arXiv subjects

Heshou Zhang

Publications and source records attributed to Heshou Zhang.

12 recordsLinked to original sources

arXiv2024

Discovery of the Goat Horn complex: a $\sim 1000$ deg$^2$ diffuse X-ray source connected to radio loop XII

A dozen of patches of polarized radio emission spanning tens of degrees in the form of coherent and stationary loops are observed at radio frequencies across the sky. Their origin is usually associated to nearby shocks, possibly arising from close supernovae explosions. The origin of the radio Loop XII remains so far unknown. We report an anti-correlation of the radio polarized emission of loop XII with a large patch of soft X-ray emission found with SRG/eROSITA in excess of the background surface brightness, in the same region. The soft X-ray seemingly coherent patch in excess of the background emission, which we dub as the Goat Horn complex, extends over a remarkable area of $\sim 1000$ deg$^2$ and includes an arc-shaped enhancement potentially tracing a cold front. An anti-correlation of the X-ray intensity with the temperature of the plasma responsible for the X-ray emission is also observed. The X-ray bright arc seems to anticipate the radio loop XII by some degrees on the sky. This behavior can be recast in terms of a correlation between X-ray surface brightness and radio depolarization. We explore and discuss different possible scenarios for the source of the diffuse emission in the Goat Horn complex: a large supernova remnant; an outflow from active star formation regions in nearby Galactic spiral arms; a hot atmosphere around the Large Magellanic Cloud. In order to probe these scenarios further, a more detailed characterization on the velocity of the hot gas is required.

astro-ph.GA↗

arXiv2022

Diffuse $γ$-ray emission in Cygnus X: Comments to Yan & Pavaskar

It has been pointed out in arXiv:2211.17057 that our recent (published) paper might be revised, due to an incorrect evaluation of the diffusion coefficients, $D(E)$, employed in the calculations. Unfortunately, there is no indication as to where the incorrectness might be. Here we offer the opportunity to be more specific, by providing the community with the whole description of the equations involved in the calculation of $D(E)$, which is missing in the {\tt arXiv} note. In this context, we mention that no \textit{ad hoc} parameterisation has been used in our paper. Furthermore, any assumption on the injection mechanisms is explicitly described in the paper as an input factor and is obviously part of the modelisation procedure, hence the final outcome is subject to it. Finally, we discuss in a more broad context what, in this calculation of the diffusion coefficient, we believe is the key message.

astro-ph.HE↗

arXiv2022

MHD decomposition explains diffuse $γ$-ray emission in Cygnus X

Cosmic-ray (CR) diffusion is the result of the interaction of such charged particles against magnetic fluctuations. These fluctuations originate from large-scale turbulence cascading towards smaller spatial scales, decomposed into three different modes, as described by $magneto-hydro-dynamics$ (MHD) theory. As a consequence, the description of particle diffusion strongly depends on the model describing the injected turbulence. Moreover, the amount of energy assigned to each of the three modes is in general not equally divided, which implies that diffusion properties might be different from one region to another. Here, motivated by the detection of different MHD modes inside the Cygnus-X star-forming region, we study the 3D transport of CRs injected by two prominent sources within a two-zone model that represents the distribution of the modes. Then, by convolving the propagated CR-distribution with the neutral gas, we are able to explain the $γ$-ray diffuse emission in the region, observed by the Fermi-LAT and HAWC Collaborations. Such a result represents an important step in the long-standing problem of connecting the CR observables with the micro-physics of particle transport.

astro-ph.HE↗

arXiv2022

Cosmic ray Transport in Magnetohydrodynamic turbulence

This paper studies cosmic ray (CR) transport in magneto hydrodynamic (MHD) turbulence. CR transport is strongly dependent on the properties of the magnetic turbulence. We perform test particle simulations to study the interactions of CR with both total MHD turbulence and decomposed MHD modes. The spatial diffusion coefficients and the pitch angle scattering diffusion coefficients are calculated from the test particle trajectories in turbulence. Our results confirm that the fast modes dominate the CR propagation, whereas Alfvén and slow modes are much less efficient and have shown similar pitch angle scattering rates. We investigate the cross field transport on large and small scales. On large/global scales, normal diffusion is observed and the diffusion coefficient is suppressed by $M_A^ζ$ compared to the parallel diffusion coefficients, with $ζ$ closer to 4 in Alfvén modes than that in total turbulence as theoretically expected. For the CR transport on scales smaller than the turbulence injection scale, both the local and global magnetic reference frames are adopted. Super diffusion is observed on such small scales in all the cases. Particularly, CR transport in Alfvén modes show clear Richardson diffusion in the local reference frame. Our results have broad applications to CRs in various astrophysical environments.

