arXiv subjects

K. S. Cheng

Publications and source records attributed to K. S. Cheng.

At least 109 records · Page 6Linked to original sources

arXiv2011

Inflating wormholes in the braneworld models

The braneworld model, in which our Universe is a three-brane embedded in a five-dimensional bulk, allows the existence of wormholes, without any violation of the energy conditions. A fundamental ingredient of traversable wormholes is the violation of the null energy condition (NEC). However, in the brane world models, the stress energy tensor confined on the brane, threading the wormhole, satisfies the NEC. In conventional general relativity, wormholes existing before inflation can be significantly enlarged by the expanding spacetime. We investigate the evolution of an inflating wormhole in the brane world scenario, in which the wormhole is supported by the nonlocal brane world effects. As a first step in our study we consider the possibility of embedding a four-dimensional brane world wormhole into a five dimensional bulk. The conditions for the embedding are obtained by studying the junction conditions for the wormhole geometry, as well as the full set of the five dimensional bulk field equations. For the description of the inflation we adopt the chaotic inflation model. We study the dynamics of the brane world wormholes during the exponential inflation stage, and in the stage of the oscillating scalar field. A particular exact solution corresponding to a zero redshift wormhole is also obtained. The resulting evolution shows that while the physical and geometrical parameters of a zero redshift wormhole decay naturally, a wormhole satisfying some very general initial conditions could turn into a black hole, and exist forever.

gr-qc↗

arXiv2011

Exploring the dark accelerator HESS J1745-303 with Fermi Large Area Telescope

We present a detailed analysis of the gamma-ray emission from HESS J1745-303 with the data obtained by the Fermi Gamma-ray Space Telescope in the first ~29 months observation.The source can be clearly detected at the level of ~18-sigma and ~6-sigma in 1-20 GeV and 10-20 GeV respectively. Different from the results obtained by the Compton Gamma-ray Observatory, we do not find any evidence of variability. Most of emission in 10-20 GeV is found to coincide with the region C of HESS J1745-303. A simple power-law is sufficient to describe the GeV spectrum with a photon index of ~2.6. The power-law spectrum inferred in the GeV regime can be connected to that of a particular spatial component of HESS J1745-303 in 1-10 TeV without any spectral break. These properties impose independent constraints for understanding the nature of this "dark particle accelerator".

astro-ph.HE↗

arXiv2011

Population study for gamma-ray pulsars: (III) Radiation characteristics and viewing geometry

We have performed a Monte-Calro simulation for Galactic population of pulsars and for the $γ$-ray observations. We apply outer gap model for the $γ$-ray emission process, and study the radiation characteristics as a function of the magnetic inclination angle $(α)$ and the Earth viewing angle ($ζ$). In our model, the $γ$-ray flux and the spectral cut-off energy tend to decrease as the inclination and viewing angles deviate from $90^{\circ}$. The emerging spectrum above 100 MeV becomes soft with a photon index $p\sim 1.8-2$ for $ζ\rightarrow 90^{\circ}$ and $p\sim 1.2-1.3$ for $ζ\ll 90^{\circ}$. Our simulation predicts that the pulsars with larger inclination angles ($α=70-90^{\circ}$) and larger viewing angles ($ζ=70-90^{\circ}$) have been preferentially detected by the $Fermi$ $γ$-ray telescope, and hence the observed pulse profiles of the $γ$-ray pulsars have the double peak structure rather than single peak. In the simulation, most $γ$-ray millisecond pulsars are categorized as the radio-quiet $γ$-ray pulsars, because its radio fluxes are under the sensitivities of the major radio surveys. Even we drastically increase the radio sensitivity by a factor of ten, the number of radio-selected millisecond pulsars detected by the $Fermi$ ten years observations is still much less than the expected $γ$-ray-selected millisecond pulsars, indicating the radio-quiet millisecond pulsars must contribute to the $Fermi$ unidentified sources and/or the $γ$-ray background radiations. The Unique radiation properties of the low-efficient $γ$-ray pulsar, PSR J0659+1414, can be explained by the present our gap model with a viewing geometry of $α\sim ζ=40^{\circ}-50^{\circ}$.

