Searcharxiv⌕ Search

arXiv subjects

Y. Lu

Publications and source records attributed to Y. Lu.

105 records · Page 6Linked to original sources
arXiv2009

The energy dependence of the centroid frequency and phase lag of the QPOs in GRS 1915+105

We present a study of the centroid frequencies and phase lags of the quasi-periodic oscillations (QPOs) as functions of photon energy for GRS 1915+105. It is found that the centroid frequencies of the 0.5-10 Hz QPOs and their phase lags are both energy dependent, and there exists an anti-correlation between the QPO frequency and phase lag. These new results challenge the popular QPO models, because none of them can fully explain the observed properties. We suggest that the observed QPO phase lags are partially due to the variation of the QPO frequency with energy, especially for those with frequency higher than 3.5 Hz.

astro-ph.HE↗
arXiv2009

Structure and evolution of rotationally and tidally distorted stars

This paper aims to study the configuration of two components caused by rotational and tidal distortions in the model of a binary system. The potentials of the two distorted components can be approximated to 2nd-degree harmonics. Furthermore, both the accretion luminosity ($σ_{i}$) and the irradiative luminosity are included in stellar structure equations. The equilibrium structure of rotationally and tidally distorted star is exactly a triaxial ellipsoids. A formula describing the isobars is presented, and the rotational velocity and the gravitational acceleration at the primary surface simulated. The results show the distortion at the outer layers of the primary increases with temporal variation and system evolution. Besides, it was observed that the luminosity accretion is unstable, and the curve of the energy-generation rate fluctuates after the main sequence in rotation sequences. The luminosity in rotation sequences is slightly weaker than that in non-rotation sequences. As a result, the volume expands slowly. Polar ejection is intensified by the tidal effect. The ejection of an equatorial ring may be favoured by both the opacity effect and the $g_{e}(θ,φ)$-effect in the binary system.

astro-ph.SR↗
arXiv2008

A tidal disruption model for the gamma-ray burst of GRB 060614

The combination of a long duration and the absence of any accompanying supernova clearly shows that GRB 060614 can not be grouped into the two conventional classes of gamma-ray bursts, i.e. the long/soft bursts deemed to be collapsars and the short/hard bursts deemed to be merging binary compact stars. A new progenitor model is required for this anomalous gamma-ray burst. We propose that GRB 060614 might be produced through the tidal disruption of a star by an intermediate mass black hole. In this scenario, the long duration and the lack of any associated supernova are naturally expected. The theoretical energy output is also consistent with observations. The observed 9-s periodicity in the $γ$-ray light curve of GRB 060614 can also be satisfactorily explained.

astro-ph↗
arXiv2008

Study on the spectrum of the injected relativistic protons

About 10TeV gamma-ray emission within 10 pc region from the Galactic Center had been reported by 4 independent groups. Considering that this TeV gamma-ray emission is produced via a hadronic model, and the relativistic protons came from the tidal disruption of stars by massive black holes, we investigate the spectral nature of the injected relativistic protons required by the hadronic model. The calculation was carried on the tidal disruption of the different types of stars and the different propagation mechanisms of protons in the interstellar medium. Compared with the observation data from HESS, we find for the best fitting that the power-law index of the spectrum of the injected protons is about -1.9, when a red giant star is tidally disrupted, and the effective confinement of protons diffusion mechanism is adopted.

astro-ph↗
arXiv2007

A Detailed Study on the Equal Arrival Time Surface Effect in Gamma-Ray Burst Afterglows

Due to the relativistic motion of gamma-ray burst remnant and its deceleration in the circumburst medium, the equal arrival time surfaces at any moment are not spherical, but should be distorted ellipsoids. This will leave some imprints in the afterglows. In this article, we study the effect of equal arrival time surfaces numerically under various conditions, i.e., for isotropic fireballs, collimated jets, density jump conditions, and energy injection events. For each condition, direct comparison between the two instances when the effect is and is not included, is presented. For isotropic fireballs and jets viewed on axis, the effect slightly hardens the spectra and postpones the peak time of afterglows, but does not change the shapes of the spectra and light curves significantly. In the cases when a density jump or an energy injection is involved, the effect smears the variability of the afterglows markedly.

