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Ye-Fei Yuan

Publications and source records attributed to Ye-Fei Yuan.

At least 73 records · Page 4Linked to original sources
arXiv2012

Vertical Structure of Neutrino Dominated Accretion Disks and Neutrino Transport in the disks

We investigate the vertical structure of neutrino dominated accretion disks by self-consistently considering the detailed microphysics, such as the neutrino transport, vertical hydrostatic equilibrium, the conservation of lepton number, as well as the balance between neutrino cooling, advection cooling and viscosity heating. After obtaining the emitting spectra of neutrinos and antineutrinos by solving the one dimensional Boltzmann equation of neutrino and antineutrino transport in the disk, we calculate the neutrino/antineutrino luminosity and their annihilation luminosity. We find that the total neutrino and antineutrino luminosity is about $10^{54}$ ergs/s and their annihilation luminosity is about $5\times10^{51}$ ergs/s with an extreme accretion rate $10 M_{\rm {sun}}$/s and an alpha viscosity $α=0.1$. In addition, we find that the annihilation luminosity is sensitive to the accretion rate and will not exceed $10^{50}$ ergs/s which is not sufficient to power the most fireball of GRBs, if the accretion rate is lower than $1 M_{\rm {sun}}$/s. Therefore, the effects of the spin of black hole or/and the magnetic field in the accretion flow might be introduced to power the central engine of GRBs.

astro-ph.HE↗
arXiv2012

Neutrino and anti-neutrino transport in accretion disks

We numerically solve the one dimensional Boltzmann equation of the neutrino and anti-neutrino transport in accretion disks and obtain the fully energy dependent and direction dependent neutrino and anti-neutrino emitting spectra, under condition that the distribution of the mass density,temperature and chemical components are given. Then, we apply the resulting neutrino and anti-neutrino emitting spectra to calculate the corresponding annihilation rate of neutrino pairs above the neutrino dominated accretion disk and find that the released energy resulting from the annihilation of neutrino pairs can not provide sufficient energy for the most energetic short gamma ray bursts whose isotropic luminosity can be as high as $10^{52}$ ergs/s unless the high temperature zone where the temperature is beyond 10 MeV can stretch over 200 km in the disk. We also compare the resulting luminosity of neutrinos and anti-neutrinos with the results from the two commonly used approximate treatment of the neutrino and anti-neutrino luminosity: the Fermi-Dirac black body limit and a simplified model of neutrino transport, i.e., the gray body model, and find that both of them overestimate the neutrino/anti-neutrino luminosity and their annihilation rate greatly. Additionally, as did in Sawyer (2003), we also check the validity of the two stream approximation, and find that it is a good approximation to high accuracy.

astro-ph.HE↗
arXiv2011

A general relativistic external Compton-scattering model for TeV emission from M87

M87 is the first detected non-blazar extragalactic Tera-Electron-Volt (TeV) source with rapid variation and very flat spectrum in the TeV band. To explain the two-peaks in the spectral energy distribution (SED) of the nucleus of M87 which is similar to those of blazars, the most commonly adopted models are the synchrotron self-Compton scattering (SSC) models and the external inverse Compton scattering (EIC) models. Considering that there is no correlated variation in the soft band (from radio to X-ray) matching the TeV variation, and the TeV sources should not suffer from the gamma-gamma absorption due to the flat TeV spectrum, the EIC models are advantageous in modeling the TeV emission from M87. In this paper, we propose a self-consistent EIC model to explain the flat TeV spectrum of M87 within the framework of fully general relativity, where the background soft photons are from the advection-dominated accretion flow (ADAF) around the central black hole, and the high energy electrons are from the mini-jets which are powered by the magnetic reconnection in the main jet (Giannios et al. 2010). In our model, both the TeV flares observed in the years of 2005 and 2008 could be well explained: the gamma-gamma absorption for TeV photons is very low, even inside the region very close to the black hole 20Rg~50Rg; at the same region, the average EIC cooling time (~ 10^2-10^3s) is short, which is consistent with the observed time scale of TeV variation. Furthermore, we also discuss the possibility that the accompanying X-ray flare in 2008 is due to the direct synchrotron radiation of the mini-jets.

astro-ph.HE↗
arXiv2010

Instabilities in the Gamma Ray Burst central engine. What makes the jet variable?

