arXiv subjects

Z. J. Jiang

Publications and source records attributed to Z. J. Jiang.

At least 37 records · Page 2Linked to original sources

arXiv2023

Does or did the supernova remnant Cassiopeia A operate as a PeVatron?

For decades, supernova remnants (SNRs) have been considered the prime sources of Galactic Cosmic rays (CRs). But whether SNRs can accelerate CR protons to PeV energies and thus dominate CR flux up to the knee is currently under intensive theoretical and phenomenological debate. The direct test of the ability of SNRs to operate as CR PeVatrons can be provided by ultrahigh-energy (UHE; $E_γ\geq 100$~TeV) $γ$-rays. In this context, the historical SNR Cassiopeia A (Cas A) is considered one of the most promising target for UHE observations. This paper presents the observation of Cas A and its vicinity by the LHAASO KM2A detector. The exceptional sensitivity of LHAASO KM2A in the UHE band, combined with the young age of Cas A, enabled us to derive stringent model-independent limits on the energy budget of UHE protons and nuclei accelerated by Cas A at any epoch after the explosion. The results challenge the prevailing paradigm that Cas A-type SNRs are major suppliers of PeV CRs in the Milky Way.

astro-ph.HE↗

arXiv2023

Measurement of ultra-high-energy diffuse gamma-ray emission of the Galactic plane from 10 TeV to 1 PeV with LHAASO-KM2A

The diffuse Galactic $γ$-ray emission, mainly produced via interactions between cosmic rays and the interstellar medium and/or radiation field, is a very important probe of the distribution, propagation, and interaction of cosmic rays in the Milky Way. In this work we report the measurements of diffuse $γ$-rays from the Galactic plane between 10 TeV and 1 PeV energies, with the square kilometer array of the Large High Altitude Air Shower Observatory (LHAASO). Diffuse emissions from the inner ($15^{\circ} 10$~TeV). The energy spectrum in the inner Galaxy regions can be described by a power-law function with an index of $-2.99\pm0.04$, which is different from the curved spectrum as expected from hadronic interactions between locally measured cosmic rays and the line-of-sight integrated gas content. Furthermore, the measured flux is higher by a factor of $\sim3$ than the prediction. A similar spectrum with an index of $-2.99\pm0.07$ is found in the outer Galaxy region, and the absolute flux for $10\lesssim E\lesssim60$ TeV is again higher than the prediction for hadronic cosmic ray interactions. The latitude distributions of the diffuse emission are consistent with the gas distribution, while the longitude distributions show clear deviation from the gas distribution. The LHAASO measurements imply that either additional emission sources exist or cosmic ray intensities have spatial variations.

astro-ph.HE↗

arXiv2022

Flux Variations of Cosmic Ray Air Showers Detected by LHAASO-KM2A During a Thunderstorm on 10 June 2021

The Large High Altitude Air Shower Observatory (LHAASO) has three sub-arrays, KM2A, WCDA and WFCTA. The flux variations of cosmic ray air showers were studied by analyzing the KM2A data during the thunderstorm on 10 June 2021. The number of shower events that meet the trigger conditions increases significantly in atmospheric electric fields, with maximum fractional increase of 20%. The variations of trigger rates (increases or decreases) are found to be strongly dependent on the primary zenith angle. The flux of secondary particles increases significantly, following a similar trend with that of the shower events. To better understand the observed behavior, Monte Carlo simulations are performed with CORSIKA and G4KM2A (a code based on GEANT4). We find that the experimental data (in saturated negative fields) are in good agreement with simulations, assuming the presence of a uniform upward electric field of 700 V/cm with a thickness of 1500 m in the atmosphere above the observation level. Due to the acceleration/deceleration and deflection by the atmospheric electric field, the number of secondary particles with energy above the detector threshold is modified, resulting in the changes in shower detection rate.

