arXiv subjects

Jian-Fu Zhang

Publications and source records attributed to Jian-Fu Zhang.

25 records · Page 2Linked to original sources

arXiv2018

Spectral properties of magnetohydrodnamic turbulence revealed by polarization synchrotron emission with Faraday rotation

We investigate how to recover the spectral properties of underlying magnetohydrodynamic (MHD) turbulence using fluctuation statistics of synchrotron polarization radiation, based on the synthetic observations. Taking spatially coincident, separated, and compounded synchrotron emission and Faraday rotation regions into account, we extract the power spectrum of synchrotron polarization intensities integrated along the line of sight. Our results demonstrate that in the short wavelength range, the power spectra reflect fluctuation statistics of the perpendicular component of turbulent magnetic fields, and the spectra at long wavelengths reveal the fluctuation of the Faraday rotation density, which is a product of the parallel component of magnetic field and thermal electron density. We find that our numerical results (in the case of spatially coincident regions) are in agreement with the analytical prediction in Lazarian \& Pogosyan (2016), and this theoretical prediction is applicable to more complicated settings, i.e., the spatially separated and compounded regions. We simulate telescopic observations that incorporate the effects of telescope angular resolution and noise, and find that statistics of underlying MHD turbulence can be recovered successfully. We expect that the technique can be applied to a variety of astrophysical environments, with existing synchrotron data cubes and a large number of forthcoming data sets from such as the LOw-Frequency Array for Radio astronomy (LOFAR), the Square Kilometer Array (SKA) and the Five-hundred-meter Aperture Spherical radio Telescope (FAST).

astro-ph.HE↗

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.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2016

Studying Magnetohydrodynamic Turbulence with Synchrotron Polarization Dispersion

We test a new technique of studying magnetohydrodynamic (MHD) turbulence suggested by Lazarian \& Pogosyan, using synthetic synchrotron polarization observations. This paper focuses on a one-point statistics, which is termed the polarization frequency analysis, that is characterized by the variance of polarized emission as a function of the square of wavelengths along a single line of sight. We adopt a ratio $η$ of the standard deviation of the line-of-sight turbulent magnetic field to the line-of-sight mean magnetic field to depict the level of turbulence. When this ratio is either large ($η\gg1$), which characterizes a turbulent field dominated region, or small ($η\lesssim0.2$), which characterizes a mean field dominated region, we obtain the polarization variance $\left \proptoλ^{-2}$ and $\left \proptoλ^{-2-2m}$, respectively. At small $η$, i.e., the mean field dominated region, we successfully recover the turbulent spectral index by the polarization variance. We find that our simulations agree well with the theoretical prediction of Lazarian \& Pogosyan. With existing and upcoming data cubes from the Low-Frequency Array for Radio astronomy (LOFAR) and Square Kilometer Array (SKA), this new technique can be applied to study the magnetic turbulence in the Milky Way and other galaxies.

astro-ph.HE↗

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.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2013

Spectrum of jet emitting disc: Application to microquasar XTE J1118+480

Under the framework of the magnetized accretion ejection structures, we analyze the energy balance properties, and study the spectral energy distributions (SEDs) of the Jet Emitting Disc (JED) model for black hole X-ray transients. Various radiative processes are considered, i.e. synchrotron, bremsstrahlung, and their Comptonizations, and external Comptonization of radiation from outer thin disc. With these cooling terms taken into account, we solve the thermal equilibrium equation self-consistently and find three solutions, of which the cold and the hot solutions are stable. Subsequently we investigate the theoretical SEDs for these two stable solutions.We find the hot JED model can naturally explain the spectra of the Galactic microquasars in their hard states. As an example, we apply this model to the case of XTE J1118+480.

astro-ph.HE↗