arXiv subjects

Houdun Zeng

Publications and source records attributed to Houdun Zeng.

26 records · Page 2Linked to original sources

arXiv2016

Evolution of High-Energy Particle Distribution in Mature Shell-Type Supernova Remnants

Multi-wavelength observations of mature supernova remnants (SNRs), especially with recent advances in gamma-ray astronomy, make it possible to constrain energy distribution of energetic particles within these remnants. In consideration of the SNR origin of Galactic cosmic rays and physics related to particle acceleration and radiative processes, we use a simple one-zone model to fit the nonthermal emission spectra of three shell-type SNRs located within 2 degrees on the sky: RX J1713.7-3946, CTB 37B, and CTB 37A. Although radio images of these three sources all show a shell (or half-shell) structure, their radio, X-ray, and gamma-ray spectra are quite different, offering an ideal case to explore evolution of energetic particle distribution in SNRs. Our spectral fitting shows that 1) the particle distribution becomes harder with aging of these SNRs, implying a continuous acceleration process, and the particle distributions of CTB 37A and CTB 37B in the GeV range are harder than the hardest distribution that can be produced at a shock via the linear diffusive shock particle acceleration process, so spatial transport may play a role; 2) the energy loss timescale of electrons at the high-energy cutoff due to synchrotron radiation appears to be always a bit (within a factor of a few) shorter than the age of the corresponding remnant, which also requires continuous particle acceleration; 3) double power-law distributions are needed to fit the spectra of CTB 37B and CTB 37A, which may be attributed to shock interaction with molecular clouds.

astro-ph.HE↗

arXiv2016

Cosmological Tests with the FSRQ Gamma-ray Luminosity Function

The extensive catalog of $γ$-ray selected flat-spectrum radio quasars (FSRQs) produced by \emph{Fermi} during a four-year survey has generated considerable interest in determining their $γ$-ray luminosity function (GLF) and its evolution with cosmic time. In this paper, we introduce the novel idea of using this extensive database to test the differential volume expansion rate predicted by two specific models, the concordance $Λ$CDM and $R_{\rm h}=ct$ cosmologies. For this purpose, we use two well-studied formulations of the GLF, one based on pure luminosity evolution (PLE) and the other on a luminosity-dependent density evolution (LDDE). Using a Kolmogorov-Smirnov test on one-parameter cumulative distributions (in luminosity, redshift, photon index and source count), we confirm the results of earlier works showing that these data somewhat favour LDDE over PLE; we show that this is the case for both $Λ$CDM and $R_{\rm h}=ct$. Regardless of which GLF one chooses, however, we also show that model selection tools very strongly favour $R_{\rm h}=ct$ over $Λ$CDM. We suggest that such population studies, though featuring a strong evolution in redshift, may nonetheless be used as a valuable independent check of other model comparisons based solely on geometric considerations.

astro-ph.CO↗

arXiv2015

Limitation of the Least Square Method in the Evaluation of Dimension of Fractal Brownian Motions

With the standard deviation for the logarithm of the re-scaled range $\langle |F(t+τ)-F(t)|\rangle$ of simulated fractal Brownian motions $F(t)$ given in a previous paper \cite{q14}, the method of least squares is adopted to determine the slope, $S$, and intercept, $I$, of the log$(\langle |F(t+τ)-F(t)|\rangle)$ vs $\rm{log}(τ)$ plot to investigate the limitation of this procedure. It is found that the reduced $χ^2$ of the fitting decreases with the increase of the Hurst index, $H$ (the expectation value of $S$), which may be attributed to the correlation among the re-scaled ranges. Similarly, it is found that the errors of the fitting parameters $S$ and $I$ are usually smaller than their corresponding standard deviations. These results show the limitation of using the simple least square method to determine the dimension of a fractal time series. Nevertheless, they may be used to reinterpret the fitting results of the least square method to determine the dimension of fractal Brownian motions more self-consistently. The currency exchange rate between Euro and Dollar is used as an example to demonstrate this procedure and a fractal dimension of 1.511 is obtained for spans greater than 30 transactions.

physics.data-an↗

arXiv2014

The physical properties of Fermi BL Lac objects jets

We investigate the physical properties of BL Lac objects (BL Lacs) jets by modeling the quasi-simultaneous spectral energy distributions (SEDs) of 22 {\it Fermi} BL Lac objects in the frame of a simple one-zone synchrotron self-Compton (SSC) model. We obtained the best-fit model parameters and their uncertainties for each BL Lac object through the $χ^2$-minimization procedure and discussed their implications on the physical processes. The modeling results show that the one-zone SSC model can successfully fit the SEDs of high-synchrotron-peaked BL Lacs (HBLs) and intermediate-synchrotron-peaked BL Lacs (IBLs), but fails to explain the SEDs of low-synchrotron-peaked BL Lacs (LBLs). The statistical analysis results for model parameters are summarized as follows. (1) No correlation is found between magnetic field ($B$) and the broken energy of relativistic electrons distribution ($γ^{\prime}_{\rm b}$) for HBLs and IBLs, but there are inverse correlations between $γ^{\prime}_{\rm b}$ and the radius of emitting blob ($R^{\prime}_{\rm b}$) as well as the electrons number $K^{\prime}_{\rm e}$ for HBLs and IBLs. It's therefore concluded that the variation of $γ^{\prime}_{\rm b}$ is mainly caused by that of $R^{\prime}_{\rm b}$ rather than $B$ for HBLs and IBLs. (2) The Poynting flux in jets can not account for the observed radiations since the power in Poynting flux is smaller than the radiative power, and the cold protons could be the primary energy carrier in the jets.

