An updated search of steady TeV γ-ray point sources in northern hemisphere using the Tibet air shower array

Using the data taken from Tibet II High Density (HD) Array (1997 February—1999 September) and Tibet-III array (1999 November—2005 November), our previous northern sky survey for TeV γ-ray point sources has now been updated by a factor of 2.8 improved statistics. From 0.0° to 60.0° in declination (Dec) range, no new TeV γ-ray point sources with sufficiently high significance were identified while the well-known Crab Nebula and Mrk421 remain to be the brightest TeV γ-ray sources within the field of view of the Tibet air shower array. Based on the currently available data and at the 90% confidence level (C.L.), the flux upper limits for different power law index assumption are re-derived, which are approximately improved by 1.7 times as compared with our previous reported limits.     

Prospects for a multi-TeV gamma-ray sky survey with the LHAASO water Cherenkov detector array

The Water Cherenkov Detector Array(WCDA) is a major component of the Large High Altitude Air Shower Array Observatory(LHAASO), a new generation cosmic-ray experiment with unprecedented sensitivity, currently under construction. WCDA is aimed at the study of TeV γ-rays. In order to evaluate the prospects of searching for TeV γ-ray sources with WCDA, we present a projection of the one-year sensitivity of WCDA to TeV γ-ray sources from TeVCat using an all-sky approach. Out of 128 TeVCat sources observable by WCDA up to a zenith angle of 45°, we estimate that 42 would be detectable in one year of observations at a median energy of 1 TeV. Most of them are Galactic sources, and the extragalactic sources are Active Galactic Nuclei(AGN).     

Study on the separation of 100 TeV γ-rays from cosmic rays for the Tibet ASγ experiment

A γ/hadron separation analysis is described for the observed air shower events with primary energy above 100 TeV based on the Tibet ASγ detector configuration. The shower age and size parameters are fitted from the measured lateral density distribution and used as discrimination variables. According to the MC simulation while taking into account the systematic uncertainty estimated from data and MC comparison, it is found that 70% of the cosmic ray (CR) background can be rejected while more than 78% of the γ-rays can be retained. Sensitivity for 100 TeV γ-rays observation can thus be improved by at least 40%.     

Hadronic Interpretation of High-Energy Gamma-Rays from Two TeV Sources Coincident with Molecular Clouds Overtaken by Supernova Remnants

We present a possible hadronic explanation of the high-energy γ-ray emission from two very high-energy （VHE） sources, HESS 31745-303 （A ） and HESS 31714-385, which coincide with supernova remnants （SNRs） interacting with dense molecular clouds （MCs）. We calculate the proton spectra and the corresponding hadronic γ-ray spectra for different Mach numbers of the shock wave in a semi-analytical model for the non-linear shock acceleration process, then apply the model to the two newly discovered TeV sources. The results show that the γ-ray spectra for the two sources with energies above 100 MeV detected by HESS and EGRET can be reproduced with low Mach numbers about 2.5. Thus the high-energy γ-ray origin for each one of the two sources can be interpreted as proton-proton （p-p） collisions in MCs overtaken by an SNR shock wave.     

Updated study on multi-TeV cosmic-ray modulation with the Tibet Ⅲ air shower array using the east-west method

We study the sidereal and solar time modulation of multi-TeV cosmic rays using the east-west method with Tibet Ⅲ air shower array data taken from November 1999 to December 2008. The statistics are twice the amount used in our previous paper. In this analysis, the amplitude of the observed sidereal time modulation is about 0.1%, and the modulation shows an excess from about 4 to 7 hours and a deficit around 12 hours in local sidereal time. The sidereal time modulation has a weak dependence on the primary energy of the cosmic rays. However, the solar time modulation shows a large energy dependence. We find that the solar time modulation is fairly consistent with the prediction of the Compton-Getting effect for high-energy samples (6.2 TeV and 12.0 TeV), but exceeds the prediction for the low-energy sample (4.0 TeV). Such a discrepancy may be due to the solar modulation or the characteristics of the experimental device in the near threshold energy.     

