach. We determine the optical properties of a water lens with a diameter of 0.9 m, such as focal length, spot size, and transmittance. The first detection of cosmic rays (CRs) observed in coincidence with a scintillator extensive air shower (EAS) array is presented and discussed.