arXiv subjects

Dahai Yan

Publications and source records attributed to Dahai Yan.

48 records · Page 3Linked to original sources

arXiv2015

Dynamic changes of emitting electron distribution in the jet of 3C 279: signatures of acceleration and cooling

We study the dynamic changes of electron energy distribution (EED) through systematically analysing the quasi-simultaneous spectral energy distributions (SEDs) of the flat spectrum radio quasar 3C 279 in different states. With Markov chain Monte Carlo (MCMC) technique we model fourteen SEDs of 3C 279 using a leptonic model with a three-parameter log-parabola electron energy distribution (EED). The 14 SEDs can be satisfactorily fitted with the one-zone leptonic model. The observed $γ$ rays in 13 states are attributed to Compton scattering of external infrared photons from a surrounding dusty torus. The curved $γ$-ray spectrum observed during 2-8 April 2014 is well explained by the external Compton of dust radiation. It is found that there is a clear positive correlation between the curvature parameter $b$ of the EED and the electron peak energy $γ'_{\rm pk}$. No clear correlation between $b$ and the synchrotron peak frequency $ν_{\rm s}$ is found, due to the varied product of Doppler factor and fluid magnetic field from state to state. We interpret the correlation of $b-γ'_{\rm pk}$ in a stochastic acceleration scenario. This positive correlation is in agreement with the prediction in the stage when the balance between acceleration and radiative cooling of the electrons is nearly established in the case of the turbulence spectral index $q=2$.

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arXiv2015

Parameter constraints in a near-equipartition model with multi-frequency \emph{NuSTAR}, \emph{Swift} and \emph{Fermi-LAT} data from 3C 279

Precise spectra of 3C 279 in the 0.5-70 keV range, obtained during two epochs of \emph{Swift} and \emph{NuSTAR} observations, are analyzed using a near-equipartition model. We apply a one-zone leptonic model with a three-parameter log-parabola electron energy distribution (EED) to fit the \emph{Swift} and \emph{NuSTAR} X-ray data, as well as simultaneous optical and \emph{Fermi}-LAT $γ$-ray data. The Markov Chain Monte Carlo (MCMC) technique is used to search the high-dimensional parameter space and evaluate the uncertainties on model parameters. We show that the two spectra can be successfully fit in near-equipartition conditions, defined by the ratio of the energy density of relativistic electrons to magnetic field $ζ_{\rm e}$ being close to unity. In both spectra, the observed X-rays are dominated by synchrotron-self Compton photons, and the observed $γ$ rays are dominated by Compton scattering of external infrared photons from a surrounding dusty torus. Model parameters are well constrained. From the low state to the high state, both the curvature of the log-parabola width parameter and the synchrotron peak frequency significantly increase. The derived magnetic fields in the two states are nearly identical ($\sim1$\ G), but the Doppler factor in the high state is larger than that in the low state ($\sim$28 versus $\sim$18). We derive that the gamma-ray emission site takes place outside the broad-line region, at $\gtrsim$ 0.1 pc from the black hole, but within the dusty torus. Implications for 3C 279 as a source of high-energy cosmic-rays are discussed.

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arXiv2015

Near-Equipartition Jets with Log-Parabola Electron Energy Distribution and the Blazar Spectral-Index Diagrams

Fermi-LAT analyses show that the gamma-ray photon spectral indices Gamma_gamma of a large sample of blazars correlate with the vFv peak synchrotron frequency v_s according to the relation Gamma_gamma = d - k log v_s. The same function, with different constants d and k, also describes the relationship between Gamma_gamma and peak Compton frequency v_C. This behavior is derived analytically using an equipartition blazar model with a log-parabola description of the electron energy distribution (EED). In the Thomson regime, k = k_EC = 3b/4 for external Compton processes and k = k_SSC = 9b/16 for synchrotron self-Compton (SSC) processes, where b is the log-parabola width parameter of the EED. The BL Lac object Mrk 501 is fit with a synchrotron/SSC model given by the log-parabola EED, and is best fit away from equipartition. Corrections are made to the spectral-index diagrams for a low-energy power-law EED and departures from equipartition, as constrained by absolute jet power. Analytic expressions are compared with numerical values derived from self-Compton and external Compton scattered gamma-ray spectra from Ly alpha broad-line region and IR target photons. The Gamma_gamma vs. v_s behavior in the model depends strongly on b, with progressively and predictably weaker dependences on gamma-ray detection range, variability time, and isotropic gamma-ray luminosity. Implications for blazar unification and blazars as ultra-high energy cosmic-ray sources are presented. Arguments by Ghisellini et al. (2014) that the jet power exceeds the accretion luminosity depend on the doubtful assumption that we are viewing at the Doppler angle.

