arXiv subjects

Dahai Yan

Publications and source records attributed to Dahai Yan.

At least 37 records · Page 2Linked to original sources

arXiv2021

A Quasi-periodic Oscillation in the gamma-ray Emission from the Non-blazar Active Galactic Nucleus PKS 0521-36

Long-term gamma-ray variability of a non-blazar Active Galactic Nucleus (AGN) PKS 0521-36 is investigated by using Fermi-LAT pass 8 data covering from 2008 August to 2021 March. The results show that the histogram of the gamma-ray fluxes follows a log-normal distribution. Interestingly, in the analysis of about 5.8-year (from MJD 56317 to 58447) LAT data between two outbursts (occurring during 2012 October and 2019 May respectively), a quasi-periodic oscillation (QPO) with a period of about 1.1 years (about 5 sigma of significance) is found in the Lomb-Scargle Periodogram (LSP), the Weighted Wavelet Z-transform (WWZ) and the REDFIT results. This quasi-periodic signal also appears in the results of Gaussian process modeling the light curve. Therefore, the robustness of the QPO is examined by four different methods. This is the first gamma-ray QPO found in a mildly beamed jet. Our results imply that the gamma-ray outbursts play an important role in the formation of the gamma-ray QPO.

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arXiv2020

Gaussian Process Modeling $\it{Fermi}$-LAT $γ$-ray Blazar Variability: A Sample of Blazars with $γ$-ray Quasi-periodicities

Blazar variability may be driven by stochastic processes. On the other hand, quasi-periodic oscillation (QPO) behaviors are recently reported to be detected in $\it{Fermi}$-LAT data of blazars. However, the significances of these QPO signals given by traditional Fourier-like methods are still questioned. We analyze $γ$-ray light curves of the QPO blazars with two Gaussian process methods, CARMA and $\it{celerite}$, to examine the appropriateness of Gaussian processes for characterizing $γ$-ray light curves of blazars and the existence of the reported QPOs. We collect a sample of 27 blazars with possible $γ$-ray periodicity and generate their $\sim11$ years $\it{Fermi}$-LAT light curves. We apply the Gaussian process models to the $γ$-ray light curves, and build their intrinsic power spectral densities (PSDs). The results show that in general the $γ$-ray light curves can be characterized by CARMA and $\textit{celerite}$ models, indicating that $γ$-ray variabilities of blazars are essentially Gaussian processes. The resulting PSDs are generally the red noise shapes with slopes between $-0.6$ and $-1.7$. Possible evidence for the $γ$-ray QPOs in PKS 0537$-$441 and PG 1553$+$113 are found in the Gaussian process modelings.

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arXiv2020

Multicolor Optical Monitoring of the Blazar S5 0716+714 from 2017 to 2019

We continuously monitored the blazar S5 0716+714 in the optical $g$, $r$ and $i$ bands from Nov. 10, 2017 to Jun. 06, 2019. The total number of observations is 201 nights including 26973 data points. This is a very large quasi-simultaneous multicolor sample for the blazar. The average time spans and time resolutions are 3.4 hours and 2.9 minutes per night, respectively. During the period of observations, the target source in the $r$ band brightens from $14^{\rm m}.16$ to $12^{\rm m}.29$ together with five prominent sub-flares, and then first becomes fainter to $14^{\rm m}.76$ and again brightens to $12^{\rm m}.94$ with seven prominent sub-flares. For the long-term variations, we find a strong flatter when brighter (FWB) trend at a low flux state and then a weak FWB trend at a higher flux state. A weak FWB trend at a low flux state and then a strong FWB trend at a higher flux state are also reported. Most of sub-flares show the strong FWB trends, except for two flares with a weak FWB trend. The particle acceleration and cooling mechanisms together with the superposition of different FWB-slopes from sub-flares are likely to explain the optical color behaviours. A scenario of bent jet is discussed.

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arXiv2020

Searching for Quasi-Periodic Modulations in $γ$-ray Active Galactic Nuclei

We perform a systematic search of quasi-periodic variabilities in $γ$-ray active galactic nuclei (AGNs) in the third \emph{Fermi} Large Area Telescope source catalog (3FGL). We employe two techniques, Lomb-Scargle Periodogram (LSP) and Weighted Wavelet Z-transform (WWZ), to obtain power spectra of $γ$-ray light curves covering from 2008 August to 2016 December. The results show that besides several objects that have been reported in previous works, an additional source, the FSRQ PKS 0601-70 has a possible quasi-periodic variability of 450 days in its $γ$-ray light curves with the significance of $>3σ$. The physical implications of our findings are discussed.

