arXiv subjects

J. L. Xu

Publications and source records attributed to J. L. Xu.

9 recordsLinked to original sources

arXiv2023

Measurement of the Gamma-Ray Energy Spectrum beyond 100 TeV from the HESS J1843$-$033 Region

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 $100\, {\rm TeV}$ 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 $0.34^{\circ} \pm 0.12^{\circ}$ is successfully detected above $25\, {\rm TeV}$ at $(α,\, δ) = (281.09^{\circ}\pm 0.10^{\circ},\, -3.76^{\circ}\pm 0.09^{\circ})$ 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 $25\, {\rm TeV} < E < 130\, {\rm TeV}$ is described with ${\rm d}N/{\rm d}E = (9.70\pm 1.89)\times 10^{-16} (E/40\, {\rm TeV})^{-3.26\pm 0.30}\, {\rm TeV}^{-1} {\rm cm}^{-2} {\rm s}^{-1}$, 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 $49.5\pm 9.0\, {\rm TeV}$. Associations of TASG J1844-038 with SNR G28.6$-$0.1 and PSR J1844-0346 are also discussed in detail for the first time.

astro-ph.HE↗

arXiv2023

Observation of gamma rays up to 320 TeV from the middle-aged TeV pulsar wind nebula HESS J1849$-$000

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 Gaussian standard deviation $σ$. 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.

astro-ph.HE↗

arXiv2023

Designing a Transition Photonic Band with a Synthetic Moire Sphere

In recent years, twisted bilayer graphene has become a hot topic and inspired the research upsurge of photonic moiré lattice. Here, we designed a photonic moiré superlattice with two synthetic twist angles and constructed a synthetic moiré sphere based on these two angles. Thus, we have more degrees of freedom to design the band structure flexibly. A type of transition photonic bands (TPBs) is obtained in such a moiré superlattice. We investigate the influence of two twist angles on TPBs and find a series of magic angle pairs with optimal band compression of TPB. The interesting optical properties of TPBs are experimentally demonstrated, including pulse delay, nonlinear optical enhancement, and pulse width compression. Our work introduces a new path to explore multi-twist angles moiré superlattices and reveals that the designed photonic moiré superlattice based on moiré spheres has broad application prospects including optical signal processing, nonlinear optics processes ,and other light-matter interactions.

physics.optics↗

arXiv2021

Gamma-ray Observation of the Cygnus Region in the 100 TeV Energy Region

We report observations of gamma-ray emissions with energies in the 100 TeV energy region from the Cygnus region in our Galaxy. Two sources are significantly detected in the directions of the Cygnus OB1 and OB2 associations. Based on their positional coincidences, we associate one with a pulsar PSR J2032+4127 and the other mainly with a pulsar wind nebula PWN G75.2+0.1 with the pulsar moving away from its original birthplace situated around the centroid of the observed gamma-ray emission. This work would stimulate further studies of particle acceleration mechanisms at these gamma-ray sources.

astro-ph.HE↗

arXiv2020

No pulsed radio emission during a bursting phase of a Galactic magnetar

Fast radio bursts (FRBs) are mysterious millisecond-duration radio transients of unknown origin observed at extragalactic distances. It has been long speculated that magnetars are the engine powering repeating bursts from FRB sources, but no convincing evidence has been collected so far\cite{sun19}. Recently, the Galactic magnetar SGR J1935+2154 entered an active phase by emitting intense soft Gamma-ray bursts. One FRB-like event with two peaks (FRB 200428) and a luminosity slightly lower than the faintest extragalactic FRBs was detected from the source, in association with a soft Gamma-ray / hard X-ray flare. Here we report an eight-hour targeted radio observational campaign comprising four sessions and assisted by multi-wavelength (optical and hard X-rays) data. During the third session, 29 soft Gamma-ray repeater (SGR) bursts were detected in Gamma-ray energies. Throughout the observing period, we detected no single dispersed pulsed emission coincident with the arrivals of SGR bursts, but unfortunately we were not observing when the FRB was detected. The non-detection places a fluence upper limit that is eight orders of magnitude lower than the fluence of FRB 200428. Our results suggest that FRB -- SGR burst associations are rare. FRBs may be highly relativistic and geometrically beamed, or FRB-like events associated with SGR bursts may have narrow spectra and characteristic frequencies outside the observed band. It is also possible that the physical conditions required to achieve coherent radiation in SGR bursts are difficult to satisfy, and that only under extreme conditions could an FRB be associated with an SGR burst.

