arXiv subjects

Yong-Gang Zheng

Publications and source records attributed to Yong-Gang Zheng.

14 recordsLinked to original sources

arXiv2026

A Tentative Line-like GeV Excess in Fermi Blazar 4FGL J1754.2+3212: Implications for Jet Physics and Beyond

We report the detection of a tentative, narrow spectral feature in the GeV gamma-ray spectrum of the blazar 4FGL J1754.2$+$3212, using approximately 16.5 years of observations from the Fermi Large Area Telescope. A detailed likelihood analysis of the time-averaged spectrum reveals a localized, line-like excess at an energy of $\sim$31 GeV, with a local statistical significance of $\sim$3$σ$ (3 degrees of freedom), without corrections for multiple trials and the look-elsewhere effect. This feature is not well described by standard smooth continuum models such as a single log-parabola. The addition of a Gaussian component significantly improves the spectral fit. If confirmed by future observations with higher sensitivity, this feature would represent a novel phenomenon in blazar astrophysics. We explore its potential physical origins, which could involve exotic processes such as dark matter annihilation or photon-axion-like-particle oscillations, or extreme astrophysical mechanisms within a structured jet. Regardless of its ultimate nature, this finding highlights the potential of deep spectral studies of blazars to reveal non-standard physics and challenges conventional one-zone emission models. Definitive verification awaits observations by next-generation instruments, particularly the Very Large Area $γ$-ray Space Telescope (VLAST).

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arXiv2026

The Evolution of Thermal and Non-thermal Emission Components in GRB 250920C

We present a temporal and time-resolved spectral analysis of GRB 250920C using \textit{Fermi}/GBM and \textit{Swift}/BAT data. The prompt emission consists of two distinct episodes, EI and EII, separated by a significant quiescent interval. We perform Bayesian spectral fitting with empirical, thermal, composite, and physical synchrotron models. In EI, the spectra show clear evidence for an additional thermal component. The BB+PL model is preferred in most bright time bins, and joint \textit{Fermi}/GBM+\textit{Swift}/BAT fits further support the presence of this component. The blackbody temperature generally decreases with time, the blackbody flux follows the pulse profile, and the thermal flux fraction remains high. Fireball-parameter estimates give a photospheric Lorentz factor of a few hundred and an initial radius of $r_{0}\sim10^{9}$--$10^{10}$ cm, with $1+σ_{0}$ of order unity and $η\gg1$, supporting a thermally dominated baryonic outflow. In contrast, EII is dominated by non-thermal emission. Its low-energy photon indices do not significantly exceed the synchrotron line of death, and the spectra are well described by fast-cooling synchrotron radiation in a decaying magnetic field. The magnetization constraint gives $σ_{\min}\sim1.1$--$4.3$. Since these values exceed unity, they suggest that EII may be Poynting-flux dominated. These results therefore suggest a possible transition in GRB 250920C from a fireball-dominated EI phase to a Poynting-flux-dominated EII phase.

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arXiv2026

Discovery of Repeating Transitions in 16 Changing-look Active Galactic Nuclei

The repeating changing-look active galactic nuclei (RCL AGNs) exhibit multiple appearances and disappearances of broad emission lines (BELs), whose underlying mechanism remains a puzzle. Expanding the sample of RCL AGNs is valuable for constraining the transition timescale and probing the accretion physics driving CL behaviors. This study aims to identify RCL AGNs using the multi-epoch spectroscopic data of confirmed CL AGNs from the Sloan Digital Sky Survey, Large Sky Area Multi-Object Fiber Spectroscopic Telescope, and Dark Energy Spectroscopic Instrument, supplemented with mid-infrared (MIR) light curves. Through selection criteria and visual inspection, we identify 22 RCL AGNs among 299 CL AGNs, corresponding to an occurrence rate of about 7\%, indicating that repeated transitions are not extremely rare in CL AGNs. Among the 22 RCL AGNs, 16 are newly identified, which significantly expands the known RCL AGN sample. Based on the spectra and densely sampled MIR light curves, we derive MIR variability timescales for 18 RCL AGNs, and find no significant correlation between the timescale and the black hole mass.