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arXiv2020

On the Injection of Relativistic Electrons in the Jet of 3C 279

The acceleration of electrons in 3C 279 is investigated through analyzing the injected electron energy distribution (EED) in a time-dependent synchrotron self-Compton + external Compton emission model. In this model, it is assumed that relativistic electrons are continuously injected into the emission region, and the injected EED [$Q_e^\prime(γ^\prime)$] follows a single power-law form with low- and high-energy cutoffs $\rm γ_{min}'$ and $\rm γ_{max}'$, respectively, and the spectral index $n$, i.e, $Q_e^\prime(γ^\prime)\proptoγ^{\prime-n}$. This model is applied to 14 quasi-simultaneous spectral energy distributions (SEDs) of 3C 279. The Markov Chain Monte Carlo fitting technique is performed to obtain the best-fitting parameters and the uncertainties on the parameters. The results show that the injected EED is well constrained in each state. The value of $n$ is in the range of 2.5 to 3.8, which is larger than that expected by the classic non-relativistic shock acceleration. However, the large value of $n$ can be explained by the relativistic oblique shock acceleration. The flaring activity seems to be related to an increased acceleration efficiency, reflected in an increased $γ'_{\rm min}$ and electron injection power.

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arXiv2019

Gamma-ray luminosity function of BL Lac objects and contribution to the extragalactic gamma-ray background

Using a significantly enlarged \textit{Fermi}-LAT BL Lac objects (BL Lacs) sample, we construct the gamma-ray luminosity function (GLF) of BL Lacs, by the joint use of the space density distribution and source counts distribution. We use three well-studied forms of the GLF, i.e., the forms of pure density evolution (PDE), pure luminosity evolution (PLE) and luminosity-dependent density evolution (LDDE). The Markov Chain Monte Carlo (MCMC) technique is used to constrain model parameters. Our results suggest that LDDE model can give the best description for the BL Lac GLF. And the model shows that the BL Lacs with a harder GeV spectrum and a less luminosity evolve as strongly as FSRQs, and the evolution decreases as increasing luminosity. We also model the average photon spectra of BL Lacs with a double power laws model. Using this modeled spectra, BL Lacs contribute $ \sim 20\%$ of the total extragalactic gamma-ray background (EGB) at $E>100$ MeV, $\sim 100\%$ of the EGB at $E>50$ GeV, and the unresolved BL Lacs contribute $\sim 20 \%$ of the isotropic diffuse gamma-ray background (IGRB) at $E>100$ MeV. A prediction of the TeV EGB spectra are given, which may be tested by the future detectors.

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arXiv2019

Statistical analysis on X-ray flares from the nucleus and HST-1 knot in the M87 jet

The statistical properties of X-ray flares from two separate locations (nucleus and HST-1) in the M87 jet are investigated to reveal the physical origin of the flares. We analyse the archival \textit{Chandra} data for M87, and identify 14 flares in the nucleus and 9 flares in HST-1. The peak intensity ($I_{\rm{P}}$) and the flaring duration time ($T_{\rm{fl}}$) for each flare are obtained. It is found that the distributions of both $I_{\rm{P}}$ and $T_{\rm{fl}}$ for the nucleus obey a power-law form with a similar index. A similar result is also obtained for HST-1, and no significant inconsistency between the nucleus and HST-1 is found for the indices. Similar to solar X-ray flares, the power-law distributions of the flare event parameters can be well explained by a self-organized criticality (SOC) system, which are triggered by magnetic reconnection. Our results suggest that the flares from nucleus and HST-1 are possibly triggered by magnetic reconnection process. The consistent indices for the distributions of $I_{\rm{P}}$ and $T_{\rm{fl}}$ in the CORE and HST-1 indicate that the dimensions of the energy dissipation of the magnetic reconnection is identical in the two regions. A strong correlation between the flares in the two regions also suggests a similar physical origin for the flares.