astro-ph.HE↗

arXiv2017

Discriminating cosmic muons and radioactivity using a liquid scintillation fiber detector

In the case of underground experiments for neutrino physics or rare event searches, the background caused by cosmic muons contributes significantly and therefore must be identified and rejected. We proposed and optimized a new detector using liquid scintillator with wavelenghth-shifting fibers which can be employed as a veto detector for cosmic muons background rejection. From the prototype study, it has been found that the detector has good performances and is capable of discriminating between muons induced signals and environmental radiation background. Its muons detection efficiency is greater than 98$\%$, and on average, 58 photo-electrons (p.e.) are collected when a muon passes through the detector. To optimize the design and enhance the collection of light, the reflectivity of the coating materials has been studied in detail. A Monte Carlo simulation of the detector has been developed and compared to the performed measurements showing a good agreement between data and simulation results.

physics.ins-det↗

arXiv2006

Optimally Designed Digitally-Doped Mn:GaAs

We use the ab initio local-density approximation (LDA) to study of exchange interactions and Tc of Ga1−xMnxAs grown in digitally doped structures. We analyze the crystallographic dependence of exchange interactions predicted by the LDA in terms of the Mn t2 and e levels, and explain the origin of the antiferromagnetic contribution to the total exchange interactions. We exploit this dependence and consider delta-doping in specific orientations where the antiferromagnetic interactions are minimized, to optimize Tc of the system. By including hole doping with the addition of Be in the GaAs host digitally doped Ga1−xMnxAs is predicted to be significantly above room temperature.

cond-mat.mtrl-sci↗

arXiv2005

Optimizing Tc in the (Mn,Cr,Ga)As and (Mn,Ga)(As,P) Ternary Alloys

We explore two possible ways to enhance the critical temperature $T_c$ in the dilute magnetic semiconductor Mn$_{0.08}$Ga$_{0.92}$As. Within the context of the double-exchange and RKKY pictures, the ternary alloys Mn$_{x}$Cr$_{0.08-x}$Ga$_{0.92}$As and Mn$_{0.08}$Ga$_{0.92}$As$_y$P$_{1-y}$ might be expected to have $T_c$ higher than the pseudobinary Mn$_{0.08}$Ga$_{0.92}$As. To test whether the expectations from model pictures are confirmed, we employ linear response theory within the local-density approximation to search for theoretically higher critical temperatures in these ternary alloys. Our results show that neither co-doping Mn with Cr, nor alloying As with P improves $T_c$. Alloying with Cr is found to be deleterious to the $T_c$. Mn$_{0.08}$Ga$_{0.92}$As$_y$P$_{1-y}$ shows almost linear dependence of $T_c$ on $y$.

cond-mat.mtrl-sci↗

arXiv2004

Role of Disorder in Mn:GaAs, Cr:GaAs, and Cr:GaN

We present calculations of magnetic exchange interactions and critical temperature T_c in Mn:GaAs, Cr:GaAs and Cr:GaN. The local spin density approximation is combined with a linear-response technique to map the magnetic energy onto a Heisenberg hamiltonion, but no significant further approximations are made. Special quasi-random structures in large unit cells are used to accurately model the disorder. T_c is computed using both a spin-dynamics approach and the cluster variation method developed for the classical Heisenberg model. We show the following: (i) configurational disorder results in large dispersions in the pairwise exchange interactions; (ii) the disorder strongly reduces T_c; (iii) clustering in the magnetic atoms, whose tendency is predicted from total-energy considerations, further reduces T_c. Additionally the exchange interactions J(R) are found to decay exponentially with distance R^3 on average; and the mean-field approximation is found to be a very poor predictor of T_c, particularly when J(R) decays rapidly. Finally the effect of spin-orbit coupling on T_c is considered. With all these factors taken into account, T_c is reasonably predicted by the local spin-density approximation in MnGaAs without the need to invoke compensation by donor impurities.

cond-mat.mtrl-sci↗