astro-ph.HE↗

arXiv2021

Polarization of fluorescence lines: tracing magnetic field from circumstellar medium to early universe

Fluorescence emission lines are broadly applied in observation for diffuse medium in the universe. They are normally observed around strong pumping source, tracing the gas in circumstellar medium, reflection nebula, and H\,{\sc ii} regions, etc. They reside in UV/optical and infrared bands and hence could be directly observed with ground-base telescopes. In this letter, we demonstrate the polarization of fluorescence lines as a magnetic field tracer arising from ground state atomic alignment in diffuse medium, including our solar system, supernova remnants (SNRs), as well as quasi-stellar object (QSO) host galaxies. Two types of fluorescence emissions are considered: the primary fluorescence from the excited states; and the secondary fluorescence from the metastable state (forbidden lines). We find that the synergy of these lines could measure three-dimensional magnetic direction: the polarizations of the primary fluorescence lines could reveal the magnetic polar angle along the line-of-sight, whereas the polarization of forbidden lines traces the plane-of-sky magnetic direction. The expected degree of polarization is $P>10\%$. Polarizations of both types of fluorescence emissions have shown strong potential for observations, and are applicable to measure magnetic field within and beyond our galaxy.

astro-ph.GA↗

arXiv2021

Identification of plasma modes in Galactic turbulence with synchrotron polarization

Magneto-hydrodynamic (MHD) turbulence is ubiquitous and a fundamental ingredient underlying many astrophysical phenomena. The multiphase nature of interstellar medium and diversity of driving mechanisms give rise to spatial variation of turbulence properties, particularly their plasma properties. There has been no observational diagnosis of the plasma modes so far beyond the solar system. Here we report the identification of different plasma modes in various Galactic environments, including active star forming zones and supernova remnants, based on our synchrotron polarization analysis. The observed high degree of consistency between the $γ-$ray excess in Cygnus cocoon and the location of magnetosonic modes provides strong observational evidence for the long-advocated theory that magnetosonic modes dominate the cosmic ray (CR) scattering and acceleration. Our results open up a new avenue for the study of interstellar turbulence and demonstrate the indispensability to account for their plasma property in all the relevant processes including CR transport and star formation.

physics.plasm-ph↗

arXiv2020

Discovery of polarizations from ground state absorption lines: tracer of sub-Gauss magnetic field on 89Her

We report the identification of the polarization of ground state absorption lines from post-AGB 89~Hercules. Two ground state neutral iron lines are found to have counterintuitive high-amplitude polarizations and an unchanged polarization direction through the orbital period, as opposed to the pattern of polarizations of absorption lines from excited states, which are synchronized with the orbital phase owing to optical pumping. This can be explained with magnetic realignment on the ground state. The 3D mean magnetic field is thereby unveiled from the degree and direction of the polarizations of the two iron lines. The field strength is also constrained to be $\lesssim 100\,{\rm mG}$. Our result has thus improved the accuracy by orders of magnitude compared to the previous $10\,{\rm G}$ upper limit set by non-detection of the Zeeman effect.

astro-ph.GA↗

arXiv2019

Understanding the multiwavelength observation of Geminga's TeV halo: the role of anisotropic diffusion of particles

In this letter we propose that the X-ray and the TeV observations in the vicinity of Geminga can be understood in the framework of anisotropic diffusion of injected electrons/positrons. This interpretation only requires the turbulence in the vicinity of Geminga to be sub-Alfvénic with the local mean magnetic field direction approximately aligned with our line of sight towards Geminga, without invoking extreme conditions for the environment, such as an extremely small diffusion coefficient and a weak magnetic field of strength $<1μ$G as suggested in previous literature.

