arXiv subjects

Ju-Fu Lu

Publications and source records attributed to Ju-Fu Lu.

At least 19 recordsLinked to original sources

arXiv2017

Electron transport with re-acceleration and radiation in the jets of X-ray binaries

This paper studies acceleration processes of background thermal electrons in X-ray binary jets via turbulent stochastic interactions and shock collisions. By considering turbulent magnetized jets mixed with fluctuation magnetic fields and ordered, large-scale one, and numerically solving the transport equation along the jet axis, we explore the influence of such as magnetic turbulence, electron injections, location of an acceleration region, and various cooling rates on acceleration efficiency. The results show that (1) the existence of the dominant turbulent magnetic fields in the jets is necessary to accelerate background thermal electrons to relativistic energies. (2) Acceleration rates of electrons depend on magnetohydrodynamic turbulence types, from which the turbulence type with a hard slope can accelerate electrons more effectively. (3) An effective acceleration region should be located at the distance $>10^3R_{\rm g}$ away from the central black hole ($R_{\rm g}$ being a gravitational radius). As a result of acceleration rates competing with various cooling rates, background thermal electrons obtain not only an increase in their energies but also their spectra are broadened beyond the given initial distribution to form a thermal-like distribution. (4) The acceleration mechanisms explored in this work can reasonably provide the electron maximum energy required for interpreting high-energy $γ$-ray observations from microquasars, but it needs to adopt some extreme parameters in order to predict a possible very high-energy $γ$-ray signal.

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arXiv2017

Polarization radiation with turbulent magnetic fields from X-ray binaries

We study the properties of polarized radiation in turbulent magnetic fields from X-ray binary jets. These turbulent magnetic fields are composed of large- and small-scale configurations, which result in the polarized jitter radiation when the characteristic length of turbulence is less than the non-relativistic Larmor radius. On the contrary, the polarized synchrotron emission occurs, corresponding to a large-scale turbulent environment. We calculate the spectral energy distributions and the degree of polarization for a general microquasar. Numerical results show that turbulent magnetic field configurations can indeed provide a high degree of polarization, which does not mean that a uniform, large-scale magnetic field structure exists. The model is applied to investigate the properties of polarized radiation of black hole X-ray binary Cygnus X--1. Under the constraint of multiband observations of this source, our studies demonstrate that the model can explain the high polarization degree at MeV tail and predict the highly polarized properties at high-energy $γ$-ray region, and that the dominant small-scale turbulent magnetic field plays an important role for explaining the highly polarized observation at hard X-ray/soft $γ$-ray bands. This model can be tested by polarization observations of upcoming polarimeters at high-energy $γ$-ray bands.

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arXiv2016

Central Engine of Late-Time X-ray Flares with Internal Origin

This work focuses on a sample of seven extremely late-time X-ray flares with peak time $t_{\rm p} > 10^4 {\rm s}$, among which two flares can be confirmed as the late-time activity of central engine. The main purpose is to investigate the mechanism of such late-time flares based on the internal origin assumption. In the hyper-accreting black hole (BH) scenario, we study the possibility of two well-known mechanisms as the central engine to power such X-ray flares, i.e., the neutrino-antineutrino annihilation and the Blandford-Znajek (BZ) process. Our results show that the annihilation luminosity is far below the observational data. Thus, the annihilation mechanism cannot account for such late-time flares. For the BZ process, if the role of outflows is taken into consideration, the inflow mass rate near the horizon will be quite low such that the magnetic field will probably be too weak to power the observed X-ray flares. We therefore argue that, for the late-time flares with internal origin, the central engine is unlikely to be associated with BHs. On the contrary, a fast rotating neutron star with strong bipolar magnetic fields may be responsible for such flares.