astro-ph.HE↗

arXiv2021

Peta-electron volt gamma-ray emission from the Crab Nebula

The Crab pulsar and the surrounding nebula powered by the pulsar's rotational energy through the formation and termination of a relativistic electron-positron wind is a bright source of gamma-rays carrying crucial information about this complex conglomerate. We report the detection of $γ$-rays with a spectrum showing gradual steepening over three energy decades, from $5\times 10^{-4}$ to $1.1$ petaelectronvolt (PeV). The ultra-high-energy photons exhibit the presence of a PeV electron accelerator (a pevatron) with an acceleration rate exceeding 15% of the absolute theoretical limit. Assuming that unpulsed $γ$-rays are produced at the termination of the pulsar's wind, we constrain the pevatron's size, between $0.025$ and $0.1$ pc, and the magnetic field $\approx 110 μ$G. The production rate of PeV electrons, $2.5 \times 10^{36}$ erg $\rm s^{-1}$, constitutes 0.5% of the pulsar's spin-down luminosity, although we do not exclude a non-negligible contribution of PeV protons to the production of the highest energy $γ$-rays.

astro-ph.HE↗

arXiv2021

Construction and On-site Performance of the LHAASO WFCTA Camera

The focal plane camera is the core component of the Wide Field-of-view Cherenkov/fluorescence Telescope Array (WFCTA) of the Large High-Altitude Air Shower Observatory (LHAASO). Because of the capability of working under moonlight without aging, silicon photomultipliers (SiPM) have been proven to be not only an alternative but also an improvement to conventional photomultiplier tubes (PMT) in this application. Eighteen SiPM-based cameras with square light funnels have been built for WFCTA. The telescopes have collected more than 100 million cosmic ray events and preliminary results indicate that these cameras are capable of working under moonlight. The characteristics of the light funnels and SiPMs pose challenges (e.g. dynamic range, dark count rate, assembly techniques). In this paper, we present the design features, manufacturing techniques and performances of these cameras. Finally, the test facilities, the test methods and results of SiPMs in the cameras are reported here.

physics.ins-det↗

arXiv2021

Calibration of the Air Shower Energy Scale of the Water and Air Cherenkov Techniques in the LHAASO experiment

The Wide Field-of-View Cherenkov Telescope Array (WFCTA) and the Water Cherenkov Detector Arrays (WCDA) of LHAASO are designed to work in combination for measuring the energy spectra of various cosmic ray species over a very wide energy range from a few TeV to 10 PeV. The energy calibration of WCDA can be achieved with a proven technique of measuring the westward shift of the Moon shadow of galactic cosmic rays due to the geomagnetic field. This deflection angle $Δ$ is inversely proportional to the energy of the cosmic rays. The precise measurements of the shifts by WCDA allows us to calibrate its energy scale for energies as high as 35 TeV. The energy scale measured by WCDA can be used to cross calibrate the energy reconstructed by WFCTA, which spans the whole energy range up to 10 PeV. In this work, we will demonstrate the feasibility of the method using the data collected from April 2019 to January 2020 by the WFCTA array and WCDA-1 detector, the first of the three water Cherenkov ponds, already commissioned at LHAASO site.

astro-ph.IM↗

arXiv2020

The observation of the Crab Nebula with LHAASO-KM2A for the performance study

As a sub-array of the Large High Altitude Air Shower Observatory (LHAASO), KM2A is mainly designed to cover a large fraction of the northern sky to hunt for gamma-ray sources at energies above 10 TeV. Even though the detector construction is still underway, a half of the KM2A array has been operating stably since the end of 2019. In this paper, we present the pipeline of KM2A data analysis and the first observation on the Crab Nebula, a standard candle in very high energy gamma-ray astronomy. We detect gamma-ray signals from the Crab Nebula in both energy ranges of 10$-$100 TeV and $>$100 TeV with high significance, by analyzing the KM2A data of 136 live days between December 2019 and May 2020. With the observations, we test the detector performance including angular resolution, pointing accuracy and cosmic ray background rejection power. The energy spectrum of the Crab Nebula in the energy range 10-250 TeV fits well with a single power-law function dN/dE =(1.13$\pm$0.05$_{stat}$$\pm$0.08$_{sys}$)$\times$10$^{-14}$$\cdot$(E/20TeV)$^{-3.09\pm0.06_{stat}\pm0.02_{sys}}$ cm$^{-2}$ s$^{-1}$ TeV$^{-1}$. It is consistent with previous measurements by other experiments. This opens a new window of gamma-ray astronomy above 0.1 PeV through which ultrahigh-energy gamma-ray new phenomena, such as cosmic PeVatrons, might be discovered.