astro-ph.HE↗

arXiv2011

Origin of the Fermi Bubble

Fermi has discovered two giant gamma-ray-emitting bubbles that extend nearly 10kpc in diameter north and south of the galactic center (GC). The existence of the bubbles was first evidenced in X-rays detected by ROSAT and later WMAP detected an excess of radio signals at the location of the gamma-ray bubbles. We propose that periodic star capture processes by the galactic supermassive black hole, Sgr A$^*$, with a capture rate $3\times 10^{-5}$yr$^{-1}$ and energy release $\sim 3\times 10^{52}$erg per capture can produce very hot plasma $\sim 10$keV with a wind velocity $\sim 10^8$cm/s injected into the halo and heat up the halo gas to $\sim 1$keV, which produces thermal X-rays. The periodic injection of hot plasma can produce shocks in the halo and accelerate electrons to $\sim$TeV, which produce radio emission via synchrotron radiation, and gamma-rays via inverse Compton scattering with the relic and the galactic soft photons.

astro-ph.HE↗

arXiv2011

Three-dimensional Two-Layer Outer Gap Model: Fermi Energy Dependent Light Curves of the Vela Pulsar

We extend the two-dimensional two-layer outer gap model to a three-dimensional geometry and use it to study the high-energy emission of the Vela pulsar. In this model, the outer gap is divided into two parts, i.e. the main acceleration region on the top of last-open field lines and the screening region around the upper boundary of the gap. In the main acceleration region, the charge density is much lower than the Goldreich-Julian charge density and the charged particles are accelerated by the electric field along the magnetic field to emit multi-GeV photons. In the screening region, the charge density is larger than the Goldreich-Julian value to close the gap and particles in this region are responsible for multi-100MeV photon emission. We apply this three dimensional two-layer model to the Vela pulsar and compare the model light curves, the phase-averaged spectrum and the phase-resolved spectra with the recent Fermi observations, which also reveals the existence of the third peak between two main peaks. The phase position of the third peak moves with the photon energy, which cannot be explained by the geometry of magnetic field structure and the caustic effects of the photon propagation. We suggest that the existence of the third peak and its energy dependent movement results from the azimuthal structure of the outer gap.

astro-ph.HE↗

arXiv2011

Population study for gamma-ray pulsars; II Millisecond pulsars

The population of $γ$-ray emitting millisecond pulsars (MSPs) is studied by using Monte-Carlo techniques. We simulate the Galactic distributions of the MSPs, and apply the outer gap model for the $γ$-ray emission from each simulated MSP. We take into account the dependence of the observed $γ$-ray flux on the viewing angle and inclination angle, which is the angle between the rotation axis and the magnetic axis, respectively. Using the sensitivity of the six-month long observation of the $Fermi$ telescope and radio sensitivities of existing pulsar surveys, 9-13 radio-selected and 22-35 $γ$-ray-selected pulsars are detected within our simulation. The statistical properties of the simulated population are consistent with the $Fermi$ observations. Scaling the observed sensitivity $\propto \sqrt{T}$, where $T$ is the length of observation time, the present model predicts that over the 5-year mission $Fermi$ would detect 15-22 radio-selected $γ$-ray MSPs, and 95-152 $γ$-ray-selected MSPs. Our simulation also predicts that about 100 (or 200-300) $γ$-ray MSPs with a flux larger $F\ge 10^{-11}~\mathrm{erg/cm^2 s}$ (or $5\times 10^{-12}~\mathrm{erg/cm^2 s}$) irradiate the Earth. With the present sensitivities of the radio surveys, most of them are categorized as $γ$-ray-selected pulsars, indicating that most of the $γ$-ray MSPs have been missed by the present $Fermi$ observations. We argue that the Galactic $Fermi$ unidentified sources located at high latitudes should be dominated by MSPs, whereas the sources in the galactic plane are dominated by radio-quiet canonical pulsars.

astro-ph.HE↗

arXiv2011

The Fundamental Plane of Gamma-ray Globular Clusters

We have investigated the properties of a group of $γ$-ray emitting globular clusters (GCs) which have recently been uncovered in our Galaxy. By correlating the observed $γ$-ray luminosities $L_γ$ with various cluster properties, we probe the origin of the high energy photons from these GCs. We report $L_γ$ is positively correlated with the encounter rate $Γ_{c}$ and the metalicity $\left[{\rm Fe/H}\right]$ which place an intimate link between the gamma-ray emission and the millisecond pulsar population. We also find a tendency that $L_γ$ increase with the energy densities of the soft photon at the cluster location. Furthermore, the two-dimensional regression analysis suggests that $L_γ$, soft photon densities, and $Γ_{c}$/$\left[{\rm Fe/H}\right]$ possibly span fundamental planes which potentially provide better predictions for the $γ$-ray properties of GCs.

astro-ph.HE↗

arXiv2011

Gamma-ray emission from the globular clusters Liller 1, M80, NGC 6139, NGC 6541, NGC 6624, and NGC 6752