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arXiv2015

Evolutions of stellar-mass black hole hyperaccretion systems in the center of gamma-ray bursts

A neutrino-dominated accretion disk around a stellar-mass black hole (BH) can power a gamma-ray burst (GRB) via annihilation of neutrinos launched from the disk. For the BH hyperaccretion system, high accretion rate should trigger the violent evolution of the BH's characteristics, which further leads to the evolution of the neutrino annihilation luminosity. In this paper, we consider the evolution of the accretion system to analyze the mean time-dependent neutrino annihilation luminosity with the different mean accretion rates and initial BH parameters. By time-integrating the luminosity, the total neutrino annihilation energy with the reasonable initial disk mass can satisfy the most of short-duration GRBs and about half of long-duration GRBs. Moreover, the extreme Kerr BH should exist in the cental engines of some high-luminosity GRBs. GRBs with higher energy have to request the alternative magnetohydrodynamics processes in the centers, such as the Blandford-Znajek jet from the accretion system or the millisecond magnetar.

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arXiv2015

Potential Gamma-ray Emissions from Low-Mass X-ray Binary Jets

By proposing a pure leptonic radiation model, we study the potential gamma-ray emissions from jets of the low-mass X-ray binaries. In this model, the relativistic electrons that are accelerated in the jets are responsible for radiative outputs. Nevertheless, dynamics of jets are dominated by the magnetic and proton-matter kinetic energies. The model involves all kinds of related radiative processes and considers the evolution of relativistic electrons along the jet by numerically solving the kinetic equation. Numerical results show that the spectral energy distributions can extend up to TeV bands, in which synchrotron radiation and synchrotron self-Compton scattering are dominant components. As an example, we apply the model to the low-mass X-ray binary GX 339-4. The results can not only reproduce the currently available observations from GX 339-4, but also predict detectable radiation at GeV and TeV bands by Fermi and CTA telescopes. The future observations with Fermi and CTA can be used to test our model, which could be employed to distinguish the origin of X-ray emissions.

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arXiv2015

High frequency oscillations in outbursts of Kerr-metric slim disks

We numerically investigate the thermally unstable accretion disks around black holes. We adopt an evolutionary viscous stress equation to replace the standard alpha-prescription based on the results of two MHD simulations. We find a kind of interesting oscillations on some running models in limit-cycle outburst state. The oscillations arise near the inner boundary and propagate radially outwards. We deem that they are the trapped $p$-mode oscillations excited by sonic-point instability. We directly integrate the local radiation cooling fluxes to construct the mimic bolometric light-curve. We find a series of overtones beside the fundamental harmonic on the power spectra of mimic light-curves. The frequency of the fundamental harmonic is very close to the maximum epicyclic frequency of the disk and the frequency ratio of the fundamental harmonic and overtones is a regular integer series. We suggest that the code for ray-tracing calculation must be time-dependent in virtual observation and point out the robustness of the black hole spin measurement with high frequency QPOs.

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arXiv2015

Thermal Stability of Magnetized, Optically Thin, Radiative Cooling-dominated Accretion Disks

We investigate the thermal stability of optically thin, two-temperature, radiative cooling-dominated accretion disks. Our linear analysis shows that the disk is thermally unstable without magnetic fields, which agrees with previous stability analysis on the Shapiro-Lightman-Eardley disk. By taking into account the effects of magnetic fields, however, we find that the disk can be or partly be thermally stable. Our results may be helpful to understand the outflows in optically thin flows. Moreover, such radiative cooling-dominated disks may provide a new explanation of the different behaviors between black hole and neutron star X-ray binaries on the radio/X-ray correlation.

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arXiv2014

Broadband Emission Spectra from the Cygnus X-3 Jet in the Soft Spectral State

In order to understand the main observational characteristics of the Galactic X-ray binary Cygnus X-3, we propose a radiation model in which high-energy electrons accelerated in the dissipation zone of a jet produce the non-thermal broadband emissions. Broadband spectral energy distributions are computed to confront with the AGILE and\emph{ Fermi} LAT data together with the multi-band data, during soft X-ray spectral states. By fitting observations at different locations of the jet, we find that the emission region is rather compact and should be located at the distance of about one orbital radius. Our results can explain the current multi-frequency observations and also predict the TeV band emission. The model could be tested by a polarization measurement at IR band, and/or by a correlation study between the GeV and TeV bands once very high-energy observations are available.