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arXiv2019

Using the Extragalactic Gamma-Ray Background to Constrain the Hubble Constant and Matter Density of the Universe

The attenuation produced by extragalactic background light (EBL) in $γ$-ray spectra of blazars has been used to constrain the Hubble constant ($H_0$) and matter density ($Ω_{\rm m}$) of the Universe. We propose to estimate $H_0$ and $Ω_{\rm m}$ using the well measured $>$10 GeV extragalactic $γ$-ray background (EGB). This suggestion is based on the facts that the $>$10 GeV EGB is totally explained by the emissions from blazars, and an EBL-absorption cutoff occurs at $\sim$50 GeV in the EGB spectrum. We fit the $>$10 GeV EGB data with modeled EGB spectrum. This results in $H_0=64.9^{+4.6}_{-4.3}\rm \ km\ s^{-1}\ Mpc^{-1}$ and $Ω_{\rm m}=0.31^{+0.13}_{-0.14}$. Note that the uncertainties may be underestimated due to the limit of our realization for EBL model. $H_0$ and $Ω_{\rm m}$ are degenerate in our method. Independent determination of $Ω_{\rm m}$ by other methods would improve the constraint on $H_0$.

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arXiv2018

Impact of Plasma Instability on Constraint of the Intergalactic Magnetic Field

A relativistic electron-positron pair beam can be produced in the interaction of TeV photons from a blazar with the extragalactic background light (EBL). The relativistic $e^{\pm}$ pairs would loss energy through inverse-Compton scattering (ICS) photons of cosmic microwave background (CMB) or plasma instabilities. The dominant energy-loss process is under debate. Based on the assumption that the dominant energy-loss process is ICS, the resulted cascade GeV radiation is usually used to constrain the intergalactic magnetic field (IGMF). Here, we include the energy-loss due to plasma oblique instability in the calculation of cascade gamma-ray flux, and investigate the impact of the plasma instability on the constraint of IGMF. The up-to-date GeV data and archival TeV data of the blazar 1ES 0229+200 are used. The results indicate that even if the oblique instability cooling is dominating over ICS cooling, the cascade flux could be still used to constrain the IGMF. It is found that with the ratio between the cooling rates of the oblique instability and the ICS varying from 0.1, 1 to 10, the lower limit of the IGMF putted by the cascade flux and the gamma-ray data changes from $8\times10^{-18}\ $G, $5\times10^{-18}\ $G to $10^{-18}\ $G. If the ratio between the two cooling rates is 30, the estimate of IGMF based on the cascade flux is invalid.

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arXiv2018

Testing relativistic boost as the cause of gamma-ray quasi-periodic oscillation in a blazar

The mechanism for producing gamma-ray quasi-periodic oscillation (QPO) in blazar is unknown. One possibility is the geometric model, in which without the need for intrinsic quasi-periodic variation, the relativistic Doppler factor changes periodically, resulting in observed gamma-ray QPO. We propose a method to test this geometric model. We analyze the {\it Fermi}-LAT data of PG 1553+113 spanning from 2008 August until 2018 February. According to 29 four-month average spectral energy distributions (SEDs) in the energy range of 0.1-300 GeV, we split the {\it Fermi}-LAT energy range into three bands: 0.1-1 GeV, 1-10 GeV, and 10-300 GeV. The spectrum in each energy range can be successfully fitted by a power-law. The light curves and photon indices in the three energy ranges are obtained. Then, light curves in three narrow energy ranges, i.e., 0.2-0.5 GeV, 2-5 GeV and 20- 40 GeV, are constructed, and the relative variability amplitudes in the three narrow energy ranges are calculated. A discrete-correlation analysis is performed for the light curves. Our results indicate that (i) the light curves in the different energy ranges follow the same pattern showed in the light curve above 0.1 GeV; (ii) the three groups of photon indices in the energy ranges of 0.1-1 GeV, 1-10 GeV, and 10-300 GeV keep nearly constant; (iii) the ratio between relative variability amplitudes in different narrow energy ranges are equal (within their errors) to the prediction by the Doppler effect. Our results support the scenario of the relativistic boost producing the gamma-ray QPO for PG 1553+113.

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arXiv2018

Statistical analysis on XMM-Newton X-ray flares of Mrk 421: distributions of peak flux and flaring time duration

The energy dissipation mechanism in blazar jet is unknown. Blazar's flares could provide insights on this problem. Here we report statistical results of XMM-Newton X-ray flares of Mrk 421. We analyze all public XMM-Newton X-ray observations for Mrk 421, and construct the light curves. Through fitting light curves, we obtain the parameters of flare-profiles, such as peak flux ($F_{\rm p}$) and flaring time duration ($T_{\rm fl}$). It is found that both the distributions of $F_{\rm p}$ and $T_{\rm fl}$ obey a power-law form, with the same index of $α_{\rm F}=α_{\rm T}\approx1$. The statistical properties are consistent with the predictions by a self-organized criticality (SOC) system with energy dissipation in one-dimensional space. This is similar to solar flare, but with different space dimensions of the energy dissipation domain. This suggests that X-ray flaers of Mrk 421 are possibly driven by a magnetic reconnection mechanism. Moreover, in the analysis, we find that variability on timescale of $\sim1000\ $s frequently appears. Such rapid variability indicates a magnetic field of $\geq 2.1δ_{\rm D}^{-1/3}$ G ($δ_{\rm D}$ is the Doppler factor) in emission region.