Globular clusters (GCs) are emerging as a new class of gamma-ray emitters, thanks to the data obtained from the Fermi Gamma-ray Space Telescope. By now, eight GCs are known to emit gamma-rays at energies >100~MeV. Based on the stellar encounter rate of the GCs, we identify potential gamma-ray emitting GCs out of all known GCs that have not been studied in details before. In this paper, we report the discovery of a number of new gamma-ray GCs: Liller 1, NGC 6624, and NGC 6752, and evidence for gamma-ray emission from M80, NGC 6139, and NGC 6541, in which gamma-rays were found within the GC tidal radius. With one of the highest metallicity among all GCs in the Milky Way, the gamma-ray luminosity of Liller 1 is found to be the highest of all known gamma-ray GCs. In addition, we confirm a previous report of significant gamma-ray emitting region next to NGC 6441. We briefly discuss the observed offset of gamma-rays from some GC cores. The increasing number of known gamma-ray GCs at distances out to ~10 kpc is important for us to understand the gamma-ray emitting mechanism and provides an alternative probe to the underlying millisecond pulsar populations of the GCs.

astro-ph.HE↗

arXiv2010

Evidence for gamma-ray emission from the low-mass X-ray binary system FIRST J102347.6+003841

The low-mass X-ray binary (LMXB) system FIRST J102347.6+003841 hosts a newly born millisecond pulsar (MSP) PSR J1023+0038 that was revealed as the first and only known rotation-powered MSP in a quiescent LMXB. While the system is shown to have an accretion disk before 2002, it remains unclear how the accretion disk has been removed in order to reveal the radio pulsation in 2007. In this Letter, we report the discovery of gamma-rays spatially consistent with FIRST J102347.6+003841, at a significance of 7 standard deviations, using data obtained by the Fermi Gamma-ray Space Telescope. The gamma-ray spectrum can be described by a power law (PL) with a photon index of 2.9+-0.2, resulting in an energy flux above 200 MeV of (5.5+-0.9)x10^{-12} erg cm^{-2}s^{-1}. The gamma-rays likely originate from the MSP PSR J1023+0038, but also possibly from an intrabinary shock between the pulsar and its companion star. To complement the gamma-ray study, we also re-investigate the XMM-Newton data taken in 2004 and 2008. Our X-ray spectral analysis suggests that a broken PL with two distinct photon indices describes the X-ray data significantly better than a single PL. This indicates that there exists two components and that both components appear to vary with the orbital phase. The evidence for gamma-ray emission conforms with a recent suggestion that gamma-rays from PSR J1023+0038 may be responsible for ejecting the disk material out of the system.

astro-ph.HE↗

arXiv2010

Population study for gamma-ray pulsars with the outer gap model

Inspired by increase of population of $γ$-ray emitting pulsars by the $Fermi$ telescope, we perform a population study for $γ$-ray emitting canonical pulsars. We use a Monte-Carlo technique to simulate the Galactic population of neutron stars and the radio pulsars. For each simulated neutron star, we consider the $γ$-ray emission from the outer gap accelerator in the magnetosphere. In our outer gap model, we apply the gap closure mechanism proposed by Takata et al., in which both photon-photon pair-creation and magnetic pair-creation processes are considered. Simulating the sensitivities of previous major radio surveys, our simulation predicts that there are $\sim 18-23$ radio loud and $\sim 26-34$ $γ$-ray-selected $γ$-ray pulsars, which can be detected with a $γ$-ray flux $F_γ\ge 10^{-10}~\mathrm{erg/cm^2 s}$. Applying the sensitivity of the six-month observation of the $Fermi$ telescope, 40-61 radio-selected and 36-75 $γ$-ray selected pulsars are detected within our simulation. We show that the distributions of various pulsar parameters for the simulated $γ$-ray pulsars can be consistent with the observed distribution of the $γ$-ray pulsars detected by the $Fermi$ telescope. We also predict that $\sim 64$ radio-loud and $\sim 340$ $γ$-ray-selected pulsars irradiate the Earth with a flux $F_γ\ge 10^{-11}~\mathrm{erg/cm^2 s}$, and most of those $γ$-ray pulsars are distributing with a distance more than 1~kpc and a flux $F_γ\sim 10^{-11}~\mathrm{erg/cm^2 s}$. The ration between the radio-selected and $γ$-ray-selected pulsars depend on the sensitivity of the radio surveys. We also discuss the Galactic distribution of the unidentified $Fermi$ sources and the canonical $γ$-ray pulsars.

astro-ph.HE↗

arXiv2010

A past capture event at Sagittarius A* inferred from the fluorescent X-ray emission of Sagittarius B clouds