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arXiv2014

Self-gravity in neutrino-dominated accretion disks

We present the effects of the self-gravity on the vertical structure and neutrino luminosity of the neutrino-dominated accretion disks in cylindrical coordinates. It is found that significant changes of the structure appear in the outer region of the disk, especially for high accretion rates (e.g., $\ga 1 M_\odot~\rm s^{-1}$), and thus causes the slight increase of the neutrino luminosity. Furthermore, the gravitational instability of the disk is reviewed by the vertical distribution of the Toomre parameter \citep{Toomre1964}, which may account for the late-time flares in gamma-ray bursts and the extended emission in short-duration gamma-ray bursts.

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arXiv2014

Variability of the giant X-ray bump in GRB 121027A and possible origin

The particular giant X-ray bump of GRB 121027A triggered by \emph{Swift} is quite different from the typical X-ray flares in gamma-ray bursts. There exhibit four parts of the observed structural variabilities in the rise and decay phase of the bump. Considering the quality of four parts of the data, we can only analyze the data from about 5300 s to about 6100 s in the bump using the stepwise filter correlation method (Gao et al. 2012), and find that the $86^{+5.9}_{-9.4}~\rm s$ periodic oscillation may exist, which is confirmed by the Lomb-Scargle method (Scargle 1982). Furthermore, a jet precession model (Liu et al. 2010) is proposed to account for such a variability.

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arXiv2014

Time evolution of flares in GRB 130925A: jet precession in black hole accretion system

GRB 130925A, composed of three gamma-ray emission episodes and a series of orderly flares, has been detected by $Swift$, Fermi, Konus-$Wind$, and $INTEGRAL$. If the third weakest gamma-ray episode can be considered as a giant flare, we find that after the second gamma-ray episode observed by $INTEGRAL$ located at about 2000 s, there exists a positive relation between the time intervals of the adjacent flares and the time since the episode. We suggest that the second gamma-ray episode and its flares originate from the resumption of the accretion process due to the fragments from the collapsar falling back, such a relation may be related to a hyperaccretion disk around a precessed black hole (BH). We propose that the origin and time evolution of the flares, and the approximately symmetrical temporal structure and spectral evolution of the single flare, can be explained well by the jet precession model \citep{Liu2010}. In addition, the mass and spin of the BH can be constrained, which indicates a stellar-mass, fast rotating BH located in the center of GRB 130925A.

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arXiv2013

Revisiting the Light Curves of Gamma-ray Bursts in the Relativistic Turbulence Model

Rapid temporal variability has been widely observed in the light curves of gamma-ray bursts (GRBs). One possible mechanism for such variability is related to the relativistic eddies in the jet. In this paper, we include the contribution of the inter-eddy medium together with the eddies to the gamma-ray emission. We show that the gamma-ray emission can either lead or lag behind the observed synchrotron emission, where the latter originates in the inter-eddy medium and provides most of seed photons for producing gamma-ray emission through the inverse-Compton scattering. As a consequence, we argue that the lead/lag found in non-stationary short-lived light curves may not reveal the intrinsic lead/lag of different emission components. In addition, our results may explain the lead of gamma-ray emission with respect to optical emission observed in GRB 080319B.

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arXiv2013

Do Intermediate-Mass Black Holes Exist in Globular Clusters?

The existence of intermediate-mass black holes (IMBHs) in globular clusters (GCs) remains a crucial problem. Searching IMBHs in GCs reveals a discrepancy between radio observations and dynamical modelings: the upper mass limits constrained by radio observations are systematically lower than that of dynamical modelings. One possibility for such a discrepancy is that, as we suggest in this work, there exist outflows in accretion flows. Our results indicate that, for most sources, current radio observations cannot rule out the possibility that IMBHs may exist in GCs. In addition, we adopt an \dot{M}-L_{R} relation to revisit this issue, which confirms the results obtained by the Fundamental Plane relation.

