We fit the (quasi-)simultaneous multi-waveband spectral energy distributions (SEDs) for a sample of low-synchrotron-peaked (LSP) blazars with a one-zone leptonic model. The seed photons that predominantly come from broad line region (BLR) and infrared (IR) molecular torus are considered respectively in external Compton process. We find that the modeling with IR seed photons is systematically better than that with BLR photons based on a χ 2 test, which suggest that γ-ray emitting region most possibly stay outside the BLR. The minimum electron Lorentz factor, γ min , is constrained from the modeling for these LSP blazars with good soft X-ray data (ranges from 5 to 160 with a median value of 55), which plays a key role in jet power estimation. Assuming one-to-one ratio of proton and electron, we find that the jet power for LSP blazars is systematically higher than that of FR II radio galaxies at given 151 MHz radio luminosity, L 151MHz , even though FR IIs are regarded as same as LSP blazars in unification scheme except the jet viewing angle. The possible reason is that there are some e ± pairs in the jet of these blazars. If this is the case, we find the number density of e ± pairs should be several times higher than that of e − − p pairs by assuming the jet power is the same for LSP blazars and FR IIs at given L 151MHz .

PKS 1424+240 is a distant very high energy gamma-ray BL Lac object with redshift z = 0.601. It was found that pure synchrotron self-Compton (SSC) process normally need extreme input parameters (e.g., very low magnetic field intensity and extraordinarily large Doppler factor) to explain its multi-wavelength spectral energy distributions (SEDs). To avoid the extreme model parameters, different models have been proposed (e.g., two-zone SSC model or lepto-hadronic model). In this work, we employ the traditional one-zone leptonic model after including a weak external Compton component to re-explore the simultaneous multi-wavelength SEDs of PKS 1424+240 in both high (2009) and low (2013) states. We find that the input parameters of magnetic field and Doppler factor are roughly consistent with those of other BL Lacs if a weak external photon field from either broad line region (BLR) or the dust torus. However, the required energy density of seed photons from BLR or torus is about 3 orders of magnitude less than that constrained in luminous quasars (e.g., flat-spectrum radio quasars, FSRQs). This result suggests that the BLR/torus in BL Lacs is much weaker than that of luminous FSRQs (but not fully disappear), and the inverse-Compton of external photons from BLR/torus may still play a role even in high synchrotron peaked blazars.

There are still some important unanswered questions about the detailed particle acceleration and escape occurring during the quiescent epoches. As a result, the particle distribution that is adopted in the blazar quiescent spectral model have numerous unconstrained shapes. To help remedy this problem, we introduce a analytical particle transport model to reproduce quiescent broadband spectral energy distribution of blazar. In this model, the exact electron distribution is solved from a generalized transport equation that contains the terms describing first-order and secondary-order Fermi acceleration, escape of particle due to both the advection and spatial diffusion, energy losses due to synchrotron emission and inverse-Compton scattering of an assumed soft photon field. We suggest that the advection is a significant escape mechanism in blazar jet. We find that in our model the advection process tends to harden the particle distribution, which enhances the high energy components of resulting synchrotron and synchrotron self-Comptom spectrum from jet. Our model is able to roughly reproduce the observed spectra of extreme BL Lac object 1ES 0414+009 with reasonable assumptions about the physical parameters.

In the third catalog of active galactic nuclei detected by the Fermi-LAT (3LAC) Clean Sample, there are 402 blazar candidates of uncertain type (BCUs). Due to the limitations of astronomical observation or intrinsic properties, it is difficult to classify blazars using optical spectroscopy. The potential classification of BCUs using machine-learning algorithms is essential. Based on the 3LAC Clean Sample, we collect 1420 Fermi blazars with eight parameters of γ-ray photon spectral index; radio flux; flux density; curve significance; the integral photon flux in 100–300 MeV, 0.3–1 GeV, and 10–100 GeV; and variability index. Here we apply four different supervised machine-learning (SML) algorithms (decision trees, random forests, support vector machines, and Mclust Gaussian finite mixture models) to evaluate the classification of BCUs based on the direct observational properties. All four methods can perform exceedingly well with more accuracy and can effectively forecast the classification of Fermi BCUs. The evaluating results show that the results of these methods (SML) are valid and robust, where about one-fourth of sources are flat-spectrum radio quasars (FSRQs) and three-fourths are BL Lacertae (BL Lacs) in 400 BCUs, which are consistent with some other recent results. Although a number of factors influence the accuracy of SML, the results are stable at a fixed ratio 1:3 between FSRQs and BL Lacs, which suggests that the SML can provide an effective method to evaluate the potential classification of BCUs. Among the four methods, Mclust Gaussian Mixture Modeling has the highest accuracy for our training sample (4/5, seed = 123).

