arXiv subjects

Jian-Fu Zhang

Publications and source records attributed to Jian-Fu Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Impact of disk magnetic fields on the propagation of stellar-scale jets in the magnetically arrested accretion disks of active galactic nuclei

It is widely recognized that active galactic nucleus (AGN) disks host numerous massive stars and compact objects. Stellar-scale jets triggered by collapses of massive stars and mergers of compact objects could propagate through the disk and produce observable electromagnetic radiation. Magnetically arrested disks (MADs), supported by both numerical simulations and observations, possess strong magnetic fields (MFs). As jets travel within such environments, the MFs should regulate jet evolution and shape radiation signatures. In this work, we explore the effects of disk MFs on jet propagation and breakout emission within the MAD framework. We employ a jet-cocoon model that accounts for potential disk-MF effects, including both magnetic pressure and magnetic energy dissipation driven by magnetic reconnection. We find that magnetic pressure effectively suppresses the lateral expansion of the cocoon, which enhances jet collimation and modestly increases the jet-head velocity. Furthermore, magnetic pressure effects are more pronounced at relatively low jet powers. In this regime, the breakout luminosity of the jet-head shock is enhanced, while its breakout time is shortened. However, the magnitude of the luminosity enhancement is sensitive to the adopted regime-dependent emission prescriptions. These findings suggest that, within the explored parameter space, disk MFs can facilitate the breakout of low-power jets arising from binary black hole mergers in AGN MADs.

astro-ph.HE↗

arXiv2026

The $\beta$-Dependence of Particle Spectra in Relativistic Turbulent Reconnection

We perform numerical simulations of particle acceleration in relativistic, self-driven turbulent magnetic reconnection using the MHD-PIC method. We systematically investigate the dependence of the non-thermal particle spectral exponent on the plasma $\beta$. We find that particle acceleration proceeds in two stages: an initial, efficient first-order Fermi phase where momentum gains are comparable in parallel and perpendicular directions, followed by a slower drift-dominated phase. The power-law slope of the non-thermal spectrum is established during the Fermi phase, as found in previous studies. Our results demonstrate a systematic steepening of the accelerated particle energy spectrum with increasing $\beta$. We derive empirical scaling relations: the spectral exponent $\alpha \propto \beta^{0.5}$ in the relativistic regime, compared to $\alpha \propto \beta^{0.3}$ in the non-relativistic case. This marked difference is rooted in relativistic physics: the increased inertial mass density ($\rho h$) in high-$\beta$ plasmas acts as an energy sink, reducing the Alfv\'en velocity and thereby altering the dynamics of magnetic energy release and its partition efficiency. The derived scaling provides a unified physical framework for interpreting the diversity of non-thermal radiation spectra observed in astrophysical sources, including black hole corona X-ray flares, gamma-ray bursts, and active galactic nucleus jets.

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arXiv2026

Revealing the Spectral Properties of Galactic Interstellar Medium by Survey Observations

Based on multi-frequency radio polarization survey datasets, we investigate the spectral characteristics of the Galactic interstellar medium (ISM) using the polarization frequency analysis (PFA) method, referred to as polarization variance. By comparing this novel PFA technique with the traditional power spectrum approach, and by cross-examining data from two distinct surveys, we aim to reinforce the robustness of our findings. Our analysis reveals that the ISM scaling slope in the Galactic disk is steeper than the classic Kolmogorov slope, whereas the ISM scaling slope in the Galactic halo aligns with the Kolmogorov slope. We suggest a distinct turbulence cascade process operating in the Galactic halo compared to the Galactic disk.