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arXiv2026

Modeling Gamma-Ray Burst Spectra with Convolutional Neural Networks: Fast-Cooling Synchrotron Emission in a Decaying Magnetic Field

The radiation mechanism of gamma-ray burst (GRB) prompt emission remains uncertain. Although the fast-cooling synchrotron model in a decaying magnetic field can account for the characteristic nonthermal spectral shape, its computational cost has limited its use in systematic observational fitting and statistical model comparison. We develop a convolutional neural network (CNN)-based spectral emulator for this physical model and train it on a large synthetic data set generated over a physically motivated parameter space. The trained network reproduces the numerical spectra with high fidelity while reducing the cost of spectral evaluation to the millisecond level. We then incorporate the emulator into a Bayesian spectral-analysis framework and apply it to the time-resolved spectra of GRB 231020A observed by Fermi/GBM. In most time intervals, the decaying-field fast-cooling synchrotron model provides better fits and smaller Bayesian information criterion values than the standard fast-cooling synchrotron model. These results suggest that a radially decaying magnetic field provides a plausible and more physically motivated interpretation of the prompt-emission spectrum of this burst, while also indicating that the emulator offers a practical route for large-sample Bayesian inference and systematic comparisons of GRB prompt-emission models.

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arXiv2026

Fast-Cooling Synchrotron in Decaying Magnetic Fields: Implications for the GRB Spectral Distribution

The prompt-emission spectra of gamma-ray bursts (GRBs) are commonly described by the empirical Band function. The typical low-energy spectral index is $\sim -1$, which poses a challenge to standard synchrotron radiation models. We systematically investigate a fast-cooling synchrotron model with a decaying magnetic field and test, within an observation-consistent pipeline, whether it reproduces the Band-fit parameter distributions in the GBM catalog, in a statistical sense. We solve the electron continuity equation with synchrotron, adiabatic, and synchrotron self-Compton cooling to obtain the time-dependent electron distribution and synthetic spectra; we then forward-fold through the GBM response matrices and recover $(α, β, E_p)$ with Band fits. We find that magnetic-field decay can harden the recovered $α$ relative to the fast-cooling limit in part of parameter space, but the effect is not robust and is sensitive to the location of $E_p$ within the finite band and to spectral curvature; varying key physical scales reshapes the recovered $α$ distribution, indicating that catalog $α$ often represents an effective in-band slope rather than the asymptotic index. SSC cooling provides modest additional hardening and, in our setups, does not stabilize $α$ near the observed peak. Using Monte Carlo samples designed to mimic the observations, the model yields $α$ mostly between $-1.5$ and $-0.8$, but remains centered around $α\approx -1.5$. Overall, while decaying-field fast-cooling synchrotron can partially alleviate overly soft spectra expected from standard fast-cooling synchrotron emission, it still falls short of reproducing the GBM $α$ distribution at the population level, implying that additional physical processes are required.

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arXiv2026

A Universal 1.5 GeV Gamma-Ray Line in Active Galactic Nuclei

We report the detection of a gamma-ray spectral line at approximately 1.5 GeV in three active galactic nuclei (AGN) using 17 years of Fermi-LAT observations. The sample includes both blazars (with relativistic jets directed toward Earth) and a radio galaxy (with a misaligned jet, free from significant beaming effects). The line is detected with local significances of $\sim$4.1$σ$, $\sim$3.9$σ$, and $\sim$2.8$σ$ in the individual sources. A joint likelihood analysis yields a combined test statistic TS $\simeq$ 57.77, corresponding to a significance well above 5$σ$. The line flux remains stable over the full observation period, in contrast to the variable continuum emission from the AGN. The appearance of an identical spectral feature in astrophysically distinct environments is difficult to reconcile with standard jet-based emission mechanisms. While a conventional astrophysical explanation remains elusive, the signal's characteristics are consistent with predictions for dark matter annihilation. This finding motivates further investigation into the nature of this spectral feature and its possible connection to particle dark matter.

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arXiv2025

Newly Discovered Changing-look Active Galactic Nuclei from SDSS and LAMOST Surveys