The fluorescent X-ray emission from neutral iron in the molecular clouds (Sgr B) indicates that the clouds are being irradiated by an external X-ray source. The source is probably associated with the Galactic central black hole (Sgr A*), which triggered a bright outburst one hundred years ago. We suggest that such an outburst could be due to a partial capture of a star by Sgr A*, during which a jet was generated. By constraining the observed flux and the time variability ($\sim$ 10 years) of the Sgr B's fluorescent emission, we find that the shock produced by the interaction of the jet with the dense interstellar medium represents a plausible candidate for the X-ray source emission.

astro-ph.HE↗

arXiv2010

Electron-positron energy deposition rate from neutrino pair annihilation on the rotation axis of neutron and quark stars

We investigate the deposition of energy due to the annihilations of neutrinos and antineutrinos on the rotation axis of rotating neutron and quark stars, respectively. The source of the neutrinos is assumed to be a neutrino-cooled accretion disk around the compact object. Under the assumption of the separability of the neutrino null geodesic equation of motion we obtain the general relativistic expression of the energy deposition rate for arbitrary stationary and axisymmetric space-times. The neutrino trajectories are obtained by using a ray tracing algorithm, based on numerically solving the Hamilton-Jacobi equation for neutrinos by reversing the proper time evolution. We obtain the energy deposition rates for several classes of rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the MIT bag model equation of state and in the CFL (Color-Flavor-Locked) phase, respectively. The electron-positron energy deposition rate on the rotation axis of rotating neutron and quark stars is studied for two accretion disk models (isothermal disk and accretion disk in thermodynamical equilibrium). Rotation and general relativistic effects modify the total annihilation rate of the neutrino-antineutrino pairs on the rotation axis of compact stellar, as measured by an observer at infinity. The differences in the equations of state for neutron and quark matter also have important effects on the spatial distribution of the energy deposition rate by neutrino-antineutrino annihilation.

astro-ph.HE↗

arXiv2010

On the Transition from Accretion Powered to Rotation Powered Millisecond Pulsars

The heating associated with the deposition of $γ$-rays in an accretion disk is proposed as a mechanism to facilitate the transformation of a low mass X-ray binary to the radio millisecond pulsar phase. The $γ$-ray emission produced in the outer gap accelerator in the pulsar magnetosphere likely irradiates the surrounding disk, resulting in its heating and to the possible escape of matter from the system. We apply the model to PSR J1023+0038, which has recently been discovered as a newly born rotation powered millisecond pulsar. The predicted $γ$-ray luminosity $\sim 6 \times 10^{34}~\mathrm{erg~s^{-1}}$ can be sufficient to explain the disappearance of the truncated disk existing during the 8~month$\sim 2$~yr period prior to the 2002 observations of J1023+0038 and the energy input required for the anomalously bright optical emission of its companion star.

astro-ph.HE↗

arXiv2010

The Origin of Gamma-Rays from Globular Clusters

Fermi has detected gamma-ray emission from eight globular clusters. We suggest that the gamma-ray emission from globular clusters may result from the inverse Compton scattering between relativistic electrons/positrons in the pulsar wind of MSPs in the globular clusters and background soft photons including cosmic microwave/relic photons, background star lights in the clusters, the galactic infrared photons and the galactic star lights. We show that the gamma-ray spectrum from 47 Tuc can be explained equally well by upward scattering of either the relic photons, the galactic infrared photons or the galactic star lights whereas the gamma-ray spectra from other seven globular clusters are best fitted by the upward scattering of either the galactic infrared photons or the galactic star lights. We also find that the observed gamma-ray luminosity is correlated better with the combined factor of the encounter rate and the background soft photon energy density. Therefore the inverse Compton scattering may also contribute to the observed gamma-ray emission from globular clusters detected by Fermi in addition to the standard curvature radiation process. Furthermore, we find that the emission region of high energy photons from globular cluster produced by inverse Compton scattering is substantially larger than the core of globular cluster with a radius >10pc. The diffuse radio and X-rays emitted from globular clusters can also be produced by synchrotron radiation and inverse Compton scattering respectively. We suggest that future observations including radio, X-rays, and gamma-rays with energy higher than 10 GeV and better angular resolution can provide better constraints for the models.