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arXiv2026

Cascade Processes of Strong and Weak MHD Turbulence

On the framework of relativistic force-free magnetohydrodynamic (MHD) turbulence, we explore the fundamental properties of strong and weak turbulent cascades using high-resolution numerical simulations in the presence of a uniform background magnetic field. We find that (1) power spectra and scale-dependent anisotropies both for the strong and weak turbulence resemble those observed in the non-relativistic MHD turbulence; (2) intermittency of magnetic fields in strong turbulence is stronger than that in the weak one; (3) generated Alfv\'en modes show similar energy spectra and scale-dependent anisotropies to those of non-relativistic case; (4) generated fast modes present a power spectrum similar to that of Alfv\'en modes, with a strong (for strong turbulence) or weak (for weak turbulence) scale-dependent anisotropy, which are significantly different from non-relativistic turbulence; and (5) applications of our numerical results to neutron star magnetospheres show that the strong (or moderately weak) turbulent cascade can explain the X-ray radiation of the Vela pulsar. Our study is of great significance for understanding energy transfer, magnetic field evolution, and particle acceleration mechanisms in extreme astrophysical environments.

astro-ph.HE↗

arXiv2025

Studying the properties of reconnection-driven turbulence

Magnetic reconnection, often accompanied by turbulence interaction, is a ubiquitous phenomenon in astrophysical environments. However, the current understanding of the nature of turbulent magnetic reconnection remains insufficient. We investigate the statistical properties of reconnection turbulence in the framework of the self-driven reconnection. Using the open-source software package AMUN, we first perform numerical simulations of turbulent magnetic reconnection. We then obtain the statistical results of reconnection turbulence by traditional statistical methods such as the power spectrum and structure function. Our numerical results demonstrate: (1) the velocity spectrum of reconnection turbulence follows the classical Kolmogorov type of $E\propto k^{-5/3}$, while the magnetic field spectrum is steeper than the Kolmogorov spectrum, which are independent of limited resistivity, guide field, and isothermal or adiabatic fluid states; (2) most of the simulations show the anisotropy cascade, except that the presence of a guide field leads to an isotropic cascade; (3) reconnection turbulence is incompressible in the adiabatic state, with energy distribution dominated by the velocity solenoidal component; (4) different from pure magnetohydrodynamic (MHD) turbulence, the intermittency of the velocity field is stronger than that of the magnetic field in reconnection turbulence. The steep magnetic field spectrum, together with the velocity spectrum of Kolmogorov type, can characterize the feature of the reconnection turbulence. In the case of the presence of the guide field, the isotropy of the reconnection turbulence cascade is also different from the cascade mode of pure MHD turbulence. Our experimental results provide new insights into the properties of reconnection turbulence, which will contribute to advancing the self-driven reconnection theory.

astro-ph.HE↗

arXiv2025

Anisotropic diffusion of high-energy cosmic rays in magnetohydrodynamic turbulence

The origin of cosmic rays (CRs) and how they propagate remain unclear. Studying the propagation of CRs in magnetohydrodynamic (MHD) turbulence can help to comprehend many open issues related to CR origin and the role of turbulent magnetic fields. To comprehend the phenomenon of slow diffusion in the near-source region, we study the interactions of CRs with the ambient turbulent magnetic field to reveal their universal laws. We numerically study the interactions of CRs with the ambient turbulent magnetic field, considering pulsar wind nebula as a general research case. Taking the magnetization parameter and turbulence spectral index as free parameters, together with radiative losses, we perform three group simulations to analyze the CR spectral, spatial distributions, and possible CR diffusion types. Our studies demonstrate that (1) CR energy density decays with both its effective radius and kinetic energy in the form of power-law distributions; (2) the morphology of the CR spatial distribution strongly depends on the properties of magnetic turbulence and the viewing angle; (3) CRs suffer a slow diffusion near the source and a fast/normal diffusion away from the source; (4) the existence of a power-law relationship between the averaged CR energy density and the magnetization parameter is independent of both CR energy and radiative losses; (5) radiative losses can suppress CR anisotropic diffusion and soften the power-law distribution of CR energy density. The distribution law established between turbulent magnetic fields and CRs presents an intrinsic property, providing a convenient way to understand complex astrophysical processes related to turbulence cascades.