astro-ph.HE↗

arXiv2013

Emitting electrons spectra and acceleration processes in the jet of Mrk 421: from low state to giant flare state

We investigate the electron energy distributions (EEDs) and the acceleration processes in the jet of Mrk 421 through fitting the spectral energy distributions (SEDs) in different active states in the frame of a one-zone synchrotron self-Compton (SSC) model. After assuming two possible EEDs formed in different acceleration models: the shock accelerated power-law with exponential cut-off (PLC) EED and the stochastic turbulence accelerated log-parabolic (LP) EED, we fit the observed SEDs of Mrk 421 in both low and giant flare states by using the Markov Chain Monte Carlo (MCMC) method which constrains the model parameters in a more efficient way. Our calculating results indicate that (1) the PLC and LP models give comparably good fits for the SED in low state, but the variations of model parameters from low state to flaring can be reasonably explained only in the case of the PLC in low state; and (2) the LP model gives better fits compared to the PLC model for the SED in flare state, and the intra-day/night variability observed at GeV-TeV bands can be accommodated only in the LP model. The giant flare may be attributed to the stochastic turbulence re-acceleration of the shock accelerated electrons in low state. Therefore, we may conclude that shock acceleration is dominant in low state, while stochastic turbulence acceleration is dominant in flare state. Moreover, our result shows that the extrapolated TeV spectra from the best-fit SEDs from optical through GeV with the two EEDs are different. It should be considered in caution when such extrapolated TeV spectra are used to constrain extragalactic background light (EBL) models.

astro-ph.HE↗

arXiv2012

Non-variable TeV emission from the extended jet of a blazar in the stochastic acceleration scenario: the case of the hard TeV emission of 1ES 1101-232

The detections of X-ray emission from the kiloparsec-scale jets of blazars and radio galaxies may imply the existence of high energy electrons in these extended jets, and these electrons could produce high energy emission through inverse Compton (IC) process. In this paper we study the non-variable hard TeV emission from a blazar. The multi-band emission consists of two components: one is the traditional synchrotron self-Compton (SSC) emission from the inner jet, and the other is the emission produced via SSC and IC scattering of cosmic microwave background (CMB) photons (IC/CMB) and extragalactic background light (EBL) photons (IC/EBL) by relativistic electrons in the extended jet under the stochastic acceleration scenario. Such a model is applied to 1ES 1101-232. The results indicate that (1) the non-variable hard TeV emission of 1ES 1101-232 can be reproduced well, which is dominated by IC/CMB emission from the extended jet, using three characteristic values of Doppler factor ($δ_{\rm D}=5,10,15$) for the TeV emitting region in the extended jet; and (2) in the cases of $δ_{\rm D}=15$ and 10, the physical parameters can achieve the equipartition (or quasi-equipartition) between the relativistic electrons and the magnetic field; In contrast, the physical parameters largely deviate from the equipartition for the case of $δ_{\rm D}=5$. It is therefore concluded that the TeV emission region of 1ES 1101-232 in the extended jet should be moderately or highly beamed.

astro-ph.HE↗

arXiv2012

The contribution from blazar cascade emission to the extragalactic gamma-ray background: What a role does the extragalactic magnetic field play?

We estimate the contribution to the extragalactic gamma-ray background (EGRB) from both intrinsic and cascade emissions produced by blazars using a simple semi- analysis method for two models of the blazar gamma-ray luminosity function (GLF). For the cascade emission, we consider two possible contributions: one is due to that the flux of the cascade emission is lower than the Fermi LAT sensitivity (case I), which is independent on the extragalactic magnetic field (EGMF), another is due to the fact that the flux of the cascade emission is larger than the Fermi LAT sensitivity but the emission angle is larger than LAT point-spread-function (PSF) angle (case II), which depends on the EGMF. Our results indicate that (1) blazar contribution to the EGRB is dominant although it depends on the GLF model and the EGMF; (2) the EGMF plays an important role in estimating the contribution from the cascade emission produced by blazars, the contribution from the cascade emission will significantly alter the EGRB spectrum when the strength of the EGMF is large enough (say BEGMF > 10-12 G); and (3) since the cascade emission in case II reaches a saturation when the strength of the EGMF is ? 10-11 G, it is very possible that the contribution from the cascade emission produced by blazars can be considered as another method to probe the upper limit of the strength of the EGMF.

astro-ph.HE↗

arXiv2012

The estimate of emission region locations of {\it Fermi} FSRQs

We study the locations of emission regions through modelling the quasi-simultaneous multi-frequency spectral energy distributions of 21 {\it Fermi} flat spectrum radio quasars (FSRQs) in the frame of a multi-component one-zone leptonic model. In our calculations, we take the detailed broad line region (BLR) structure into account and discuss the effect of the uncertainty of the BLR structure on constraining the location of the emission regions for each FSQR, meanwhile we also include both the internal and external absorptions. Our results indicate that (1) the contribution of external Compton-BLR component is important to $γ$-ray emission, and the energy density of external target photon fields depends on the location of the emission region, which can be derived through reproducing the observed $γ$-ray emission, and (2) the emission regions of FSRQs with relative low accretion disk luminosity lie in the region of $7.9\times10^{16}$\ --\ $1.3\times10^{18}$\ cm (300 -- 4300 Schwarzschild radii) from central black hole, and for FSRQs with high accretion disk luminosity, the emission regions locate in the larger region of $2.6\times10^{17}$\ --\ $4.2\times10^{18}$\ cm (300 -- 5600 Schwarzschild radii).

astro-ph.HE↗