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Significance map of the deficit event densities observed by the Tibet-III array for 1318.9 live days, made using the events with ∑ρFT > 101.25 (2 TeV), in the square area of 6° × 6° whose origin is at the apparent center of the Moon. The scale at right shows the level of significance of the deficit event density in terms of the standard deviation σ.
Source publication
The Tibet-III air shower array, consisting of 533 scintillation detectors, has been operating successfully at Yangbajing in Tibet, China since 1999. Using the data set collected by this array from 1999 November through 2005 November, we obtained the energy spectrum of γ-rays from the Crab Nebula, expressed by a power law as (dJ/dE) = (2.09 ± 0.32)...
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Context 1
... this partition is commonly used in observations of both the Moon's shadow and the Crab Nebula. Figure 2 shows the experimental significance map of the deficit event densities observed with the Tibet-III array for 1318.9 live days. This map is smoothed using the events with ρ FT > 10 1.25 (2 TeV) within a circle of radius 0. ...
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... 9, corresponding to the overall angular resolution for events with ρ FT > 10 1.25 . The MC simulation well reproduces the observed Moon's shadow, as shown in Figure 2. ...
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... investigate the PSF of the Tibet-III array, we compared the θ distribution of the Crab Nebula between the experimental data and the MC events, where θ is the opening angle relative to the Crab Nebula direction. Figure 12 shows the distribution of the excess events as a function of θ for events with ρ FT > 10 1.25 . The experimental data agree well with the MC simulation assuming the pointlike source. ...
Citations
... The energy resolution is approximately 40%, 20%, and 10% for 10, 100, and 400 TeV gamma rays, respectively [1,15]. The systematic uncertainty in the absolute energy scale for ∼10 TeV is less than ±12% [16]. Until 2014, observations were conducted only with the Tibet air shower array. ...
The Tibet ASγ experiment has been observing cosmic gamma rays and cosmic rays in the energy range from teraelectron volts to several tens of petaelectron volts with a surface detector array since 1990. The derivation of cosmic gamma-ray flux is made by finding the excess distribution of the arrival direction of air showers above background cosmic rays. In 2014, the underground water Cherenkov muon detector (MD) was added to separate cosmic gamma rays from the background on the basis of the muon-less feature of the air showers of gamma-ray origin; hybrid observations using these two detectors were started at this time. In the present study, we developed methods to separate gamma-ray-induced air showers and hadronic cosmic-ray-induced ones using the measured particle number density distribution to improve the sensitivity of cosmic gamma-ray measurements using the Tibet air shower array data alone before the installation of the MD. We tested two approaches based on neural networks. The first method used feature values representing the lateral spread of the secondary particles, and the second method used the shower image data. To compare the separation performance of each method, we analyzed Monte Carlo air shower events in the vertically incident direction with mono-initial-energy gamma rays and protons. When discriminated by a single feature, the feature with the highest separation performance has an area under the curve (AUC) value of 0.701 for a gamma-ray energy of 10 TeV and 0.808 for 100 TeV. A separation method with a multilayer perceptron (MLP) based on multiple features has AUC values of 0.761 for a gamma-ray energy of 10 TeV and 0.854 for 100 TeV, which represents an improvement of approximately 5% in the AUC value compared with the single-feature case. We also found that the feature values that effectively contribute to the separation vary depending on the energy. A separation method with a convolutional neural network (CNN) using the shower image data has AUC values of 0.781 for a gamma-ray energy of 10 TeV and 0.901 for 100 TeV, which are approximately 5% higher than those of the MLP method. We applied the CNN separation method to Monte Carlo gamma-ray and cosmic-ray events from the Crab Nebula in the energy range 10–100 TeV. The AUC values range from 0.753 to 0.879, and the significance of the observed gamma-ray excess is improved by 1.3 to 1.8 times compared with the case without the separation procedure.
... The Tibet AS array has been observing CRs above the TeV energy range since 1991 (Amenomori et al. 1992(Amenomori et al. , 1999(Amenomori et al. , 2009(Amenomori et al. , 2019 at Yangbajing (90.522°E, 30.102°N, 4300 m a.s.l.) in Tibet, China. The surface array consists of 597 plastic scintillation detectors covering a total geometrical area of 65,700 m 2 . ...