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arXiv2015

A self-consistent interpretation of the GeV-TeV emission from a distant blazar PKS 1424+240

We propose a scenario for self-consistent interpretation of GeV - TeV spectrum of a distant blazar PKS 1424+240. In this scenario, ultra-high energy (UHE) protons are assumed to exist in the blazar jet and produce gamma rays through synchrotron emission emitted by relativistic protons and pair cascades (resulted from $pγ$ interaction); meanwhile, some of these UHE protons may escape from the jet and are injected into intergalactic space. Therefore, we assume that UHE cosmic rays (CR) originate from relativistic protons in the jet and use energy-independent escape timescale to obtain UHECR injection spectrum. Both contributions of gamma rays injected by the source and secondary gamma rays produced in interactions of UHECRs emitted by the blazar with photon background during their propagation through intergalactic space are calculated. Our results show that this scenario is able to reproduce the GeV-TeV spectrum of PKS 1424+240 self-consistently in a broad range of redshifts $0.6<z<1.3$. The required relativistic jet power $L_p\simeq3\times10^{46}\ \rm erg\ s^{-1}$ only moderately depends on the assumed source redshift, while the proton escape timescale (and the injected UHECR luminosity) strongly depends on $z$. We compare the integral TeV fluxes predicted in our scenario with the sensitivity of the planned Cherenkov Telescope Array (CTA), and discuss the implications to future observations.

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arXiv2014

Understanding the TeV emission from a distant blazar PKS 1424+240 in a lepto-hadronic jet model

We investigate the formation of TeV spectrum of a distant blazar PKS 1424+240 residing at a redshift $z\geq0.6$ in two scenarios in the frame of a lepto-hadronic jet model taking both the uncertainties of the extragalactic background light (EBL) and its redshift into account. In the first scenario, TeV emission is attributed to the synchrotron emission of pair cascades resulting from $pγ$ interaction; in the second scenario, TeV emission is attributed to the proton-synchrotron emission, and an internal absorption due to interaction with the photons around the jet is included. The results show that in the first scenario the 68\% upper limit of its redshift within which this scenario can explain the VERITAS TeV spectrum in 2009 well is $\sim0.75$; in the second scenario, this upper limit of the redshift becomes $\sim1.03$. However, the second scenario can be excluded because it requires an unreasonable photon field around the jet with a luminosity of $\sim10^{43}\rm \ erg\ s^{-1}$. In conclusion, the jet model can explain its TeV spectrum with a low EBL density if $0.6<z<0.75$.

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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.

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arXiv2013

Emitting Electron Spectra and Acceleration Processes in the Jet of PKS 0447-439

We investigate the electron energy distributions (EEDs) and the corresponding acceleration processes in the jet of PKS 0447$-$439 and estimate its redshift through modeling its observed spectral energy distribution (SED) in the frame of a one-zone synchrotron-self Compton (SSC) model. Three EEDs formed in different acceleration scenarios are assumed: the power-law with exponential cut-off (PLC) EED (shock-acceleration scenario or the case of the EED approaching equilibrium in the stochastic-acceleration scenario), the log-parabolic (LP) EED (stochastic-acceleration scenario and the acceleration dominating) and the broken power law (BPL) EED (no acceleration scenario), and then the corresponding fluxes of both synchrotron and SSC are calculated. The model is applied to PKS 0447-439 and modeling SEDs are compared to the observed SED of this object by using the Markov Chain Monte Carlo (MCMC) method. Calculating results show that PLC model fails to fit the observed SED well, while the LP and BPL models give comparably good fits for the observed SED. The results indicate that it is possible that stochastic acceleration process acts in the emitting region of PKS 0447-439 and the EED is far from equilibrium (acceleration dominating) or no acceleration process works (in the emitting region). The redshift of PKS 0447-439 is also estimated in our fitting, and $z=0.16\pm0.05$ for LP case and $z=0.17\pm0.04$ for BPL case.

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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.

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arXiv2012

Multi-wavelength emission from 3C 66A: clues on its redshift and gamma-ray emission location

The quasi-simultaneous multi-wavelength emission of TeV blazar 3C 66A is studied by using a one-zone multi-component leptonic jet model. It is found that the quasi-simultaneous spectral energy distribution (SED) of 3C 66A can be well reproduced, especially its {\it Fermi}-LAT first 3 months average spectrum can be well reproduced by the synchrotron-self Compton (SSC) component plus external Compton (EC) component of the broad line region (BLR). Clues on its redshift and gamma-ray emission location are obtained. The results indicate the following. (i) On the redshift; The theoretical intrinsic TeV spectra can be predicted by extrapolating the reproduced GeV spectra. Through comparing this extrapolated TeV spectra with the extragalactic background light (EBL) corrected observed TeV spectra, it is suggested that the redshift of 3C 66A could be between 0.1 and 0.3, the most likely value is $\sim$ 0.2. (ii) On the gamma-ray emission location; To well reproduce the GeV emission of 3C 66A under different assumptions on BLR, the gamma-ray emission region is always required to be beyond the inner zone of BLR. The BLR absorption effect on gamma-ray emission confirms this point.

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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.

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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.

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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).

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