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arXiv2018

A Method for Locating High Energy Dissipation Region in Blazar

The production site of gamma-rays in blazar jet is an unresolved problem. We present a method to locate gamma-ray emission region in the framework of one-zone emission model. From measurements of core-shift effect, the relation between the magnetic field strengths ($B'$) in the radio cores of jet and the distances ($R$) of these radio cores from central supermassive black hole (SMBH) can be inferred. Therefore once the magnetic field strength in gamma-ray emission region ($B'_{\rm diss}$) is obtained, one can use the relation of $B'$-$R$ to derive the distance ($R_{\rm diss}$) of gamma-ray emission region from SMBH. Here we evaluate the lower limit of $B'_{\rm diss}$ by using the criteria that the optical variability timescale $t_{\rm var}$ should be longer or equal to the synchrotron radiation cooling timescale of the electrons that emit optical photons. We test the method with the observations of PSK 1510-089 and BL Lacertae, and derive $R_{\rm diss}<0.15δ_{\rm D}^{1/3}(1+A)^{2/3}\ $pc for PSK 1510-089 with $t_{\rm var}\sim$ a few hours, and $R_{\rm diss}<0.003δ_{\rm D}^{1/3}(1+A)^{2/3}\ $pc for BL Lacertae with $t_{\rm var}\sim$ a few minutes. Here $δ_{\rm D} $ is the Doppler factor and $A$ is the Compton dominance (i.e., the ratio of the Compton to the synchrotron peak luminosities).

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arXiv2018

Constraints on the location of $γ$-ray emitting region for a sample of blazars with radio core-shift measurements

We model simultaneous or quasi-simultaneous multi-band spectral energy distributions (SEDs) for a sample of 25 blazars that have radio core-shift measurements, where a one-zone leptonic model and Markov chain Monte Carlo technique are adopted. In the SED fitting for 23 low-synchrotron-peaked (LSP) blazars, the seed photons from broad line region (BLR) and molecular torus are considered respectively in the external Compton process. We find that the SED fitting with the seed photons from the torus are better than those utilizing BLR photons, which suggests that the $γ$-ray emitting region may be located outside the BLR. Assuming the magnetic field strength in the $γ$-ray emitting region as constrained from the SED fitting follows the magnetic field distribution as derived from the radio core-shift measurements (i.e., $B(R)\simeq B_{\rm 1 pc}(R/\rm 1pc)^{-1}$, $R$ is the distance from the central engine and $B_{\rm 1 pc}$ is the magnetic field strength at 1 pc), we further calculate the location of the $γ$-ray emitting region, $R_γ$, for these blazars. We find that $R_γ\sim2\times10^{4}R_{\rm S}\simeq10 R_{\rm BLR}$ ($R_{\rm S}$ is the Schwarzschild radius and $R_{\rm BLR}$ is the BLR size), where $R_{\rm BLR}$ is estimated from the broad line luminosities using the empirical correlations obtained using the reverberation mapping methods.

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arXiv2017

Constraining the redshifts of TeV BL Lac objects

We present a model-dependent method to estimate the redshifts of three TeV BL Lac objects (BL Lacs) through fltting their (quasi-) simultaneous multi-waveband spectral energy distributions (SEDs) by one-zone leptonic synchrotron self-Compton (SSC) model. Considering the impact of electron energy distributions (EEDs) on the results, we use three types of EEDs, such as the power-law EED with exponential cut-ofi (PLC), the log-parabola (PLLP) and the broken power-law (BPL) EEDs, to flt the SEDs. We also use a parameter fi to describe the uncertainties of the extragalactic background light (EBL) models, as in (Abdo et al. 2010b). We then use Markov Chain Monte Carlo (MCMC) method to explore multi-dimensional parameter space and obtain the uncertainties of the model parameters based on the observational data. We apply our method to obtain the redshifts of three TeV BL Lac objects in the marginalized 68% confldence, and flnd that the PLC EED does not flt the SEDs. For 3C66A, the redshift is 0.14 - 0.31 and 0.16 - 0.32 in the BPL and PLLP EEDs; for PKS1424+240, the redshift is 0.55 - 0.68 and 0.55 - 0.67 in the BPL and PLLP EEDs; for PG1553+113, the redshift is 0.22 - 0.48 and 0.22 - 0.39 in the BPL and PLLP EEDs. We also estimate the redshift of PKS1424+240 in the high stage to be 0.46 - 0.67 in the PLLP EED, roughly consistent with that in the low stage.