Both types of long and short gamma ray bursts involve a stage of a hyper-Eddington accretion of hot and dense plasma torus onto a newly born black hole. The prompt gamma ray emission originates in jets at some distance from this 'central engine' and in most events is rapidly variable, having a form of spikes and subpulses. This indicates at the variable nature of the engine itself, for which a plausible mechanism is an internal instability in the accreting flow. We solve numerically the structure and evolution of the neutrino-cooled torus. We take into account the detailed treatment of the microphysics in the nuclear equation of state that includes the neutrino trapping effect. The models are calculated for both Schwarzschild and Kerr black holes. We find that for sufficiently large accretion rates (> 10 Msun/s for non-rotating black hole, and >1 Msun/s for rotating black hole, depending on its spin), the inner regions of the disk become opaque, while the helium nuclei are being photodissociated. The sudden change of pressure in this region leads to the development of a viscous and thermal instability, and the neutrino pressure acts similarly to the radiation pressure in sub-Eddington disks. In the case of rapidly rotating black holes, the instability is enhanced and appears for much lower accretion rates. We also find the important and possibly further destabilizing role of the energy transfer from the rotating black hole to the torus via the magnetic coupling.

astro-ph.HE↗
arXiv2010

Emergent Spectra From Disks Surrounding Kerr Black Holes: Effect of Photon Trapping and Disk Self-Shadowing

Based on a new estimation of their thickness, the global properties of relativistic slim accretion disks are investigated in this work. The resulting emergent spectra are calculated using the relativistic ray-tracing method, in which we neglect the self-irradiation of the accretion disk. The angular dependence of the disk luminosity, the effects of the heat advection and the disk thickness on the estimation of the black hole spin are discussed. Compare to the previous works, our improvements are that we use the self-consistent disk equations and we consider the disk self-shadowing effect. We find that at the moderate accretion rate, the radiation trapped in the outer region of the accretion disks will escape in the inner region of the accretion disk and contribute to the emergent spectra. At the high accretion rate, for the large inclination and large black hole spin, both the disk thickness and the heat advection have significant influence on the emergent spectra. Consequently, these effects will influence the measurement of the black hole spin based on the spectra fitting and influence the angular dependence of the luminosity. For the disks around Kerr black holes with $a=0.98$, if the disk inclination is greater than $60^\circ$, and their luminosity is beyond 0.2 Eddington luminosity, the spectral model which is based on the relativistic standard accretion disk is no longer applicable for the spectra fitting. We also confirm that the effect of the self-shadowing is significantly enhanced by the light-bending, which implies that the non-relativistic treatment of the self-shadowing is inaccurate. According to our results, the observed luminosity dependence of the measured spin suggests that the disk self-shadowing significantly shapes the spectra of GRS 1915+105, which might lead to the underestimation of the black hole spin for the high luminosity states.

astro-ph.HE↗
arXiv2009

The role of black hole spin and magnetic field threading the unstable neutrino disk in Gamma Ray Bursts

We report on the third phase of our study of the neutrino-cooled hyperaccreting torus around a black hole that powers the jet in Gamma Ray Bursts. We focus on the influence of the black hole spin on the properties of the torus. The structure of a stationary torus around the Kerr black hole is solved numerically. We take into account the detailed treatment of the microphysics in the nuclear equation of state that includes the neutrino trapping effect. We find, that in the case of rapidly rotating black holes, the thermal instability discussed in our previous work is enhanced and develops for much lower accretion rates. We also find the important role of the energy transfer from the rotating black hole to the torus, via the magnetic coupling.

astro-ph.HE↗
arXiv2009

Effect of Electron Screening on the Collapsing Process of Core-Collapse Supernovae

By using an average heavy nuclei model, the effects of the electron screening on electron capture (EC) in core-collapse supernovae are investigated. A one-dimension code based on the Ws15$M_{\sun}$ progenitor model is utilized to test the effects of electron screening during the collapsing process. The results show that, at high densities, the effects of EC on electron capture becomes significant. During the collapsing stage, the EC rate is decreased, the collapse timescale is prolonged and the leakage of the neutrino energy is increased. These effects lead to an appreciable decrease in the initial energy of the bounce shock wave. The effects of electron screeening in the other progenitor models are also discussed.

astro-ph.HE↗
arXiv2009

Polarized Emission of Sagittarius A*

We explore the parameter space of the two temperature pseudo-Newtonian Keplerian accretion flow model for the millimeter and shorter wavelength emission from Sagittarius A*. A general relativistic ray-tracing code is used to treat the radiative transfer of polarized synchrotron emission from the flow. The synchrotron self-Comptonization and bremsstrahlung emission components are also included. It is shown that the model can readily account for the millimeter to sub-millimeter emission characteristics with an accretion rate of ~6x10^17g.s^-1 and an inclination angle of ~40 deg. However, the corresponding model predicted near-infrared and X-ray fluxes are more than one order of magnitude lower than the observed 'quiescent' state values. While the extended quiescent-state X-ray emission has been attributed to thermal emission from the large-scale accretion flow, the NIR emission and flares are likely dominated by emission regions either within the last stable orbit of a Schwarzschild black hole or associated with outflows. With the viscous parameter derived from numerical simulations, there is still a degeneracy between the electron heating rate and the magnetic parameter. A fully general relativistic treatment with the black hole spin incorporated will resolve these issues.

