arXiv subjects

Xiang-Er Fang

Publications and source records attributed to Xiang-Er Fang.

5 recordsLinked to original sources

arXiv2026

Propagation Diagnostics of Supernova Remnant Environments around Young Repeating FRBs. I. Hydrodynamic Evolution of the Source-Local Dispersion Measure

Repeating fast radio bursts may reside in young supernova remnant (SNR) environments whose evolving plasma contributes to the observed dispersion measure (DM). We use two-dimensional axisymmetric hydrodynamic simulations to study the interaction between a continuous anisotropic wind from a young neutron star and homologously expanding supernova ejecta. We follow the evolution to approximately 160 yr and calculate the source-local DM along different viewing directions, using a passive tracer to separate wind and non-wind contributions. In the fiducial model, the strongly polar-focused wind inflates a low-density cavity, while the swept-up shell remains broadly rounded and the DM shows moderate angular variation. The DM is dominated by ejecta and swept-up non-wind material. The solid-angle-averaged ambient-subtracted excess DM declines throughout the evolution, approximately following $t_{\rm age}^{-2}$ during the first several tens of years and becoming modestly steeper later. Variations in wind and ejecta parameters modify the normalization, early evolution, and viewing-angle dependence, but the angle-averaged DM declines in all models, while different bipolar wind profiles produce similar long-term evolution. For FRB 20190520B, the fiducial model reaches a decline rate comparable to the source-frame value inferred from observations at approximately 20 yr, when the mean excess DM is approximately $1.8 \times 10^2$ pc cm$^{-3}$. Thus, such a young environment can retain a substantial electron column while producing a rapid secular decrease. Repeater diversity suggests that SNR-driven expansion may coexist with additional time-dependent plasma structures or ionization changes.

astro-ph.HE↗

arXiv2020

Constraining the Milky Way Mass with Its Hot Gaseous Halo

We propose a novel method to constrain the Milky Way (MW) mass $M_{\rm vir}$ with its corona temperature observations. For a given corona density profile, one can derive its temperature distribution assuming a generalized equilibrium model with non-thermal pressure support. While the derived temperature profile decreases substantially with radius, the X-ray-emission-weighted average temperature, which depends most sensitively on $M_{\rm vir}$, is quite uniform toward different sight lines, consistent with X-ray observations. For an Navarro-Frenk-White (NFW) total matter distribution, the corona density profile should be cored, and we constrain $M_{\rm vir}=(1.19$ - $2.95) \times 10^{12} M_{\rm sun}$. For a total matter distribution contributed by an NFW dark matter profile and central baryons, the corona density profile should be cuspy and $M_{\rm vir,dm}=(1.34$ - $5.44) \times 10^{12} M_{\rm sun}$. Non-thermal pressure support leads to even higher values of $M_{\rm vir}$, while a lower MW mass may be possible if the corona is accelerating outward. This method is independent of the total corona mass, its metallicity, and temperature at very large radii.

astro-ph.GA↗

arXiv2020

On the Cooling Flow Problem in the Gaseous Halo of the Milky Way

Theoretical and observational arguments suggest that there is a large amount of hot ($\sim 10^6$ K), diffuse gas residing in the Milky Way's halo, while its total mass and spatial distribution are still unclear. In this work, we present a general model for the gas density distribution in the Galactic halo, and investigate the gas evolution under radiative cooling with a series of 2D hydrodynamic simulations. We find that the mass inflow rate in the developed cooling flow increases with gas metallicity and the total gas mass in the halo. For a fixed halo gas mass, the spatial gas distribution affects the onset time of the cooling catastrophe, which starts earlier when the gas distribution is more centrally-peaked, but does not substantially affect the final mass inflow rate. The gravity from the Galactic bulge and disk affects gas properties in inner regions, but has little effect on the final inflow rate either. We confirm our results by investigating cooling flows in several density models adopted from the literature, including the Navarro-Frenk-White (NFW) model, the cored-NFW model, the Maller & Bullock model, and the $β$ model. Typical mass inflow rates in our simulations range from $\sim 5 M_{\odot}$ yr$^{-1}$ to $\sim 60 M_{\odot}$ yr$^{-1}$, and are much higher than the observed star formation rate in our Galaxy, suggesting that stellar and active galactic nucleus feedback processes may play important roles in the evolution of the Milky Way (MW) and MW-type galaxies.