Radio core dominance of Fermi/LAT-detected AGNs

We present a sample of 4388 AGNs with available radio core-dominance parameters—defined as the ratio of the core flux densities to the extended ones, R = S_core/S_ext.—which includes 630 Fermi-detected AGNs from the fourth source catalog(4FGL) of the Fermi Large Area Telescope(Fermi/LAT);the rest are non-Fermi-detected AGNs. In our sample, 584 blazars are Fermi-detected and 1310 are not. The sample also contains other subclasses, such as Seyferts, Fanaroff-Riley I/II galaxies, and normal galaxies.We investigate various properties of the Fermi-detected and non-Fermi-detected AGNs by using core-dominance parameters,capitalizing on a previous study which showed that R is a good indicator of beaming. We then calculate radio spectral indices for the whole sample, and adopt γ-ray-photon indices for the Fermi AGNs from the 4FGL catalog to discuss the properties of different subclasses. We obtain a relation between the core-dominance parameters and the radio spectral indices for both Fermi and non-Fermi sources, assuming a two-component model in the radio band. Our previous study ruled out the assumption that the core-dominance parameters and radio spectral indices are quite different for different AGN subclasses. This holds not only for Fermi sources but also for non-Fermi sources. In particular, R is, on average, greater for the former AGNs than for the latter.In this study, we enlarge our sample with available values of R to 4388 AGNs, and the obtained conclusions are consistent with our previous study. We assume that the same two-component model holds for the γ-ray band as for the radio band, and therefore,adopt the same relation between the core-dominance parameters and the γ-ray-photon indices for Fermi AGNs. Our fitting results indicate that the γ-ray emissions of Fermi blazars originate mainly from the jet, and therefore, we conclude that the Fermi blazars are beamed.     

Evaluation of the 1077 keV γ-ray emission probability from ~(68)Ga decay

68Ga decays to the excited states of68Zn through the electron capture decay mode. New recommended values for the emission probability of 1077 keV γ-ray given by the ENSDF and DDEP databases all use data from absolute measurements. In 2011, JIANG Li-Yang deduced a new value for 1077 keV γ-ray emission probability by measuring the69Ga(n,2n)68Ga reaction cross section. The new value is about 20% lower than values obtained from previous absolute measurements and evaluations. In this paper, the discrepancies among the measurements and evaluations are analyzed carefully and the new values are re-recommended. Our recommended value for the emission probability of 1077 keV γ-ray is(2.72±0.16)%.     

Evaluation of the 1077 keV γ-ray emission probability from 6SGa decay

6SGa decays to the excited states of aSzn through the electron capture decay mode. New recommended values for the emission probability of 1077 keV γ-ray given by the ENSDF and DDEP databases all use data from absolute measurements. In 2011, JIANG Li-Yang deduced a new value for 1077 keV γ-ray emission probability by measuring the 69Ga（n,2n） 6SGa reaction cross section. The new value is about 20% lower than values obtained from previous absolute measurements and evaluations. In this paper, the discrepancies among the measurements and evaluations are analyzed carefully and the new values are re-recommended. Our recommended value for the emission probability of 1077 keV γ-ray is （2.72±0.16）%.     

Pulsars as candidates of LHAASO sources J2226+6057, J1908+0621, and J1825-1326: The leptonic origin

Recently, from 12 γ-ray Galactic sources, the LHAASO has detected ultrahigh-energy photons up to 1.4 PeV. The γ-ray spectra of the sources J2226+6057, J1908+0621, and J1825-1326 and the suggested origin pulsars near the sources have been published. In our previous work, we studied the hadronic γ-ray spectra of the sources J2226+6057, J1908+0621, and J1825-1326 in terms of the Hertzian dipole model of pulsars. In this paper,we investigate the possibility of the leptonic origin of the γ-ray. We us...     