astro-ph.HE↗

arXiv2016

A universal description of pseudorapidity distributions in both nucleus-nucleus and p-p collisions at now available energy regions

Investigations have shown that the collective motion not only appears in nucleus-nucleus but also in p-p collisions. The best tool for depicting such collective motion is relativistic hydrodynamics. In this paper, the collective motion is assumed obeying the hydro model which integrates the features of Landau and Hwa-Bjorken theory and is one of a very few analytically solvable models. The fluid is then supposed freezing out into charged particles from a time-like hypersurface with a fixed time of t_FO . The researches of present paper show that this part of charged particles together with leading particles, which, by conventional definition, carry on the quantum numbers of colliding nucleons and take away the most part of incident energy, can give a proper universal description to the pseudorapidity distributions of charged particles measured in both nucleus-nucleus and p-p collisions at now available energy regions.

nucl-th↗

arXiv2015

A description of the pseudorapidity distributions in heavy ion collisions at RHIC and LHC energies

The charged particles produced in nucleus-nucleus collisions are classified into two parts,One is from the hot and dense matter created in collisions. The other is from leading particles. The hot and dense matter is assumed to expand and generate particles according to BJP hydrodynamics, a theory put forward by A. Bialas, R. A. Janik and R. Peschanski. The leading particles are argued to possess a Gaussian rapidity distribution with the normalization constant equaling the number of participants. A comparison is made between the theoretical results and the experimental measurements performed by BRAHMS and PHOBOS Collaboration at BNL-RHIC in Au-Au and Cu-Cu collisions at sqrt(s_NN)=200 GeV and by ALICE Collaboration at CERN-LHC in Pb-Pb collisions at sqrt(s_NN) =2.76 TeV. The theoretical results are well consistent with experimental data.

nucl-th↗

arXiv2015

The Unified Hydrodynamics and the Pseudorapidity Distributions in Heavy Ion Collisions at BNL-RHIC and CERN-LHC Energies

The charged particles produced in nucleus-nucleus collisions are divided into two parts. One is from the hot and dense matter created in collisions. The other is from leading particles. The hot and dense matter is assumed to expand according to unified hydrodynamics and freezes out into charged particles from a space-like hypersurface with a fixed proper time of Tau_FO.The leading particles are conventionally taken as the particles which inherit the quantum numbers of colliding nucleons and carry off most of incident energy. The rapidity distributions of the charged particles from these two parts are formulated analytically, and a comparison is made between the theoretical results and the experimental measurements performed in Au-Au and Pb-Pb collisions at the respective BNL-RHIC and CERN-LHC energies. The theoretical results are well consistent with experimental data.

nucl-th↗

arXiv2015

An extended view of the Pisces Overdensity from the SCUSS survey

SCUSS is a u-band photometric survey covering about 4000 square degree of the South Galactic Cap, reaching depths of up to 23 mag. By extending around 1.5 mag deeper than SDSS single-epoch u data, SCUSS is able to probe much a larger volume of the outer halo, i.e. with SCUSS data blue horizontal branch (BHB) stars can trace the outer halo of the Milky Way as far as 100-150 kpc. Utilizing this advantage we combine SCUSS u band with SDSS DR9 gri photometric bands to identify BHB stars and explore halo substructures. We confirm the existence of the Pisces overdensity, which is a structure in the outer halo (at around 80 kpc) that was discovered using RR Lyrae stars. For the first time we are able to determine its spatial extent, finding that it appears to be part of a stream with a clear distance gradient. The stream, which is ~5 degrees wide and stretches along ~25 degrees, consists of 20-30 BHBs with a total significance of around 6sigma over the background. Assuming we have detected the entire stream and that the progenitor has fully disrupted, then the number of BHBs suggests the original system was similar to smaller classical or a larger ultra-faint dwarf galaxy. On the other hand, if the progenitor still exists, it can be hunted for by reconstructing its orbit from the distance gradient of the stream. This new picture of the Pisces overdensity sheds new light on the origin of this intriguing system.

astro-ph.GA↗

arXiv2014

The pseudorapidity distributions of the produced charged particles in p-p collisions at center-of-mass energies from 23.6 to 900 GeV