The recently published fourth Fermi Large Area Telescope source catalog (4FGL) a)b) , reports 5065 gamma-ray sources in terms of direct observational gamma-ray properties. Among the sources, the largest population is the Active Galactic Nuclei (AGN), which consists of 3137 blazars, 42 radio galaxies, and 28 other AGNs. The blazar sample comprises 694 flat-spectrum radio quasars (FSRQs), 1131 BL Lac-type objects (BL Lacs), and 1312 blazar candidates of an unknown type (BCUs). The classification of blazars is difficult using optical spectroscopy given the limited knowledge with respect to their intrinsic properties, and the limited availability of astronomical observations. To overcome these challenges, machine learning algorithms are being investigated as alternative approaches. Using the 4FGL catalog, a sample of 3137 Fermi blazars with 23 parameters is systematically selected. Three established supervised machine learning algorithms (random forests (RFs), support vector machines (SVMs), artificial neural networks (ANNs)) are employed to general predictive models to classify the BCUs. We analyze the results for all of the different combinations of parameters. Interestingly, a previously reported trend the use of more parameters leading to higher accuracy is not found. Considering the least number of parameters used, combinations of eight, 12 or 10 parameters in the SVM, ANN, or RF generated models achieve the highest accuracy (Accuracy ≃ 91.8%, or ≃ 92.9%). Using the combined classification results from the optimal combinations of parameters, 724 BL Lac type candidates and 332 FSRQ type candidates are predicted; however, 256 remain without a clear prediction.

Context. Very high-energy (VHE) γ-ray measurements of distant TeV blazars can be nicely explained by TeV spectra induced by ultra high-energy cosmic rays. Aims. We develop a model for a plausible origin of hard spectra in distant TeV blazars.Methods. In the model, the TeV emission in distant TeV blazars is dominated by two mixed components. The first is the internal component with the photon energy around 1 TeV produced by inverse Compton scattering of the relativistic electrons on the synchrotron photons (SSC) with a correction for extragalactic background light absorbtion and the other is the external component with the photon energy more than 1 TeV produced by the cascade emission from high-energy protons propagating through intergalactic space.Results. Assuming suitable model parameters, we apply the model to observed spectra of distant TeV blazars of 1ES 0229+200. Our results show that 1) the observed spectrum properties of 1ES 0229+200, especially the TeV γ-ray tail of the observed spectra, could be reproduced in our model and 2) an expected TeV γ-ray spectrum with photon energy >1 TeV of 1ES 0229+200 should be comparable with the 50-h sensitivity goal of the Cherenkov Telescope Array (CTA) and the differential sensitivity curve for the one-year observation with the Large High Altitude Air Shower Observatory (LHAASO). Conclusions. We argue that strong evidence for the Bethe-Heitler cascades along the line of sight as a plausible origin of hard spectra in distant TeV blazars could be obtained from VHE observations with CTA, LHAASO, HAWC, and HiSCORE.

In this paper, we have selected a sample of 64 teraelectronvolt blazars, with redshift, from those classified in the fourth Fermi Large Area Telescope source catalog. 3 3 https://fermi.gsfc.nasa.gov/ssc/data/access/lat/8yr_catalog/ We have obtained the values of the relevant physical parameters by performing a log-parabolic fitting of the average-state multiwavelength spectral energy distributions. We estimate the range of the radiation zone parameters, such as the Doppler factor (D), the magnetic field strength (B), the radiative zone radius (R), and the peak Lorentz factor (γ p) of nonthermal electrons. Here, we show that (1) there is a strong linear positive correlation between the intrinsic synchrotron peak frequency and the intrinsic inverse Compton scattering (ICs) peak frequency among different types of blazars; (2) if radio bands are excluded, the spectral index of each band is negatively correlated with the intrinsic peak frequency; (3) there is a strong linear negative correlation between the curvature at the peak and the intrinsic peak frequency of the synchrotron bump, and a weak positive correlation between the curvature at the peak and the intrinsic peak frequency of the ICs bump; (4) there is a strong linear positive correlation between the intrinsic ICs peak luminosity and intrinsic γ-ray luminosity and between the intrinsic ICs peak frequency and peak Lorentz factor; (5) there is a strong negative linear correlation between log B and log γ p ; and (6) there is no correlation between log R and log γ p .

We employ a single-zone leptonic jet model, with synchrotron, synchrotron self-Compton (SSC) and external Compton (EC) process, to reproduce the quasi-simultaneous multi-wavelength spectral energy distributions in active and quiescent states of the narrow-line gamma-ray-loud radio source GB 1310+487. In the case of the EC process, the external seed photons from both broad line region (BLR) and dust torus are considered by assuming that the gamma-ray emission region is located at the outside boundary of the BLR and inside the dust torus. Comparing the energy density of external photon fields U BLR obtained by model fitting with that constrained from the BLR observations. We find that the location of the gamma-ray emitting region of GB 1310+487 can be tightly constrained at the outer edge of the BLR (the dissipation distance of the γ-ray emission region from central black hole r diss ∼ a few times of R BLR ). The ratio of magnetic energy and emitting-electron energy in the radiation blob (ǫ B = L B /L e ) is gradually increased from Flare 1, Flare 2 to Post-Flare, where the magnetic energy increase and matter energy decrease. These results suggest that the conversion of the magnetic field and the matter (radiation electrons) energy and the location of the γ-ray emission region (or ambient photon field) may play an important role in different radiation states of GB 1310+487.