On the detectability of Galactic dark matter annihilation into monochromatic gamma-rays

Monochromatic γ-rays are thought to be the smoking gun signal for identifying dark matter annihilation. However, the flux of monochromatic γ-rays is usually suppressed by virtual quantum effects since dark matter should be neutral and does not couple with γ-rays directly. In this work, we study the detection strategy of the monochromatic γ-rays in a future space-based detector. The flux of monochromatic γ-rays between 50 GeV and several TeV is calculated by assuming the supersymmetric neutralino as a typical dark matter candidate. The detection both by focusing on the Galactic center and in a scan mode that detects γ-rays from the whole Galactic halo are compared. The detector performance for the purpose of monochromatic γ-ray detection, with different energy and angular resolution, field of view, and background rejection efficiencies, is carefully studied with both analytical and fast Monte-Carlo methods.     

Radiography of a K_α X-ray source generated through ultrahigh picosecond laser–nanostructure target interaction

Laser-driven ultrafast X-ray sources are widely used for diagnostic radiography. However, there is a large divergence of fast electrons when they are generated by an intense short-pulse laser interacting with a foil target.We design a nanowire array target to achieve a more compact point X-ray source. Fast electrons are confined and guided by the nanowire array structure in order to generate a Kαsource with a small spot size. In our work, the smallest measured source size is comparable to the laser spot size, while the conversion efficiency can reach2.4 × 10-4.     

γ-and α-Ce phase diagram: First-principle calculation

Controversies about the phase diagram for the isostructural γ ? α phase transition of cerium have long been standing out for several decades. To seek insight into the problems, high-precision equations of state(EOS) for γ-and α-cerium are constructed based on first-principle calculation. Versus previous works, the strong anharmonic effects of ion vibration and the variation of magnetism of γ-cerium are stressed. The new EOS generally agrees well with experimental data regarding thermodynamics, phase diagrams, and phase transitions. However, new EOS predicts that another part of phase boundary in pressure-temperature space may exist except for the commonly known boundary. In addition, the well-known critical point seems to be a critical point for γ-cerium to translate from a stable state to an unstable state.     

基于小波变换的1—10TeV宇宙线周期变化研究

利用小波变换, 结合折叠周期分析方法, 对羊八井宇宙线观测站Tibet AS γ阵列1998年4—6月的实验记录数据进行了周期分析, 发现 TeV和10 TeV宇宙线流强观测数据中包含0.5 和1 d周期变化, 其信噪比分别达到10和6. TeV宇宙线0.5 和1 d周期变化的最大变化幅度分别约为0.3%和0.5%, 10 TeV宇宙线约为0.4%和0.7%. We use the technique of wavelet analysis and epoch folding methods to search for the superimposed periodic signals of 1—10 TeV cosmic ray in the data obtained with Tibet Ⅱ/HD AS Array for April to June of 1998. Solar time semi diurnal and diurnal variations have been detected with about signal noise ratio 10 and 6 for the 1 TeV and 10 TeV cosmic ray flux respectively. The semi diurnal variations are of an amplitude 0.3%, a phase 0.9 for 1 TeV cosmic ray and 0.4%, 0.9 for 10 TeV. The diurnal variations are of amplitude 0.5%, 0.7%, and phase 0.8 for 1 TeV and 10 TeV cosmic ray respectively.     

Search for the doubly heavy baryon ■ decaying to ■

A first search for the ■decay is performed by the LHCb experiment with a data sample of proton-proton collisions, corresponding to an integrated luminosity of 9 fb^-1 recorded at centre-of-mass energies of 7, 8, and 13 TeV. Two peaking structures are seen with a local(global) significance of 4.3(2.8) and 4.1(2.4) standard deviations at masses of 6571 and 6694 MeV/c², respectively. Upper limits are set on the ■ baryon production cross-section times the branching fraction rela...     