astro-ph↗
arXiv2006

On the nature of the flares from three candidate tidal disruption events

The X-ray flares of NGC 5905, RX J1242.6-1119A, and RX J1624.9+7554 observed by Chandra in 2001 and 2002 have been suggested as the candidate tidal disruption events. The distinct features observed from these events may be used to determine the type of a star tidally disrupted by a massive black hole. We investigate these three events, focusing on the differences for the tidal disruption of a giant star and a main sequence, resulted from their different relation between the mass and the radius. We argue that their X-ray flare properties could be modeled by the partial stripping of the outer layers of a solar type star. The tidal disruption of a giant star is excluded completely. This result may be useful for understanding the growth of a supermassive black hole by capturing stars, versus the growth mode through continuous mass accretion.

astro-ph↗
arXiv2006

Anisotropic flow at RHIC: How unique is the number-of-constituent-quark scaling?

The transverse momentum dependence of the anisotropic flow $v_2$ for $π$, $K$, nucleon, $Λ$, $Ξ$ and $Ω$ is studied for Au+Au collisions at $\sqrt{s_{\rm NN}} = 200$ GeV within two independent string-hadron transport approaches (RQMD and UrQMD). Although both models reach only 60% of the absolute magnitude of the measured $v_2$, they both predict the particle type dependence of $v_2$, as observed by the RHIC experiments: $v_2$ exhibits a hadron-mass hierarchy (HMH) in the low $p_T$ region and a number-of-constituent-quark (NCQ) dependence in the intermediate $p_T$ region. The failure of the hadronic models to reproduce the absolute magnitude of the observed $v_2$ indicates that transport calculations of heavy ion collisions at RHIC must incorporate interactions among quarks and gluons in the early, hot and dense phase. The presence of an NCQ scaling in the string-hadron model results suggests that the particle-type dependencies observed in heavy-ion collisions at intermediate $p_T$ might be related to the hadronic cross sections in vacuum rather than to the hadronization process itself.

nucl-th↗
arXiv2006

Capture of a Red Giant by the Black Hole Sagittarius A* as a Possible Origin for the TeV Gamma Rays from the Galactic Center

Non-thermal TeV $γ$-ray emission within a multiparsec has been observed from the center region of our Galaxy. We argue that these $γ$-rays are the result of transient activity of the massive black hole Sgr A$^*$ that resides at the Galactic Center. Several thousand years ago, the black hole may have experienced an active phase by capturing a red giant star and forming an accretion disk, temporarily behaving like an active galactic nucleus. A powerful jet, which contains plenty of high speed protons, was launched during the process. These runaway protons interact with the dense ambient medium, producing TeV $γ$-ray emission through the $π^0$-decay process. We show that the total energy deposited in this way is large enough to account for observations. The diffusion length of protons is also consistent with the observed size of the TeV source.

astro-ph↗
arXiv2005

The Black Hole Mass and Magnetic Field Correlation in Active Galactic Nuclei

The observed optical luminosity in 5100 angstorm and black hole mass correlation is used to probe the magnetic field of black holes harbored in active galactic nuclei(AGNs). The model is based on the assumption that the disk is heated by energy injection due to the magnetic coupling(MC) process and the gravitational dissipation due to accretion. The MC process can transfer energy and angular momentum from a rotating Kerr black hole to its surrounding disk. The relation of optical luminosity in 5100 angstorm and black hole mass as functions of the spin and magnetic field of the black hole is modelled. The model predicts that optical luminosity in 5100 angstorm emitted from the disk is sensitive to the strength of the poloidal component of the magnetic field on the BH horizon. Based on the observations of optical luminosity in 5100 angstorm for 143 AGN sources, we obtain the correlation between mass and magnetic field of black hole. And we compared out result with the approximate result between mass and magnetic field of black hole derived from the condition in the standard accretion disc theory.