In p-p collisions, there are two leading particles. One is in projectile fragmentation region. The other is in target fragmentation region. The investigations of present paper show that, just like in nucleus-nucleus collisions, the revised Landau hydrodynamic model alone is not enough to have a good description to the measured pseudorapidity distributions of the produced charged particles in p-p collisions. Only after the leading particles are taken into account as well, can the experimental data be matched up properly. The theoretical model works well in p-p collisions in the whole available energy region from sqrt(s_NN) =23.6 to 900 GeV.

nucl-th↗

arXiv2014

The evolution-dominated hydrodynamic model and the pseudorapidity distributions in high energy physics

By taking into account the effects of leading particles, we discuss the pseudorapidity distributions of the charged particles produced in high energy heavy ion collisions in the context of evolution-dominated hydrodynamic model. The leading particles are supposed to have a Gaussian rapidity distribution normalized to the number of participants. A comparison is made between the theoretical results and the experimental measurements performed by BRAHMS and PHOBOS Collaboration at BNL-RHIC in Au-Au and Cu-Cu collisions at sqrt(s_NN) =200 GeV and by ALICE Collaboration at CERN-LHC in Pb-Pb collisions at sqrt(s_NN) =2.76 TeV.

nucl-th↗

arXiv2013

Phase-Averaged Gamma-Ray Spectra from MSPs

{\it Fermi}-LAT has detected pulsed gamma-ray emissions with high confidences from more than 40 millisecond pulsars (MSPs). Here we study the phase-averaged gamma-ray properties of MSPs by using revised version of a self-consistent outer gap model. In this model, a strong multipole magnetic field near the stellar surface for a MSP is assumed and such a field will be close to the surface magnetic fields ($\sim 10^{11}- 10^{12}$ G) of young ulsars; the outer gap of a MSP is controlled by photon-photon pair production process, where the effects of magnetic inclination angle ($α$) and magnetic geometry have been taken into account, therefore the fractional size of the outer gap is a function of not only pulsar's period and magnetic field strength but also magnetic inclination angle and radial distance to the neutron star, the inner boundary of the outer gap can be estimated by the pair production process of the gamma-ray photons which are produced by the back-flowing particles through the null charge surface; inside the outer gap, a Gaussian distribution of the parallel electric field along the trans-field thickness is assumed, and the gamma-ray emission is represented by the emission from the average radial distance along the central field lines of the outer gap. Using this model, the phase-averaged gamma-ray spectra are calculated and compared with the observed spectra of 37 MSPs given by the second {\it Fermi}-LAT catalog of gamma-ray pulsars, our results show that the {\it Fermi}-LAT results can be well explained by this model. The thermal X-ray emission properties from MSPs are also investigated.

astro-ph.HE↗

arXiv2005

Possible TeV Source Candidates In The Unidentified EGRET Sources

We study the $γ$-ray emission from the pulsar magnetosphere based on outer gap models, and the TeV radiation from pulsar wind nebulae (PWNe) through inverse Compton scattering using a one-zone model. We showed previously that GeV radiation from the magnetosphere of mature pulsars with ages of $\sim 10^5-10^6$ years old can contribute to the high latitude unidentified EGRET sources. We carry out Monte Carlo simulations of $γ$-ray pulsars in the Galaxy and the Gould Belt, assuming the pulsar birth rate, initial position, proper motion velocity, period, and magnetic field distribution and evolution based on observational statistics. We select from the simulation a sample of mature pulsars in the Galactic plane ($|b|\leq 5^\circ$) and in the high latitude ($|b|> 5^\circ$) which could be detected by EGRET. The TeV flux from the pulsar wind nebulae of our simulated sample through the inverse Compton scattering by relativistic electrons on the microwave cosmic background and synchrotron seed photons are calculated. The predicted fluxes are consistent with the present observational constraints. We suggest that strong EGRET sources can be potential TeV source candidates for present and future ground-based TeV telescopes.

astro-ph↗

arXiv2005

Contribution to diffuse gamma-rays in the Galactic center region from unresolved millisecond pulsars