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arXiv2024

Exploring the intermittency of magnetohydrodynamic turbulence by synchrotron polarization radiation

Magnetohydrodynamic (MHD) turbulence plays a critical role in many key astrophysical processes such as star formation, acceleration of cosmic rays, and heat conduction. However, its properties are still poorly understood. We explore how to extract the intermittency of compressible MHD turbulence from the synthetic and real observations. The three statistical methods, namely the probability distribution function, kurtosis, and scaling exponent of the multi-order structure function, are used to reveal the intermittency of MHD turbulence. Our numerical results demonstrate that: (1) the synchrotron polarization intensity statistics can be used to probe the intermittency of magnetic turbulence, by which we can distinguish different turbulence regimes; (2) the intermittency of MHD turbulence is dominated by the slow mode in the sub-Alfv{é}nic turbulence regime; (3) the Galactic interstellar medium (ISM) at the low latitude region corresponds to the sub-Alfvénic and supersonic turbulence regime. We have successfully measured the intermittency of the Galactic ISM from the synthetic and realistic observations.

astro-ph.HE↗

arXiv2024

Studying magnetic reconnection with synchrotron polarization statistics

Magnetic reconnection is a fundamental process for releasing magnetic energy in space physics and astrophysics. At present, the usual way to investigate the reconnection process is through analytical studies or first-principles numerical simulations. This paper is the first to understand the turbulent magnetic reconnection process by exploring the nature of magnetic turbulence. From the perspective of radio synchrotron polarization statistics, we study how to recover the properties of the turbulent magnetic field by considering the line of sight along different directions of the reconnection layer. We find that polarization intensity statistics can reveal the spectral properties of reconnection turbulence. This work opens up a new way of understanding turbulent magnetic reconnection.

astro-ph.HE↗

arXiv2024

Measurement of Interstellar Magnetization by Synchrotron Polarization Variance

Since synchrotron polarization fluctuations are related to the fundamental properties of the magnetic field, we propose the polarization intensity variance to measure the Galactic interstellar medium (ISM) magnetization. We confirm the method's applicability by comparing it with the polarization angle dispersion and its reliability by measuring the underlying Alfvénic Mach number of MHD turbulence. With the finding of the power-law relation of $\mathcal{A} \propto M_{\rm A}^{2}$ between polarization intensity variance $\mathcal{A}$ and Alfvénic Mach number $M_{\rm A}$, we apply the new technique to the Canadian Galactic Plane Survey (CGPS) data, achieving Alfvénic Mach number of the Galactic ISM. Our results show that the low-latitude Galactic ISM is dominated by sub-Alfénic turbulence, with $M_{\rm A}$ approximately between 0.5 and 1.0.

astro-ph.GA↗

arXiv2023

Particle acceleration in self-driven turbulent reconnection

The theoretical prediction that magnetic reconnection spontaneously drives turbulence has been supported by magnetohydrodynamic (MHD) and kinetic simulations. While reconnection with externally driven turbulence is accepted as an effective mechanism for particle acceleration, the acceleration during the reconnection with self-driven turbulence is studied for the first time in this work. By using high-resolution 3D MHD simulations of reconnection with self-generated turbulence, we inject test particles into the reconnection layer to study their acceleration process. We find that the energy gain of the particles takes place when they bounce back and forth between converging turbulent magnetic fields. The particles can be efficiently accelerated in self-driven turbulent reconnection with the energy increase by about 3 orders of magnitude in the range of the box size. The acceleration proceeds when the particle gyroradii become larger than the thickness of the reconnection layer. We find that the acceleration in the direction perpendicular to the local magnetic field dominates over that in the parallel direction. The energy spectrum of accelerated particles is time-dependent with a slope that evolves toward -2.5. Our findings can have important implications for particle acceleration in high-energy astrophysical environments.