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arXiv2017

Possible Quasi-Periodic modulation in the z = 1.1 $γ$-ray blazar PKS 0426-380

We search for $γ$-ray and optical periodic modulations in a distant flat spectrum radio quasar (FSRQ) PKS 0426-380 (the redshift $z=1.1$). Using two techniques (i.e., the maximum likelihood optimization and the exposure-weighted aperture photometry), we obtain $γ$-ray light curves from \emph{Fermi}-LAT Pass 8 data covering from 2008 August to 2016 December. We then analyze the light curves with the Lomb-Scargle Periodogram (LSP) and the Weighted Wavelet Z-transform (WWZ). A $γ$-ray quasi-periodicity with a period of 3.35 $\pm$ 0.68 years is found at the significance-level of $\simeq3.6\ σ$. The optical-UV flux covering from 2005 August to 2013 April provided by ASI SCIENCE DATA CENTER is also analyzed, but no significant quasi-periodicity is found. It should be pointed out that the result of the optical-UV data could be tentative because of the incomplete of the data. Further long-term multiwavelength monitoring of this FSRQ is needed to confirm its quasi-periodicity.

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arXiv2017

Revisiting Quasi-Periodic modulation in $γ$-ray Blazar PKS 2155-304 with Fermi Pass 8 data

We examine the gamma-ray quasi-periodic variability of PKS 2155-304 with the latest publicly available \emph{Fermi}-LAT Pass 8 data which covers the years from 2008 August to 2016 October. We produce the light curves in two ways, i.e., the exposure-weighted aperture photometry and the maximum likelihood optimization. The light curves are then analyzed by using Lomb-Scargle Periodogram (LSP) and Weighted Wavelet Z-transform (WWZ); and the results reveal a significant quasi-periodicity with a period of $1.74\pm0.13$ years and a significance of $\sim$ 4.9 $σ$. The constraint of multifrequencies quasi-periodic variabilities on blazar emission model is discussed.

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arXiv2016

Testing one-zone synchrotron-self-Compton models with spectral energy distributions of Mrk 421

We test one-zone synchrotron self-Compton (SSC) models with high-quality multiwavelength spectral energy distribution (SED) data of Mrk 421. We use Markov chain Monte Carlo (MCMC) technique to fit twelve day-scale SEDs of Mrk 421 with one-zone SSC models. Three types of electron energy distribution (EED), a log-parabola (LP) EED, a power-law log-parabola (PLLP) EED and a broken power-law (BPL) EED, are assumed in fits. We find that the one-zone SSC model with the PLLP EED provides successful fits to all the twelve SEDs. However, the one-zone SSC model with the LP and BPL EEDs fail to provide acceptable fits to the highest energy X-ray data or GeV data in several states. We therefore conclude that the one-zone SSC model works well in explaining the SEDs of Mrk 421, and the PLLP EED is preferred over the LP and BPL EEDs for Mrk 421 during the flare in March 2010. We derive magnetic field $B'\sim0.01$\ G, Doppler factor $δ_{\rm D}\sim$30--50, and the curvature parameter of EED $r\sim1$--$10$ in the model with the PLLP EED. The evolutions of model parameters are explored. The physical implications of our results are discussed.

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arXiv2016

Formation of very hard electron and gamma-ray spectra of flat spectrum radio quasar in fast-cooling regime

In external Compton scenario, we investigate the formation of the very hard electron spectrum in the fast-cooling regime, using a time-dependent emission model. It is shown that a very hard electron distribution $N'_{\rm e}(\gp)\propto\gp^{-p}$ with the spectral index $p\sim1.3$ is formed below the minimum energy of injection electron when inverse Compton scattering takes place in the Klein-Nishina regime, i.e., inverse Compton scattering of relativistic electrons on broad-line region radiation in flat spectrum radio quasars. This produces a very hard gamma-ray spectrum, and can reasonably explain the very hard \emph{Fermi}-LAT spectrum of the flat spectrum radio quasar 3C 279 during the extreme gamma-ray flare in 2013 December.

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