Changing-look Active Galactic Nuclei (CL AGNs) exhibit drastic variations in broad emission lines (BELs), the mechanism of which remains unclear. Expanding the sample of CL AGNs is helpful to reveal the mechanism. This study aims to identify more CL AGNs by cross-matching spectroscopic data from the Sloan Digital Sky Survey (SDSS) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Our approach to identify CL AGNs is based on the automatic spectral fitting, followed by detailed visual inspections. We identify a sample of $51$ CL AGNs through this method, in which $40$ CL AGNs are newly discovered. Within this sample, $41$ CL AGNs primarily show the variability of the H$β$ line, nine exhibit obvious changes in both the H$β$ and H$α$ lines, and one source mainly displays variations in the H$α$ line. Our findings reveal that the sequence of appearance and disappearance of the BELs aligns with the known CL sequence. In addition, we identify 31 candidates exhibiting drastic variations in BELs without accompanying significant photometric variability. We estimate the black hole mass and Eddington ratio for all sources, which range from $2.5\times 10^6$ to $8.0\times 10^8 M_\odot$ and from $0.001$ to $0.13$, respectively. Similar to other studies, we also find that the Eddington ratios of CL AGNs and candidates are lower than those of typical AGNs. Our results support the hypothesis that the CL behavior is driven by the state transitions of the accretion disk.

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arXiv2024

Hunting for the candidates of Changing-Look Blazar using Mclust Clustering Analysis

The changing-look blazars (CLBs) are the blazars that their optical spectral lines at different epochs show a significant changes and present a clear transition between the standard FSRQ and BL Lac types. The changing-look phenomena in blazars are highly significant for enhancing our understanding of certain physical problems of active galactic nuclei (AGNs), such as the potential mechanism of the state transition in the accretion process of the supermassive black holes in the central engine of AGNs, the possible intrinsic variation of the jet, and the connection between the accretion disk and the jet. Currently, the CLBs reported in the literature are still rare astronomical objects. In our previous work, we found that there are 8 physical properties parameters of CLBs located between those of FSRQs and those of BL Lacs. In order to search more CLB candidates (CLBCs), we employed the $mclust$ Gaussian Mixture Modelling clustering algorithm to perform clustering analysis for the 255 subsets of the 8 physical properties parameters with 2250 blazars from the 4FGL-DR3. We find that there are 29 subsets with 3 groups (corresponding to bl lacs, fsrqs, and CLBCs), in which there are 4 subsets with the adjusted Rand index greater then 0.610 (ARI $>$ 0.610). The combined clustering results from 4 subsets report that there are 111 CLBCs that includes 44 CLBs reported in previous literature and 67 new CLBCs, where 11 CLBCs labeled as BL Lac and 56 CLBCs labeled as FSRQ in 4FGL catalog.

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arXiv2024

The Physical Properties of Changing-look Blazars

Changing-look active galactic nuclei (AGNs) are a special class of AGNs that change their spectral type from type 1 to type 2 or vice versa. In recent years, a number of changing-look blazars (CLBs) were also reported, which transition between flat-spectrum radio quasars and BL Lacs. The physical properties of CLBs are still unclear. Using the $mclust$ R package for Gaussian Mixture Modeling, we performed a clustering analysis for a sample of 105 CLBs selected from the literature. Three kinds of analysis found that CLBs lie in between the parameter distributions of FSRQs and BL Lacs: (i) univariate analysis; (ii) bivariate analysis; and (iii) multivariate analysis, carried out with a dimension reduction approach of the physical properties of the three types of blazars. Our results suggest that CLBs belong to a transition type between FSRQs and BL Lacs, which may be regulated by the change of accretion process and may be similar to other changing-look AGNs.

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arXiv2023

Hunting for the candidates of misclassified sources in LSP BL Lacs using Machine learning

An equivalent width (EW) based classification may cause the erroneous judgement to the flat spectrum radio quasars (FSRQs) and BL Lacerate objects (BL Lac) due to the diluting the line features by dramatic variations in the jet continuum flux. To help address the issue, the present paper explore the possible intrinsic classification on the bias of a random forest supervised machine learning algorithm. In order to do so, we compile a sample of 1680 Fermi blazars that have both gamma-rays and radio-frequencies data available from the 4LAC-DR2 catalog, which includes 1352 training and validation samples and 328 forecast samples. By studying the results for all of the different combinations of 23 characteristic parameters, we found that there are 178 optimal parameters combinations (OPCs) with the highest accuracy ($\simeq$ 98.89\%). Using the combined classification results from the nine combinations of these OPCs to the 328 forecast samples, we predict that there are 113 true BL Lacs (TBLs) and 157 false BL Lacs (FBLs) that are possible intrinsically FSRQs misclassified as BL Lacs. The FBLs show a clear separation from TBLs and FSRQs in the $γ$-ray photon spectral index, $Γ_{\rm ph}$, and X-band radio flux, ${\rm log}{F_{R}}$, plot. Phenomenally, existence a BL Lac to FSRQ (B-to-F) transition zone is suggested, where the FBLs are in the stage of transition from BL Lacs to FSRQs. Comparing the LSP Changing-Look Blazars (CLBs) reported in the literatures, the majority of LSP CLBs are located at the B-to-F zone. We argue that the FBLs located at B-to-F transition zone are the most likely Candidates of CLBs.