Gamma rays from HESS J1849−000, a middle-aged TeV pulsar wind nebula (PWN), are observed by the Tibet air shower array and the muon detector array. The detection significance of gamma rays reaches 4.0 σ and 4.4 σ levels above 25 TeV and 100 TeV, respectively, in units of the Gaussian standard deviation σ . The energy spectrum measured between 40 TeV < E < 320 TeV for the first time is described with a simple power-law function of dN / dE = ( 2.86 ± 1.44 ) × 10 − 16 ( E / 40 TeV ) − 2.24 ± 0.41 TeV − 1 cm − 2 s − 1 . The gamma-ray energy spectrum from the sub-TeV ( E < 1 TeV) to sub-PeV (100 TeV < E < 1 PeV) ranges, including the results of previous studies, can be modeled with the leptonic scenario, i.e., inverse Compton scattering by high-energy electrons accelerated by the PWN of PSR J1849−0001. On the other hand, the gamma-ray energy spectrum can also be modeled with the hadronic scenario in which gamma rays are generated from the decay of neutral pions produced by collisions between accelerated cosmic-ray protons and the ambient molecular cloud found in the gamma-ray-emitting region. The cutoff energy of cosmic-ray protons E p,cut is estimated as log 10 ( E p , cut / TeV ) = 3.73 − 0.66 + 2.98 , suggesting that protons are accelerated up to the PeV energy range. Our study thus proposes that HESS J1849−000 should be further investigated as a new candidate as a Galactic PeV cosmic-ray accelerator, or “PeVatron.”
... Tibet air shower (AS) array has been observing CRs above the TeV energy range since 1991 (Amenomori et al. 1992(Amenomori et al. , 1999(Amenomori et al. , 2009(Amenomori et al. , 2019 at Yangbajing (90. • 522 E, 30. ...
... = 0.47/3. The experiment's absolute energy scale uncertainty of 12% (Amenomori et al. 2009) dominates the systematic uncertainty in the flux normalization of ≃ 27%. The fraction of contamination to the number of events above 100 TeV from the lower energy range due to the finite energy resolution is estimated at ≃ 20%. ...
Gamma rays from HESS J1849$-$000, a middle-aged TeV pulsar wind nebula (PWN), are observed by the Tibet air shower array and the muon detector array. The detection significance of gamma rays reaches $4.0\, \sigma$ and $4.4\, \sigma$ levels above 25 TeV and 100 TeV, respectively, in units of Gaussian standard deviation $\sigma$. The energy spectrum measured between $40\, {\rm TeV} < E < 320\, {\rm TeV}$ for the first time is described with a simple power-law function of ${\rm d}N/{\rm d}E = (2.86 \pm 1.44) \times 10^{-16}(E/40\, {\rm TeV})^{-2.24 \pm 0.41}\, {\rm TeV}^{-1}\, {\rm cm}^{-2}\, {\rm s}^{-1}$. The gamma-ray energy spectrum from the sub-TeV ($E < 1\, {\rm TeV}$) to sub-PeV ($100\, {\rm TeV} < E < 1\, {\rm PeV}$) ranges including the results of previous studies can be modeled with the leptonic scenario, inverse Compton scattering by high-energy electrons accelerated by the PWN of PSR J1849$-$0001. On the other hand, the gamma-ray energy spectrum can also be modeled with the hadronic scenario in which gamma rays are generated from the decay of neutral pions produced by collisions between accelerated cosmic-ray protons and the ambient molecular cloud found in the gamma-ray emitting region. The cutoff energy of cosmic-ray protons $E_{\rm p\, cut}$, cut is estimated at ${\rm log}_{10}(E_{\rm p,\, cut}/{\rm TeV}) = 3.73^{+2.98}_{-0.66}$, suggesting that protons are accelerated up to the PeV energy range. Our study thus proposes that HESS J1849$-$000 should be further investigated as a new candidate for a Galactic PeV cosmic-ray accelerator, PeVatron.
... The Tibet ASgamma experiment, situated at an altitude of 4300 meters in Yangbajing, Tibet, China, covers an area of 65,700 2 [1]. Comprising three sub-arrays, namely, the Tibet airshower array (Tibet-III), air-shower-core detector-grid (YAC-II), and underwater Cherenkov muon detector array (MD) extending over 3,400 2 [2,3], this study focuses on the application of Graph Neural Networks (GNN) and automated machine learning (autoML) trained on simulated data from Tibet-III and MD array. ...
... The other method is to leverage the muon-poor nature of gamma-ray-induced air showers to separate them from hadronic cosmic-ray-induced air showers [11]. Tibet ASγ deploys a 65,700 m 2 surface-type particle detector array comprising 597 plastic scintillation detectors arranged in a grid pattern on the Tibetan plateau at an altitude of 4,300 m to continuously observe hadronic cosmic-ray/gamma-ray-induced air showers above several TeV [4,12,13]. A large underground water Cherenkov muon detector is installed 2.4 m below the surface detector array on the ground to separate gamma-ray-induced muon-poor air showers from hadronic-cosmic-ray-induced muonrich air showers [14,15]. ...