astro-ph.HE↗

arXiv2018

The influence of atomic alignment on absorption and emission spectroscopy

Spectroscopic observations play essential roles in astrophysics. They are crucial for determining physical parameters in the universe, providing information about the chemistry of various astronomical environments. The proper execution of the spectroscopic analysis requires accounting for all the physical effects that are compatible to the signal-to-noise ratio. We find in this paper the influence on spectroscopy from the atomic/ground state alignment owing to anisotropic radiation and modulated by interstellar magnetic field, has significant impact on the study of interstellar gas. In different observational scenarios, we comprehensively demonstrate how atomic alignment influences the spectral analysis and provide the expressions for correcting the effect. The variations are even more pronounced for multiplets and line ratios. We show the variation of the deduced physical parameters caused by the atomic alignment effect, including alpha-to-iron ratio ([X/Fe]) and ionisation fraction. Synthetic observations are performed to illustrate the visibility of such effect with current facilities. A study of PDRs in $ρ$ Ophiuchi cloud is presented to demonstrate how to account for atomic alignment in practice. Our work has shown that due to its potential impact, atomic alignment has to be included in an accurate spectroscopic analysis of the interstellar gas with current observational capability.

astro-ph.GA↗

arXiv2018

The influence of magnetic fields on absorption and emission spectroscopies

Spectroscopic observations play a fundamental role in astrophysics. They are crutial to determine important physical parameters, provide information about the composition of various objects in the universe, as well as depict motions in the universe. However, spectroscopic studies often do not consider the influence of magnetic fields. In this paper, we explore the influence of magnetic fields on the spectroscopic observations using the concept of atomic alignment. Synthetic spectra are generated to show the measurable changes of the spectra due to atomic alignment. The influences of atomic alignment on absorption from DLAs, emission from H\,{\sc ii} Regions, submillimeter fine-structure lines from star forming regions are presented as examples to show this effect in diffuse gas. Furthermore, we demonstrate the influence of atomic alignment on physical parameters derived from atomic line ratios, such as the alpha-to-iron ratio([X/Fe]), interstellar temperature, and ionization rate. We conclude that Ground State Alignment (GSA) should be taken into consideration in the error budget of spectroscopic studies with high signal-to-noise(S/N) ratio.

astro-ph.IM↗

arXiv2017

Polarisation of submillimetre lines from interstellar medium

Magnetic fields play important roles in many astrophysical processes. However, there is no universal diagnostic for the magnetic fields in the interstellar medium (ISM) and each magnetic tracer has its limitation. Any new detection method is thus valuable. Theoretical studies have shown that submillimetre fine-structure lines are polarised due to atomic alignment by Ultraviolet (UV) photon-excitation, which opens up a new avenue to probe interstellar magnetic fields. We will, for the first time, perform synthetic observations on the simulated three-dimensional ISM to demonstrate the measurability of the polarisation of submillimetre atomic lines. The maximum polarisation for different absorption and emission lines expected from various sources, including Star-Forming Regions (SFRs) are provided. Our results demonstrate that the polarisation of submillimetre atomic lines is a powerful magnetic tracer and add great value to the observational studies of the submilimetre astronomy.

astro-ph.GA↗

arXiv2015

Tracing Magnetic Fields by Atomic Alignment in Extended Radiation Fields

Tracing magnetic fields is crucial as magnetic fields play an important role in many astrophysical processes. Earlier studies have demonstrated that Ground State Alignment (GSA) is a unique way to detect weak magnetic fields (1G> B> 1exp(-15)G) in diffuse media, they consider the situation when the pumping source is a point source, which applies when the star is very far away from the diffuse media. In this paper, we explore the GSA in the presence of extended radiation fields. For the radiation fields with a clear geometric structure, we consider the alignment in circumstellar medium, binary systems, disc, and Local Interstellar Medium (LISM). For the radiation fields with unidentified pumping sources, we apply the method of multipole expansion and discuss the GSA induced by each component. We demonstrate that for general radiation fields, it is adequate to consider the contribution from dipole and quadrupole radiation components. We find that in general polarization of absorption arizing from GSA coincides with the projection of magnetic field in the 2D sky with 90 degree degeneracy. We conclude that the GSA is a unique tool to detect the direction of weak magnetic field, and it can be applied to diffuse media in any radiation field.

astro-ph.IM↗