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arXiv2013

Neutrino-cooled Accretion Model with Magnetic Coupling for X-ray Flares in GRBs

The neutrino-cooled accretion disk, which was proposed to work as the central engine of gamma-ray bursts, encounters difficulty in interpreting the X-ray flares after the prompt gamma-ray emission. In this paper, the magnetic coupling between the inner disk and the central black hole is taken into consideration. For mass accretion rates around $0.001 \sim 0.1 M_{\sun} s^{-1}$, our results show that the luminosity of neutrino annihilation can be significantly enhanced due to the coupling effects. As a consequence, after the gamma-ray emission, a remnant disk with mass $M_{disk} \la 0.5 M_{\sun}$ may power most of the observed X-ray flares with the rest frame duration less than 100 seconds. In addition, a comparison between the magnetic coupling process and the Blandford-Znajek mechanism is shown on the extraction of black hole rotational energy.

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arXiv2013

Relativistic global solutions of neutrino-dominated accretion flows

Neutrino-dominated accretion flows (NDAFs) around rotating stellar-mass black holes are plausible candidates for the central engines of gamma-ray bursts (GRBs). We investigate one-dimensional global solutions of NDAFs, taking account of general relativity in Kerr metric, neutrino physics and nucleosynthesis more precisely than previous works. We calculate sixteen solutions with different characterized accretion rates and black hole spins to exhibit the radial distributions of various physical properties in NDAFs. \iffalse We find that the gas pressure and the neutrino cooling always become dominated in the inner region for large accretion rate, $\gtrsim 0.1 M_\odot$ $\rm s^{-1}$. \fi We confirm that the electron degeneracy has important effects in NDAFs and we find that the electron fraction is about 0.46 in the outer region for all the sixteen solutions. From the perspective of the mass fraction, free nucleons, $\rm ^4He$, and $\rm ^{56}Fe$ dominate in the inner, middle, and outer region, respectively. The influence of neutrino trapping on the annihilation is of importance for the superhigh accretion ($\dot M=10M_{\odot} \rm s^{-1}$) and most of the sixteen solutions have an adequate annihilation luminosity for GRBs.

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arXiv2012

The vertical composition of neutrino-dominated accretion disks in gamma-ray bursts

We investigate the vertical structure and elements distribution of neutrino-dominated accretion flows around black holes in spherical coordinates with the reasonable nuclear statistical equilibrium. According our calculations, heavy nuclei tend to be produced in a thin region near the disk surface, whose mass fractions are primarily determined by the accretion rate and the vertical distribution of temperature and density. In this thin region, we find that $^{56}\rm Ni$ is dominant for the flow with low accretion rate (e.g., $0.05 $$M_{\odot}$ $\rm s^{-1}$) but $^{56}\rm Fe$ is dominant for the high counterpart (e.g., $1 M_{\odot}$ $\rm s^{-1}$). The dominant $^{56}\rm Ni$ in the special region may provide a clue to understand the bumps in the optical light curve of core-collapse supernovae.

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arXiv2012

Understanding Simulations of Thin Accretion Disks by Energy Equation

We study the fluctuations of standard thin accretion disks by linear analysis of the time-dependent energy equation together with the vertical hydrostatic equilibrium and the equation of state. We show that some of the simulation results in Hirose et al. (2009b), such as the time delay, the relationship of power spectra, and the correlation between magnetic energy and radiation energy, can be well understood by our analytic results.

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arXiv2012

Is There a Relation between Duration and E_iso in Gamma-Ray Bursts?

The system of accretion disk and black hole is usually considered as the central engine of Gamma-ray Bursts (GRBs). It is usually thought that the disk in the central engine of GRBs is the advection-dominated accretion disk, which is developed from a massive (mass M_disk) torus at radius r_disk. We find a positive correlation between the isotropic gamma-ray energy E_iso and duration (so-called T_90) for GRBs. We interpret this correlation within the advection-dominated accretion disk model, associating E_iso and T_90 with M_ disk and viscous timescale respectively.

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