astro-ph.HE↗

arXiv2010

Multiwavelength properties of a new Geminga-like pulsar: PSR J2021+4026

In this paper, we report a detailed investigation of the multiwavelength properties of a newly detected gamma-ray pulsar, PSR J2021+4026, in both observational and theoretical aspects. We firstly identify an X-ray source in the XMM-Newton serendipitous source catalogue, 2XMM J202131.0+402645, located within the 95% confidence circle of PSR J2021+4026. With an archival Chandra observation, this identification provides an X-ray position with arcsecond accuracy which is helpful in facilitating further investigations. Searching for the pulsed radio emission at the position of 2XMM J202131.0+402645 with a 25-m telescope at Urumqi Astronomical Observatory resulted in null detection and places an upper-limit of 0.1~mJy for any pulsed signal at 18~cm. Together with the emission properties in X-ray and gamma-ray, the radio quietness suggests PSR J2021+4026 to be another member of Geminga-like pulsars. In the radio sky survey data, extended emission features have been identified in the gamma-ray error circle of PSR J2021+4026. We have also re-analyzed the gamma-ray data collected by FERMI's Large Area Telescope. We found that the X-ray position of 2XMM J202131.0+402645 is consistent with that of the optimal gamma-ray timing solution. We have further modeled the results in the context of outer gap model which provides us with constraints for the pulsar emission geometry such as magnetic inclination angle and the viewing angle. We have also discussed the possibility of whether PSR J2021+4026 has any physical association with the supernova remnant G78.2+2.1 (gamma-Cygni).

astro-ph.HE↗

arXiv2010

Gamma-ray spectral properties of mature pulsars: a two-layer model

We use a simple two-layer outer gap model, whose accelerator consists of a primary region and a screening region, to discuss $γ$-ray spectrum of mature pulsars detected by $Fermi$. By solving the Poisson equation with an assumed simple step-function distributions of the charge density in these two regions, the distribution of the electric field and the curvature radiation process of the accelerated particles can be calculated. In the our model, the properties of the phase-averaged spectrum can be completely specified by three gap parameters, i.e. the fractional gap size in the outer magnetosphere, the gap current in the primary region and the gap size ratio between the primary region and the total gap size. We discuss how these parameters affect the spectral properties. We argue that although the radiation mechanism in the outer gap is curvature radiation process, the observed gamma-ray spectrum can substantially deviate from the simple curvature spectrum because the overall spectrum consists of two components, i.e. the primary region and screening region. In some pulsars the radiation from the screening region is so strong that the photon index from 100MeV to several GeV can be as flat as $\sim 2$. We show the fitting fractional gap thickness of the canonical pulsars increases with the spin down age. We find that the total gap current is about 50~\% of the Goldreich-Julian value and the thickness of the screening region is a few percent of the total gap thickness. We also find that the predicted γ$-ray luminosity is less dependent on the spin down power ($L_{sd}$) for the pulsars with $L_{sd}\ga 10^{36}$~erg/s, while the $γ$-ray luminosity decreases with the spin down power for the pulsars with $L_{sd}\la 10^{36}$~erg/s.

astro-ph.HE↗

arXiv2010

High Redshift Gamma-Ray Bursts: Observational Signatures of Superconducting Cosmic Strings?

The high-redshift gamma-ray bursts (GRBs), GRBs 080913 and 090423, challenge the conventional GRB progenitor models by their short durations, typical for short GRBs, and their high energy releases, typical for long GRBs. Meanwhile, the GRB rate inferred from high-redshift GRBs also remarkably exceeds the prediction of the collapsar model, with an ordinary star formation history. We show that all these contradictions could be eliminated naturally, if we ascribe some high-redshift GRBs to electromagnetic bursts of superconducting cosmic strings. High-redshift GRBs could become a reasonable way to test the superconducting cosmic string model, because the event rate of cosmic string bursts increases rapidly with increasing redshifts, whereas the collapsar rate decreases.

astro-ph.HE↗

arXiv2010

Neutrino and Electron-positron Pair Emission from Phase-induced Collapse of Neutron Stars to Quark Stars

We study the energy released from phase-transition induced collapse of neutron stars, which results in large amplitude stellar oscillations. To model this process we use a Newtonian hydrodynamic code, with a high resolution shock-capturing scheme. The physical process considered is a sudden phase transition from normal nuclear matter to a mixed phase of quark and nuclear matter. We show that both the temperature and the density at the neutrinosphere oscillate with time. However, they are nearly 180 degree out of phase. Consequently, extremely intense, pulsating neutrino/antineutrino and leptonic pair fluxes will be emitted. During this stage several mass ejecta can be ejected from the stellar surface by the neutrinos and antineutrinos. These ejecta can be further accelerated to relativistic speeds by the electron/positron pairs, created by the neutrino and antineutrino annihilation outside the stellar surface. We suggest that this process may be a possible mechanism for short Gamma-Ray Bursts.

astro-ph.HE↗