astro-ph.GA↗
arXiv2009

Constraining spins of supermassive black holes from TeV variability. II. fully general relativistic calculations

The fast variability of energetic TeV photons from the center of M87 has been detected, offering a new clue to estimate spins of supermassive black holes (SMBHs). We extend the study of Wang et al. (2008) by including all of general relativistic effects. We numerically solve the full set of relativistic hydrodynamical equations of the radiatively inefficient accretion flows (RIAFs) and then obtain the radiation fields around the black hole. The optical depth of the radiation fields to TeV photons due to pair productions are calculated in the Kerr metric. We find that the optical depth strongly depends on: (1) accretion rates as $\tautev\propto \dot{M}^{2.5-5.0}$; (2) black hole spins; and (3) location of the TeV source. Jointly considering the optical depth and the spectral energy distribution radiated from the RIAFs, the strong degeneration of the spin with the other free parameters in the RIAF model can be largely relaxed. We apply the present model to M87, wherein the RIAFs are expected to be at work, and find that the minimum specific angular momentum of the hole is $a\sim0.8$. The present methodology is applicable to M87-like sources with future detection of TeV emissions to constrain the spins of SMBHs.

astro-ph.HE↗
arXiv2009

Images of the radiatively inefficient accretion flow surrounding a Kerr black hole: application in Sgr A*

In fully general relativity, we calculate the images of the radiatively inefficient accretion flow (RIAF) surrounding a Kerr black hole with arbitrary spins, inclination angles, and observational wavelengths. For the same initial conditions, such as the fixed accretion rate, it is found that the intrinsic size and radiation intensity of the images become larger, but the images become more compact in the inner region, while the size of the black hole shadow decreases with the increase of the black hole spin. With the increase of the inclination angles, the shapes of the black hole shadows change and become smaller, even disappear at all due to the obscuration by the thick disks. For median inclination angles, the radial velocity observed at infinity is larger because of both the rotation and radial motion of the fluid in the disk, which results in the luminous part of the images is much brighter. For larger inclination angles, such as the disk is edge on, the emission becomes dimmer at longer observational wavelengths (such as at 7.0mm and 3.5mm wavelengths), or brighter at shorter observational wavelengths (such as at 1.3 mm wavelength) than that of the face on case, except for the high spin and high inclination images. These complex behaviors are due to the combination of the Lorentz boosting effect and the radiative absorption in the disk. We hope our results are helpful to determine the spin parameter of the black hole in low luminosity sources, such as the Galactic center. A primary analysis by comparison with the observed sizes of Sgr A* at millimeters strongly suggests that the disk around the central black hole at Sgr A* is highly inclined or the central black hole is rotating fast.

astro-ph.HE↗
arXiv2008

Negative feedback effects on star formation history and cosmic reionization

After considering the effects of negative feedback on the process of star formation, we explore the relationship between star formation process and the associated feedback, by investigating how the mechanical feedback from supernovae(SNe) and radiative feedback from luminous objects regulate the star formation rate and therefore affect the cosmic reionization.Based on our present knowledge of the negative feedback theory and some numerical simulations, we construct an analytic model in the framework of the Lambda cold dark matter model. In certain parameter regions, our model can explain some observational results properly. In large halos(T_vir>10000 K), both mechanical and radiative feedback have a similar behavior: the relative strength of negative feedback reduces as the redshift decreases. In contrast, in small halos (T_vir<10000 K$) that are thought to breed the first stars at early time, the radiative feedback gets stronger when the redshift decreases. And the star formation rate in these small halos depends very weakly on the star-formation efficiency. Our results show that the radiative feedback is important for the early generation stars. It can suppress the star formation rate considerably. But the mechanical feedback from the SNe explosions is not able to affect the early star formation significantly. The early star formation in small-halo objects is likely to be self-regulated. The radiative and mechanical feedback dominates the star formation rate of the PopII/I stars all along. The feedback from first generation stars is very strong and should not be neglected. However, their effects on the cosmic reionization are not significant, which results in a small contribution to the optical depth of Thomson scattering.

astro-ph↗
arXiv2007

Quantum Decoherence Modulated by Special Relativity

By investigating the evolution of a moving spin-1/2 Dirac electron coupled with a background magnetic noise, we demonstrate that the effects of special relativity will significantly modify the decoherence properties of the spin state. The dephasing could be much suppressed, and for a sufficiently long time the decoherence even seems to halt. This interesting phenomenon stems from the dressed environment induced by special relativity.