astro-ph↗
arXiv2003

Spectral properties of anomalous X-ray pulsars

In this paper, the spectra of the persistent emission from anomalous X-ray pulsars (AXPs) and their variation with spin-down rate $\dotΩ$ is considered. Firstly, based on an accretion-powered model, the influences of both magnetic field and mass accretion rate on the spectra properties of AXPs are addressed. Subsequently, the relation between the spectral property of AXPs and mass accretion rate $\dot{M}$ is investigated. The result shows that there exists a linear correlation between the photon index and mass accretion rate, and the spectral hardness increases with increasing $\dot{M}$. A possible emission mechanism for the explanation of spectral properties of AXPs is also discussed.

astro-ph↗
arXiv2001

Metallicity distribution of halo stars and minor merger processes of the Galaxy

A possible relation between the high dispersion in metallicity of metal-poor halo stars and the minor merger processes in the history of the Galaxy is presented. The foreign populations of stars in the satellites through minor merger processes, make considerable contributions to the Galactic halo, therefore alter the intrinsic distribution of metallicities of the halo stars. Our model reproduces the observed trends in the metallicity distribution of the extremely metal-poor halo field stars in the Galaxy. Taken all the parameters for our model, a density of $1.9\times 10^{-3}kpc^{-3}$ of extremely metal-poor halo stars in $-5<[Fe/H]<-4$ is predicted, which explains why no such stars have been observed so far.

astro-ph↗
arXiv2000

Status Report of Project GRAND: An Extensive Air Shower Array of Proportional Wire Chambers

Project GRAND is an extensive air shower array utilizing position sensitive detectors of proportional wire chambers. The 64 detectors deployed in a field 100 m x 100 m are located at 86.2 deg W and 41.7 deg N, at 220 m above sea level. The project was completed about two years ago and has been taking data, simultaneously on two triggers: 1) multiple-hit coincidence triggers which collect data on extensive air showers, and 2) 200 Hz triggers which collect the single tracks stored in each station during the last 5 msec.

astro-ph↗
arXiv1999

A Possible Energy Mechanism for Cosmological Gamma-ray Bursts

We suggest that an extreme Kerr black hole with a mass $\sim 10^6M_\odot$, a dimensionless angular momentum $A\sim 1$ and a marginal stable orbital radius $r_{ms}\sim 3r_s\sim 10^{12}M_6 cm$ located in a normal galaxy, may produced a Gamma-ray Burst by capturing and disrupting a star. During this period, a transient accretion disk is formed and a strong transient magnetic field $\sim 2.4\times 10^9M_6^{-1/2}$ Gauss, lasting for $r_{ms}/c\sim 30 M_6 s$, may be produced in the inner boundary of the accretion disk. A large amount of rotational energy of the black hole is extracted and released in the ultra relativistic jet with a bulk Lorentz factor $Γ$ larger than $10^3$ via Blandford-Znajek process. The relativistic jet energy can be converted into $γ$-ray radiation via internal shock mechanism. The gamma-ray burst (GRB) duration should be the same as that of the life time of the strong transient magnetic field. The maximum number of sub-bursts is estimated to be $r_{ms}/h\sim (10 - 10^2)$ because the disk material is likely broken into pieces with the size about the thickness of the disk $h$ at the cusp ($2r_s\le r \le 3r_s$). The shortest rising time of the burst estimated from this model is $\sim h/Γc\sim 3\times 10^{-4}Γ^{-1}_3(h/r)_{-2}M_6$ s. The model gamma-ray burst density rate is also estimated.

astro-ph↗
arXiv1993

Rates for the reactions antiproton-proton --> pi phi and gamma phi

We study antiproton-proton annihilation at rest into $πϕ$ and $γϕ$. Rescattering by $\overline{K^*}K+K^*\overline{K}$ and $ρ^{+}ρ^{-}$ for $\overline{p}p\rightarrowπϕ$ states is sizable, of order $(0.90\, {\rm to}\,2.6)\times 10^{-4}$ in the branching ratio, but smaller than experiment. For $\overline{p}p\rightarrowγϕ$ the rescattering contributions are negligible, but the $γϕ$ channel is well explained by a $ρϕ$ intermediate state combined with vector meson dominance.

nucl-th↗