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arXiv2021

On the origin of GeV spectral break for Fermi blazars: 3C 454.3

The GeV break in spectra of the blazar 3C 454.3 is a special observation feature that has been discovered by the {\it Fermi}-LAT. The origin of the GeV break in the spectra is still under debate. In order to explore the possible source of GeV spectral break in 3C 454.3, a one-zone homogeneous leptonic jet model, as well as the {\it McFit} technique are utilized for fitting the quasi-simultaneous multi-waveband spectral energy distribution (SED) of 3C 454.3. The outside border of the broad-line region (BLR) and inner dust torus are chosen to contribute radiation in the model as external, seed photons to the external-Compton process, considering the observed $γ$-ray radiation. The combination of two components, namely the Compton-scattered BLR and dust torus radiation, assuming a broken power-law distribution of emitted particles, provides a proper fitting to the multi-waveband SED of 3C 454.3 detected 2008 Aug 3 - Sept 2 and explains the GeV spectral break. We propose that the spectral break of 3C 454.3 may originate from an inherent break in the energy distribution of the emitted particles and the Klein-Nishina effect. A comparison is performed between the energy density of the 'external' photon field for the whole BLR $U_{\rm BLR}$ achieved via model fitting and that constrained from the BLR data. The distance from the position of the $γ$-ray radiation area of 3C 454.3 to the central black hole could be constrained at $\sim 0.78$pc ($\sim 4.00 R_{\rm BLR}$, the size of the BLR).

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arXiv2020

Searching for AGN and Pulsar Candidates in 4FGL Unassociated Sources Using Machine Learning

In the fourth \emph{Fermi} Large Area Telescope source catalog (4FGL), 5064 $γ$-ray sources are reported, including 3207 active galactic nuclei (AGNs), 239 pulsars, 1336 unassociated sources, 92 sources with weak association with blazar at low Galactic latitude and 190 other sources. We employ two different supervised machine learning classifiers, combined with the direct observation parameters given by the 4FGL fits table, to search for sources potentially classified as AGNs and pulsars in the 1336 unassociated sources. In order to reduce the error caused by the large difference in the sizes of samples, we divide the classification process into two separate steps in order to identify the AGNs and the pulsars. First, we select the identified AGNs from all of the samples, and then select the identified pulsars from the remaining. Using the 4FGL sources associated or identified as AGNs, pulsars, and other sources with the features selected through the K-S test and the random forest (RF) feature importance measurement, we trained, optimized, and tested our classifier models. Then, the models are applied to classify the 1336 unassociated sources. According to the calculation results of the two classifiers, we show the sensitivity, specificity, accuracy in each step, and the class of unassociated sources given by each classifier. The accuracy obtained in the first step is approximately $95\%$; in the second step, the obtained overall accuracy is approximately $80\%$. Combining the results of the two classifiers, we predict that there are 583 AGN-type candidates, 115 pulsar-type candidates, 154 other types of $γ$-ray candidates, and 484 of uncertain types.

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arXiv2018

On the intrinsic shape of gamma-ray spectrum for Fermi blazars

The curvature of the $γ$-ray spectrum in blazars may reflect the intrinsic distribution of the emitting electron distribution, which will further give some information on the possible acceleration and cooling processes in the emitting region. The $γ$-ray spectra of Fermi blazars are normally fitted either by a single power-law (PL) or a log-normal (call Logarithmic Parabola, LP) form. The possible reason for this differnece is not unclear. We statistically explore this issue based on the different observational properties of 1419 Fermi blazars in the 3LAC Clean sample. We find that the $γ$-ray flux (100 MeV-100 GeV) and variability index follow bimodal distributions for PL and LP blazars, where $γ$-ray flux and variability index show {a positive correlation}. However, the distributions of the $γ$-ray luminosity and redshift follow a unimodal distribution. Our results suggest that the bimodal distribution of $γ$-ray flux for LP and PL blazars may be not intrinsic and all blazars may have an intrinsic curved $γ$-ray spectrum and the PL spectrum is just caused by the fitting effect due to the less photons.

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arXiv2016

On the origin of the soft photons of the high synchrotron peaked blazar : PKS 1424+240

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.

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