Observation techniques of high-energy gamma rays using air showers have remarkably progressed via the Tibet ASγ, HAWC, and LHAASO experiments. These observations have significantly contributed to gamma-ray astronomy in the northern sky’s sub-PeV region. Moreover, in the southern sky, the ALPACA experiment is underway at 4,740 m altitude on the Chacaltaya plateau in Bolivia. This experiment estimates the gamma-ray flux from the difference between the number of on-source and off-source events by real data, utilizing the gamma-ray detection efficiency calculated through Monte Carlo simulations, which in turn depends on the hadronic interaction models. Even though the number of cosmic-ray background events can be experimentally estimated, this model dependence affects the estimation of gamma-ray detection efficiency. However, previous reports have assumed that the model dependence is negligible and have not included it in the error of gamma-ray flux estimation. Using ALPAQUITA, the prototype experiment of ALPACA, we quantitatively evaluated the model dependence on hadronic interaction models for the first time. We evaluate the model dependence on hadronic interactions as less than 3.6 % in the typical gamma-ray flux estimation performed by ALPAQUITA; this is negligible compared with other uncertainties such as energy scale uncertainty in the energy range from 6 to 300 TeV, which is dominated by the Monte Carlo statistics. This upper limit of 3.6 % model dependence is expected to apply to ALPACA.
... The Tibet ASγ group has been performing a continuous observation to investigate CR composition around the knee region. Its detector is located 4300 m above sea level and comprises the surface detector array (Tibet air-shower array; Tibet-III) of 65 700 m 2 [23,24] and large underground water Cherenkov muon detector array (MD) of 3400 m 2 [25][26][27][28]. In this study we report on the sensitivity of proton spectrum measurements at energies from 40 to 630 TeV using MD and Tibet-III. ...
The Tibet ASγ group has been continuously observing cosmic rays and cosmic gamma rays above several TeV using the muon detector array (MD) and high-density Tibet air-shower array (Tibet-III) installed on the Tibet plateau at an altitude of 4300 m. The MD is a water Cherenkov pool array with a large effective area of 3400 m2 and has an excellent capability of primary selection using the number of muons in the shower. We report the sensitivity of the proton spectrum measurements for energies 40–630 TeV obtained via Monte Carlo simulations for an air-shower experiment. It was found that protons could be separated with a purity of 90%, and the survival ratio of protons including model dependence was 14.2%–19.1% and 3.7%–7.4% at about 35 TeV and about 450 TeV, respectively. The maximum total systematic error of the proton flux depending on interaction models in air-shower development and composition models was ±37%. With a large effective area and high proton separation capability, the Tibet ASγ experiment can measure the proton spectrum in the energy range from tens to hundreds of TeV with high statistical accuracy.
... The Tibet air shower array (AS array) has been operating since 1990 in Yangbajing (90°. 522 E, 30°.102N, 4300 m a.s.l.) in Tibet, China (Amenomori et al. 1992(Amenomori et al. , 1999(Amenomori et al. , 2009) and currently consists of a surface AS array and an underground muon detector array (MD array; Amenomori et al. 2019). The AS array comprises 597 plastic scintillation detectors, each with a detection area of 0.5 m 2 , and covers an area of 65,700 m 2 . ...
... As a possible systematic uncertainty, the uncertainty in the source extension σ ext affects the flux normalization by 19% and the spectral index by 0.02. The absolute energy-scale uncertainty of 12% (Amenomori et al. 2009) also affects the flux normalization by 39%. Summing up these two in quadrature, the total systematic uncertainty in the flux normalization is calculated as 43%. ...
HESS J1843–033 is a very high energy gamma-ray source whose origin remains unidentified. This work presents, for the first time, the energy spectrum of gamma rays beyond 100 TeV from the HESS J1843–033 region using the data recorded by the Tibet air shower array and its underground muon detector array. A gamma-ray source with an extension of 0.°34 ± 0.°12 is successfully detected above 25 TeV at ( α , δ ) = (281.°09 ± 0.°10, −3.°76 ± 0.°09) near HESS J1843–033 with a statistical significance of 6.2 σ , and the source is named TASG J1844–038. The position of TASG J1844–038 is consistent with those of HESS J1843–033, eHWC J1842–035, and LHAASO J1843–0338. The measured gamma-ray energy spectrum in 25 TeV < E < 130 TeV is described with dN / dE = ( 9.70 ± 1.89 ) × 10 − 16 ( E /40 TeV) −3.26±0.30 TeV ⁻¹ cm ⁻² s ⁻¹ , and the spectral fit to the combined spectra of HESS J1843–033, LHAASO J1843–0338, and TASG J1844–038 implies the existence of a cutoff at 49.5 ± 9.0 TeV. Associations of TASG J1844–038 with SNR G28.6–0.1 and PSR J1844–0346 are also discussed in detail for the first time.