The diffuse gamma-rays in the Galactic center region have been studied. We propose that there exists a population of millisecond pulsars (MSPs) in the Galactic Center, which will emit GeV gamma-rays through the synchrotron-curvature radiation as predicted by outer gap models. These GeV gamma-rays from unresolved millisecond pulsars probably contribute to the diffuse gamma-ray spectrum detected by EGRET which displays a break at a few GeV. We have used the Monte Carlo method to obtain the simulated samples of millisecond pulsars in the Galactic center region covered by EGRET ($\sim 1.5^\circ$) according to the different period and magnetic field distributions from the observed millisecond pulsars in the Galactic field and globular clusters, and superposed their synchrotron-curvature spectra to derive the total GeV flux. Our simulated results suggest that there probably exist about 6000 unresolved millisecond pulsars in the region of the angular resolution for EGRET, whose emissions could contribute significantly to the observed diffuse gamma-rays in the Galactic center.

astro-ph↗

arXiv2004

Gamma-ray Emission Properties from Mature Pulsars in the Galaxy and in the Gould Belt

We study the $γ$-ray emission properties of pulsars by using a new self-consistent outer gap model. The outer gap can exist in pulsars with age over million years old if the effect of magnetic inclination angle as well as the average properties of the outer gap are considered. The mature $γ$-ray pulsars, whose ages are between 0.3 to 3 million years old, are able to move up to high galactic latitude. Moreover, their $γ$-ray luminosity are weaker and their spectra are softer than those of younger $γ$-ray pulsars in the galactic plane significantly. We use a Monte Carlo method to simulate the statistical properties of $γ$-ray pulsars in the Galaxy as well as in the Gould Belt. We find that $γ$-ray pulsars located at $\mid b \mid < 5^{\circ}$ and located at $\mid b \mid > 5^{\circ}$ have very different properties. High galactic latitude $γ$-ray pulsars are dominated by mature pulsars with longer periods, weaker fluxes and softer spectra. If the pulsar birth rate in the Galaxy and the Gould Belt are $\sim 10^{-2}yr^{-1}$ and $\sim 2\times 10^{-5}yr^{-1}$ respectively, there are 42 and 35 radio-quiet $γ$-ray pulsars for $\mid b \mid < 5^{\circ}$ and $\mid b \mid > 5^{\circ}$ respectively. Radio-quiet $γ$-ray pulsars from the Gould Belt are 2 and 13 for $\mid b \mid < 5^{\circ}$ and $\mid b \mid > 5^{\circ}$ respectively. We suggest that a good fraction of unidentified EGRET $γ$-ray sources may be these radio-quiet $γ$-ray pulsars. Furthermore $γ$-ray pulsars located at $\mid b \mid > 5^{\circ}$ satisfies $L_γ \propto L^β_{sd}$ whereas $L_γ \propto L^δ_{sd}$ for $γ$-ray pulsars in the galactic plane, where $β\sim 0.6$ and $δ\sim 0.3$ respectively.

astro-ph↗

arXiv2004

Gamma-ray Luminosity and Death Lines of Pulsars with Outer Gaps

We re-examine the outer gap size by taking the geometry of the dipole magnetic field into account. Furthermore, we also consider that instead of taking the gap size at half of the light cylinder radius to represent the entire outer gap it is more appropriate to average the entire outer gap size over the distance. When these two factors are considered, the derived outer gap size f is not only the function of period P and magnetic field B of the neutron star, but also the function of the average radial distance to the neutron star . We use this new outer gap model to study $γ$-ray luminosity of pulsars, and to study the death lines of $γ$-ray emission of the pulsars. Our model can predict the $γ$-ray luminosity of individual pulsar if its $P, B$ and $α$ are known. Since different pulsars have different $α$, this explains why some $γ$-ray pulsars have very similar $P$ and $B$ but have very different $γ$-ray luminosities. In determining the death line of $γ$-ray pulsars, we have used a new criterion based on concrete physical reason. In estimate of the fractional size of the outer gap, two possible X-ray fields are considered: (i) X-rays are produced by the neutron star cooling and polar cap heating, and (ii)X-rays are produced by the bombardment of the relativistic particles from the outer gap on the stellar surface. Since it is very difficult to measure $α$ in general, we use a Monte Carlo method to simulate the properties of $γ$-ray pulsars in our galaxy. We find that this new outer gap model predicts many more weak $γ$-ray pulsars. For all simulated $γ$-ray pulsars with self-sustained outer gaps, $γ$-ray luminosity $L_γ$ satisfies $L_γ\propto L^δ_{sd}$; where the value of $δ$ depends on the ensitivity of the $γ$-ray detector.

astro-ph↗