physics.plasm-ph↗

arXiv2022

Studying the properties of compressible MHD turbulence by synchrotron fluctuation statistics

We study the observable properties of compressible MHD turbulence covering different turbulence regimes, based on synthetic synchrotron observations arising from 3D MHD numerical simulations. Using the synchrotron emissivity and intensity, we first explore how the cosmic ray spectral indices affect the measurements of turbulence properties by employing normalized correlation functions. We then study how the anisotropy of synchrotron total and polarization intensities arising from three fundamental MHD modes vary with the viewing angle, i.e., the angle between the mean magnetic field and the line of sight. We employ the ratio of quadrupole moment to the monopole one (QM) for this purpose. Our numerical results demonstrate that: (1) the two-point correlation function of synchrotron statistics for the arbitrary cosmic ray spectral index is related to the special case of magnetic field index $γ=2$ in agreement with the analytical formulae provided by Lazarian \& Pogosyan (2012); (2) the anisotropy of synchrotron total and polarization intensities arising from Alfvén and slow modes increases with the increase of the viewing angle, while that of fast mode remains almost unchanged with the viewing angle; (3) the analytical formulae of synchrotron intensities for studying turbulence can be applied to describing statistics of polarization intensities, and the QM can be successfully used to recover turbulence anisotropy. This study validates Lazarian \& Pogosyan's analytical approach and opens a way to study turbulence from observations.

astro-ph.HE↗

arXiv2021

Gradient measurement of synchrotron polarization diagnostic: Application to spatially separated emission and Faraday rotation regions

Considering the spatially separated polarization radiation and Faraday rotation regions to simulate complex interstellar media, we study synchrotron polarization gradient techniques' measurement capabilities. We explore how to trace the direction of projected magnetic field of emitting-source region at the multi-frequency bands, using the gradient technique compared with the traditional polarization vector method. Furthermore, we study how Faraday rotation density in the foreground region, i.e., a product of electron number density and parallel component of magnetic fields along the line of sight, affects the measurement of projected magnetic field. Numerical results show that synchrotron polarization gradient technique could successfully trace projected magnetic field within emitting-source region independent of radio frequency. Accordingly, the gradient technique can measure the magnetic field properties for a complex astrophysical environment.

astro-ph.HE↗

arXiv2020

Studying the local magnetic field and anisotropy of magnetic turbulence by synchrotron polarization derivative

Due to the inevitable accumulation of the observed information in the direction of the line of sight, it is difficult to measure the local magnetic field of MHD turbulence. However, the correct understanding of the local magnetic field is a prerequisite for reconstructing the Galactic 3D magnetic field. We study how to reveal the local magnetic field direction and the eddy anisotropy on the basis of the statistics of synchrotron polarization derivative with respect to the squared wavelength $dP/dλ^2$. In the low frequency and strong Faraday rotation regime, we implement numerical simulations in the combination of multiple statistic techniques, such as structure function, quadrupole ratio modulus, spectral correlation function, correlation function anisotropy and spatial gradient techniques. We find that (1) statistic analysis of $dP/dλ^2$ indeed reveals the anisotropy of underlying MHD turbulence, the degree of which increases with the increase of the radiation frequency; (2) the synergy of both correlation function anisotropy and gradient calculation of $dP/dλ^2$ enables the measurement of the local magnetic field direction.

astro-ph.HE↗

arXiv2020

Studying Anisotropy of Compressible Magnetohydrodynamic Turbulence by Synchrotron Polarization Intensity