... In this detector, a lead plate with a thickness of one radiation length is used to convert secondary gamma rays in an air shower into electrons, which amplifies the number of detected particles about two-fold. This method improves the angular resolution by about √ 1.9 times compared to the case of the plastic scintillator alone, attaining an angular resolution of about 0.5 ∘ at 10 TeV and about 0.2 ∘ at 100 TeV [19][20][21]. One of the advantages of a high-density air shower array such as those of the HAWC and ARGO experiments is that high particle density measurements can achieve high angular resolution. ...
For research on cosmic gamma rays with energies in the range of several tens of teraelectronvolts or more, we investigated a method to improve the angular resolution of an air shower. In an air shower, the density of secondary gamma rays is several times higher than that of electrons and those measurement is important for determining the shower direction. It was found that the angular resolution in the shower front-fit method decreases in inverse proportion to the square root of the number of measured particles. Even if the total number of measured particles is the same, secondary gamma rays contribute more to the improvement of angular resolution than electrons. If secondary gamma rays could be measured at an altitude of 4,740 m with a sensitivity of 100 %, an improvement of approximately 40 % was determined for a 500 TeV shower. A water Cherenkov detector with high gamma-ray sensitivity was investigated through Monte Carlo simulation. Detection efficiencies of approximately 0.38 and 0.76 were obtained for vertically incident gamma rays and electrons, respectively, using 19 8-inch diameter PMTs inside a detector installed in a water tank of radius 4.5 m and water depth 1.6 m. The detection time error for secondary gamma rays is approximately 2.18 ns at an incident angle of 0∘ and the standard error in the detection time for shower front particles was found to be approximately 10 times lower than that obtained by using a plastic scintillation detector with an area of 1 m².
... The Σρ can be a good indicator for the true energy of CRs. The correlation of Σρ and energy has been demonstrated by the YBJ-HA [10,21] and Tibet ASγ experiment [7]. In addition, the particle density at a certain distance from the shower core can also be used as a good energy estimation parameter. ...
The observation of γ-ray sources above 100 TeV is one of the most intriguing fields in the investigation of the origin of cosmic rays. The YangBaJing Hybrid Array (YBJ-HA) experiment consist of scintillation detectors and underground muon detectors, the main objective is to measure gamma rays >10 TeV. In this study, we investigated an energy estimation method based on particle density and fixed distance from the shower axis by conducting a detailed Monte Carlo simulation of an extensive air shower using the YBJ-HA in Tibet. By comparing the results of the new method with those of other previously used estimators, we found that the new method can effectively improve the energy resolution of gamma rays. At an energy of 100 TeV for a zenith angle θ < 30°, the new energy estimation method has an improvement about 10% compared for the other estimators.
... The uncertainty in the absolute energy scale is estimated to be 12% [ref. 40 ]. We evaluate our pointing precision at the declination of G106.3+2.7 by re-analyzing the source location of the Crab Nebula. ...
Cosmic rays (protons and other atomic nuclei) are believed to gain energies of petaelectronvolts (PeV) and beyond at astrophysical particle accelerators called 'PeVatrons' inside our Galaxy. Although a characteristic feature of a PeVatron is expected to be a hard gamma-ray energy spectrum that extends beyond 100 teraelectronvolts (TeV) without a cutoff, none of the currently known sources exhibits such a spectrum due to the low maximum energy of accelerated cosmic rays or insufficient detector sensitivity around 100 TeV. Here we report the observation of gamma-ray emission from the supernova remnant G106.3+2.7 above 10 TeV. This work provides flux data points up to and above 100 TeV and indicates that the very-high-energy gamma-ray emission above 10 TeV is well correlated with a molecular cloud rather than the pulsar PSR J2229+6114. Regarding the gamma-ray emission mechanism of G106.3+2.7, this morphological feature appears to favor a hadronic origin via the {\pi}0 decay caused by accelerated relativistic protons over a leptonic one via the inverse-Compton scattering by relativistic electrons. Furthermore, we point out that an X-ray flux upper limit on the synchrotron spectrum would provide important information to firmly establish the hadronic scenario as the mechanism of particle acceleration at the source.