quant-ph↗
arXiv2007

Instabilities in the time-dependent neutrino disc in Gamma-Ray Bursts

We investigate the properties and evolution of accretion tori formed after the coalescence of two compact objects. At these extreme densities and temperatures, the accreting torus is cooled mainly by neutrino emission produced primarily by electron and positron capture on nucleons (beta reactions). We solve for the disc structure and its time evolution by introducing a detailed treatment of the equation of state which includes photodisintegration of helium, the condition of beta-equilibrium, and neutrino opacities. We self-consistently calculate the chemical equilibrium in the gas consisting of helium, free protons, neutrons and electron-positron pairs and compute the chemical potentials of the species, as well as the electron fraction throughout the disc. We find that, for sufficiently large accretion rates (> 10 solar masses per second), the inner regions of the disk become opaque and develop a viscous and thermal instability. The identification of this instability might be relevant for GRB observations.

astro-ph↗
arXiv2005

Electron Positron Capture Rates and the Steady State Equilibrium Condition for Electron-Positron Plasma with Nucleons

The reaction rates of the beta processes for all particles at arbitrary degeneracy are derived, and an {\it analytic} steady state equilibrium condition $μ_n=μ_p+2μ_e$ which results from the equality of electron and positron capture rates in the hot electron-positron plasma with nucleons is also found, if the matter is transparent to neutrinos. This simple analytic formula is valid only if electrons are nondegenerate or mildly degenerate, which is generally satisfied in the hot electron-positron plasma. Therefore, it can be used to efficiently determine the steady state of the hot matter with plenty of positrons. Based on this analytic condition, given the baryon number density and the temperature, if the nucleons are nondegenerate, only one algebraic equation for determining the electron fraction is obtained, which shows the great advantage of the analytic equilibrium condition.

astro-ph↗
arXiv2005

The Afterglows of Ultraluminous Quasars

Quasars represent a brief phase in the life-cycle of most massive galaxies, but the evolutionary connection between central black holes and their host galaxies remains unclear. While quasars are active and shining brighter than the Compton-limit luminosity, their radiation heats the surrounding medium to the Compton temperature, forming Compton spheres extending to the Strömgren radius of Fe$^{26+}$/He$^{2+}$. After the quasars shut off, their ``afterglow'' can be detected through three signatures: (1) an extended X-ray envelope, with a characteristic temperature of $\sim 3\times 10^7$ K; (2) Ly$α$ and Ly$β$ lines and the $K-$edge of Fe$^{26+}$; and (3) nebulosity from hydrogen and helium recombination emission lines. We discuss the possibility of detecting these signatures using {\em Chandra}, the planned {\em XEUS} mission, and ground-based optical telescopes. The luminosity and size of quasar afterglows can be used to constrain the lifetime of quasars.

astro-ph↗
arXiv2004

Constraining Alternate Models of Black Holes: Type I X-ray Bursts on Accreting Fermion-Fermion and Boson-Fermion Stars

The existence of black holes remains open to doubt until other conceivable options are excluded. With this motivation, we consider a model of a compact star in which most of the mass consists of dark particles of some kind, and a small fraction of the mass is in the form of ordinary nucleonic gas. The gas does not interact with the dark matter other than via gravity, but collects at the center as a separate fermionic fluid component. Depending on whether the dark mass is made of fermions or bosons, the objects may be called fermion-fermion stars or boson-fermion stars, respectively. For appropriate choices of the mass of the dark matter particles, these objects are viable models of black hole candidates in X-ray binaries. We consider models with a dark mass of 10 solar masses and a range of gas mass from 10^{-6} to nearly one solar mass, and analyse the bursting properties of the models when they accrete gas. We show that all the models would experience thermonuclear Type I X-ray bursts at appropriate mass accretion rates. Since no Type I bursts have been reported from black hole candidates, the models are ruled out. The case for identifying black hole candidates in X-ray binaries as true black holes is thus strengthened.

astro-ph↗
arXiv2000

Blob ejection from advection-dominated accretion flow: observational consequences

There is increasing evidence for the presence of an optically thin advection-dominated accretion flow (ADAF) in low luminosity active galactic nuclei and radio-loud quasars. The present paper is devoted to explore the fate of a blob ejected from an ADAF, and to discuss its observational consequences. It is inevitable for the ejected blob to drastically expand into its surroundings. Consequently, it is expected that a group of relativistic electrons should be accelerated, which may lead to nonthermal flares, since a strong shock will be formed by the interaction between the blob and its surroundings. Then the blob cools down efficiently, leading to the appearance of recombination lines about $10^5$s after its ejection from an ADAF. We apply this model to NGC 4258 for some observational prediction, and to PKS 2149--306 for the explanation of observational evidence. Future simultaneous observations of recombination X-ray lines and continuum emission are highly desired to test the present model.

astro-ph↗