Based on statistical analysis of synchrotron polarization intensity, we study the anisotropic properties of compressible magnetohydrodynamic (MHD) turbulence. The second-order normalized structure function, quadrupole ratio modulus and anisotropic coefficient are synergistically used to characterize the anisotropy of the polarization intensity. On the basis of pre-decomposition data cubes, we first explore the anisotropy of the polarization intensity in different turbulence regimes and find that the most significant anisotropy occurs in the sub-Alfvénic regime. Using post-decomposition data cubes in this regime, we then study the anisotropy of the polarization intensity from Alfvén, slow and fast modes. Statistics of polarization intensity from Alfvén and slow modes demonstrate the significant anisotropy while statistics of polarization intensity from fast modes show isotropic structures, which is consistent with the earlier results provided in Cho & Lazarian (2002). As a result, both quadrupole ratio modulus and anisotropic coefficient for polarization intensities can quantitatively recover the anisotropy of underlying compressible MHD turbulence. The synergistic use of the two methods helps enhance the reliability of the magnetic field measurement.

astro-ph.HE↗

arXiv2019

Tracing magnetic fields by the synergies of synchrotron emission gradients

This paper studies how to employ synchrotron emission gradient techniques to reveal the properties of the magnetic field within the interstellar media. Based on data cubes of three-dimensional numerical simulations of magnetohydrodynamic turbulence, we explore spatial gradients of synchrotron emission diagnostics to trace the direction of the magnetic field. According to our simulations, multifarious diagnostics for synchrotron emission can effectively determine the potential direction of projected magnetic fields. Applying the synergies of synchrotron diagnostic gradients to the archive data from the Canadian Galactic Plane Survey, we find that multifarious diagnostic techniques make consistent predictions for the Galactic magnetic field directions. With the high-resolution data presently available from Low Frequency Array for radio astronomy and those in the future from the Square Kilometer Array, the synergies of synchrotron emission gradients are supposed to perform better in tracing the actual direction of interstellar magnetic fields, especially in the low-frequency Faraday rotation regime where traditional synchrotron polarization measure fails.

astro-ph.HE↗

arXiv2019

Tracing Magnetic Field with Synchrotron Polarization Gradients: Parameter Study

We employ synthetic observations obtained with MHD simulations to study how to trace the distribution of turbulent magnetic fields using the synchrotron polarization gradient techniques suggested by Lazarian \& Yuen (2018). Both synchrotron polarization gradients and its derivative gradients with regard to the squared wavelength $λ^2$ are used to explore the probing ability of the technique in magnetic fields from sub- to super-Alfv{é}nic turbulence. We focus on studies that involve multi-frequency measurements in the presence of strong Faraday rotation and show the ways of how to recover the projected mean magnetic fields in the plane of the sky and the local magnetic fields within a tomographic slice. We conclude that the new techniques can successfully reconstruct the 3D magnetic field within our Milky Way and other galaxies. This paper opens an avenue for applying our new techniques to a large number of data cubes such as those from the Low Frequency Array for Radio astronomy and the Square Kilometer Array.

astro-ph.HE↗

arXiv2019

Ionisation state of the accretion disc in the neutron-star low-mass X-ray binary 4U 1728-34

We analysed an XMM-Newton plus a simultaneous Rossi X-ray Timing Explorer observation and a separate Suzaku observation of the neutron-star low-mass X-ray binary 4U 1728-34. We fitted the X-ray spectra with the self-consistent reflection model relxill. We found that the inclination angle of 4U 1728-34 is 49 degrees, consistent with the upper limit of 60 degrees deduced from the absence of eclipses or dips in this source. The inclination angle in the fit of the XMM-Newton/RXTE observation is larger than 85 degrees, which may be due to the possible calibration issues of the PN instrument in timing mode. We also found that the thermal emission from the accretion disc is not significant. This could be explained either by the relatively high column density of the interstellar medium along the line of sight to the source, which decreases the number of soft disc photons, or if most of the soft thermal photons from the disc are reprocessed in the corona. The ionisation parameter derived from the fits is larger than the value predicted in the framework of the standard reflection model, wherein the disc is irradiated by an X-ray source above the compact object. This inconsistency suggests that irradiation from the neutron star and its boundary layer may play an important role in the ionisation of the accretion disc, and hence in the reflection component in this source.

astro-ph.HE↗

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↗