astro-ph.GA↗

arXiv2018

On the Efficiency of Thermal Conduction in Galaxy Clusters

Galaxy clusters host a large reservoir of diffuse plasma with radially-varying temperature profiles. The efficiency of thermal conduction in the intracluster medium (ICM) is complicated by the existence of turbulence and magnetic fields, and has received a lot of attention in the literature. Previous studies suggest that the magnetothermal instability developed in outer regions of galaxy clusters would drive magnetic field lines preferentially radial, resulting in efficient conduction along the radial direction. Using a series of spherically-symmetric simulations, here we investigate the impact of thermal conduction on the observed temperature distributions in outer regions of three massive clusters, and find that thermal conduction substantially modifies the ICM temperature profile. Within 3 Gyr, the gas temperature at a representative radius of $0.3r_{500}$ typically decreases by ~10 - 20% and the average temperature slope between $0.3r_{500}$ and $r_{500}$ drops by ~ 30 - 40%, indicating that the observed ICM would not stay in a long-term equilibrium state in the presence of thermal conduction. However, X-ray observations show that the outer regions of massive clusters have remarkably similar radially-declining temperature profiles, suggesting that they should be quite stable. Our study thus suggests that the effective conductivity along the radial direction must be suppressed below the Spitzer value by a factor of 10 or more, unless additional heating sources offset conductive cooling and maintain the observed temperature distributions. Our study provides a smoking-gun evidence for the suppression of parallel conduction along magnetic field lines in low-collisionality plasmas by kinetic mirror or whistler instabilities.

astro-ph.HE↗

arXiv2016

Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. V. A New Size-Luminosity Scaling Relation for the Broad-Line Region

This paper reports results of the third-year campaign of monitoring super-Eddington accreting massive black holes (SEAMBHs) in active galactic nuclei (AGNs) between 2014-2015. Ten new targets were selected from quasar sample of Sloan Digital Sky Survey (SDSS), which are generally more luminous than the SEAMBH candidates in last two years. H$β$ lags ($τ_{_{\rm Hβ}}$) in five of the 10 quasars have been successfully measured in this monitoring season. We find that the lags are generally shorter, by large factors, than those of objects with same optical luminosity, in light of the well-known $R_{_{\rm Hβ}}-L_{5100}$ relation. The five quasars have dimensionless accretion rates of $\dot{\mathscr{M}}=10-10^3$. Combining measurements of the previous SEAMBHs, we find that the reduction of H$β$ lags tightly depends on accretion rates, $τ_{_{\rm Hβ}}/τ_{_{R-L}}\propto\dot{\mathscr{M}}^{-0.42}$, where $τ_{_{R-L}}$ is the H$β$ lag from the normal $R_{_{\rm Hβ}}-L_{5100}$ relation. Fitting 63 mapped AGNs, we present a new scaling relation for the broad-line region: $R_{_{\rm Hβ}}=α_1\ell_{44}^{β_1}\,\min\left[1,\left(\dot{\mathscr{M}}/\dot{\mathscr{M}}_c\right)^{-γ_1}\right]$, where $\ell_{44}=L_{5100}/10^{44}\,\rm erg~s^{-1}$ is 5100 Å continuum luminosity, and coefficients of $α_1=(29.6_{-2.8}^{+2.7})$ lt-d, $β_1=0.56_{-0.03}^{+0.03}$, $γ_1=0.52_{-0.16}^{+0.33}$ and $\dot{\mathscr{M}}_c=11.19_{-6.22}^{+2.29}$. This relation is applicable to AGNs over a wide range of accretion rates, from $10^{-3}$ to $10^3$. Implications of this new relation are briefly discussed.

astro-ph.GA↗