An approach to evaluate the efficiency of γ-ray detectors to determine the radioactivity in environmental samples

This work provides an approach to determine the efficiency of γ-ray detectors with a good accuracy in order to determine the concentrations of either naturally occurring or artificially prepared radionuclides. This approach is based on the efficiency transfer formula(ET), the effective solid angles, the self- absorptions of the source matrix, the attenuation by the source container and the detector housing materials on the detector efficiency. The experimental calibration process was done using radioactive(Cylindrical Marinelli) sources, in different dimensions,that contain aqueous152 Eu radionuclide. The comparison point to a fine agreement between the experimental measured and calculated efficiencies for the(NaI HPGe) detectors using volumetric radioactive sources.     

Improvements on the third generation of electron momentum spectrometer

The significant modifications to our recently constructed electron momentum spectrometer have been implemented. Compared with our previous report, the energy and the angle resolutions are significantly improved and reach △E = 0.45 eV, △θ = ±0.53° and △φ = ±0.84°, respectively. Moreover, the details of data reduction and the relation between azimuthal angle range and the sensitivity are discussed.     

Determination of the number of atoms of the long-lived nuclide ~(126)Sn by γ-ray spectrometry

By using HPGe γ-ray spectrometry, the activity of the long-lived fission product 126Sn in a SnOB2 sample was measured. The number of 126Sn atoms and the ratio of 126Sn to Sn were calculated based on the half-life value of 2.35×105a and the chemical stoichiometry. The result of the ratio of 126Sn to Sn, (1.033±0.037)×10-8, is consistent with the results measured by the accelerator mass spectrometry (AMS) within uncertainty limits, which confirms our procedures in the measurement of 126Sn by AMS and lays a foundation for the AMS measurement of 126Sn at much lower levels.     

Unified treatment for accurate and fast evaluation of the Fermi-Dirac functions

A new analytical approach to the computation of the Fermi-Dirac(FD) functions is presented,which was suggested by previous experience with various algorithms.Using the binomial expansion theorem,these functions are expressed through the binomial coefficients and familiar incomplete Gamma functions.This simplification and the use of the memory of the computer for the calculation of binomial coefficients may extend the limits to large arguments for users and result in speedier calculation,should such limits be required in practice.Some numerical results are presented for significant mapping examples and they are briefly discussed.     

Decay constants of pseudoscalar and vector mesons with improved holographic wavefunction

We calculate the decay constants of light and heavy-light pseudoscalar and vector mesons with improved soft-wall holographic wavefuntions,which take into account the effects of both quark masses and dynamical spins.We find that the predicted decay constants,especially for the ratio f V/f P,based on light-front holographic QCD,can be significantly improved,once the dynamical spin effects are taken into account by introducing the helicity-dependent wavefunctions.We also perform detailed χ~2 analyses for the holographic parameters(i.e.the mass-scale parameterκ and the quark masses),by confronting our predictions with the data for the charged-meson decay constants and the meson spectra.The fitted values for these parameters are generally in agreement with those obtained by fitting to the Regge trajectories.At the same time,most of our results for the decay constants and their ratios agree with the data as well as the predictions based on lattice QCD and QCD sum rule approaches,with only a few exceptions observed.     

Effect of Gamma-Ray Beaming on the Fluxes of Gamma-Ray Pulsars

We study the effect of γ-ray beaming on γ-ray emission of the pulsars in a self-sustained outer gap model. In this model, averaged γ-rav flux is a function of period, magnetic field, magnetic inclination angle and solid angle of γ-ray beaming for a γ-ray pulsar. We generate a sample of γ-ray pulsars with their ages less than 10^6 ears by using the Monte Carlo method, and then study the γ-ray beaming effect. The comparison of distributions of periods, magnetic fields, distances, γ-ray energy fluxes and period derivatives of the simulated γ-ray pulsars with those of observed γ-ray pulsars by the detector EGRET shows that γ-ray beaming has an important role on the detection of γ-ray pulsars. Furthermore, possible γ-ray pulsars observed by the detector GLAST are predicted.