arXiv subjects

Ze-Rui Wang

Publications and source records attributed to Ze-Rui Wang.

At least 19 recordsLinked to original sources

arXiv2026

Locating the Production Sites of High-Energy Neutrinos in Blazar Jets

The production sites of high-energy neutrinos in blazar jets remain poorly constrained. In this work, we investigate the physical conditions required for efficient neutrino production by using radio-constrained jet properties to evaluate the radial evolution of the external-to-magnetic energy density ratio (Compton dominance $Y$). We identify $Y \gg 1$ as the key physical condition for efficient neutrino production, as it simultaneously enhances photohadronic interactions and suppresses synchrotron radiation from secondary pairs, thereby avoiding an excess of hard X-ray emission. We find that such large values of $Y$ are most readily achieved near or within the broad-line region. This large-$Y$ condition is generally incompatible with reproducing the observed broadband spectral energy distribution within a single emission region, naturally indicating that the neutrino-emitting region is physically distinct from the dominant electromagnetic emission zone. We further show that such a scenario can be realized either if the jet completes its acceleration within sub-parsec scales or if the bulk Lorentz factor is intrinsically large, both of which appear uncommon based on current observations. These results offer a physically motivated framework for identifying neutrino production sites, provide a natural explanation for the rarity of blazar--neutrino associations, and underscore the importance of constraining jet property at sub-parsec scales in the search for neutrino-emitting blazars.

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arXiv2026

On the maximum neutrino flux of blazars in the one-zone leptohadronic model

The origin of extragalactic high-energy neutrinos remains a major mystery in astrophysics, with blazars as leading candidate sources. The widely adopted one-zone leptohadronic jet model, however, faces severe challenges from stringent X-ray observational constraints. In this work, we present an analytical approach that derives the maximum neutrino flux as a function of the observed X-ray flux and the corresponding physical parameters attainable within the one-zone leptohadronic framework. Applying this approach to a sample of neutrino candidate blazars, we further perform numerical modeling and find agreement between analytical and numerical results. Both approaches consistently show that the model-predicted neutrino fluxes do not significantly exceed those obtained in previous one-zone studies and remain below the flux levels inferred from IceCube observations, suggesting that the one-zone scenario alone is unlikely to fully account for high-energy neutrino-blazar associations. This highlights the importance of considering multi-zone models or alternative production sites (e.g., jet base, hot corona) to better explain high-energy neutrino origins in blazars.

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arXiv2026

Quasi-Simultaneous Broadband Spectral Energy Distributions of a Sample of Fermi Blazars -- I. Correlation Results

Blazars' non-thermal emission shows rapid variability across all wavelengths, so spectral energy distributions (SEDs) built from quasi-simultaneous data are crucial for revealing the jets physical properties. In this work, we construct quasi-simultaneous broadband SEDs for 93 Fermi blazars (56 FSRQs, 35 BL Lacs, and 2 blazar candidates of uncertain type), fit both peaks with cubic functions to allow for potential asymmetries, and examine correlations among key parameters. Our main results are summarized as follows: (1) We find that synchrotron peak frequency and curvature are only weakly related, suggesting that charged particles are accelerated by mixed acceleration mechanism. (2) The blazar sequence is confirmed in the observer's frame through negative correlations of both the bolometic luminosity $\log L_{\rm bol}$ and the Compton dominance $\log Y$ with the synchrotron peak frequency $\log \nu_{\rm syn}^{\rm peak}$. After correcting for Doppler boosting, a weak positive correlation emerges between $\log L_{\rm bol}$ and $\log \nu_{\rm syn}^{\rm peak}$. FSRQs and BL Lacs exhibit distinct correlation patterns within the blazar sequence, indicating differences in cooling mechanisms. (3) Using variability time lags between 0.1-1 GeV and 1-300 GeV light curves, we estimate lower limits of Doppler factors for 4 sources, providing a jet-speed diagnostic anchored directly to the $\gamma$-ray emission zone.

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arXiv2026

Properties and Possible Physical Origins of $\gamma$-ray Emission in Extreme Synchrotron Blazars

Extreme synchrotron blazars, characterized by a first peak in their broadband spectral energy distributions (SEDs) at frequencies exceeding $10^{17}$ Hz, often exhibit a second peak beyond 1~TeV. These sources serve as ideal laboratories for studying particle acceleration and radiation mechanisms in relativistic jets. In this work, we systematically analyze the $\sim$16-year Fermi-LAT observational data for 25 extreme high-synchrotron-peaked BL Lacs (EHBLs). The results indicate that the majority of these sources display stable or low flux levels in the GeV band, with only 6 sources showing significant variability at a confidence level exceeding 5$\sigma$. The time-averaged spectra over the 16-year period for most EHBLs are well described by a hard power-law model, with photon indices predominantly clustered between 1.7 and 1.8. Using Fermi-LAT data in conjunction with multiwavelength observations compiled from the literature, we construct broadband SEDs for these EHBLs and fit them with a one-zone synchrotron + synchrotron-self-Compton (SSC) model. We find that this simplified theoretical framework is sufficient for modeling the observed SEDs of most of these EHBLs, albeit requiring relatively higher electron energies compared to other $\gamma$-ray emitting HBLs, and at times under-representing the UV emission. Based on the SED fitting results, we investigate the physical properties of the emission regions in these EHBLs and compare them with those of other $\gamma$-ray emitting HBLs. Consistent with other GeV--TeV $\gamma$-ray-emitting BL Lacs, the jets in these EHBLs are marked by low radiation efficiency and low magnetization.

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arXiv2025

Hadronic Processes in Advection-Dominated Accretion Flow as the Origin of TeV Excesses in BL Lac Objects

The spectral energy distributions (SEDs) of certain BL Lac objects (BL Lacs) exhibit an additional hard $\gamma$-ray component in the TeV energy range that surpasses the predictions of the one-zone leptonic jet model. The origin of this excess emission remains unclear. In this study, we selected five BL Lacs whose SEDs display a very hard intrinsic spectrum in the TeV band and successfully reproduced their broadband SEDs using a two-zone lepto-hadronic model. Within this framework, the emission observed in the optical, X-ray, GeV $\gamma$-ray, and sub-TeV $\gamma$-ray bands is modeled using the synchrotron and synchrotron self-Compton radiation processes of the relativistic electrons in the jets. Meanwhile, the TeV excess is attributed to $\gamma$-ray emission resulting from the photomeson ($p\gamma$) process via $\pi^0$ decay occurring within advection-dominated accretion flows (ADAFs). This scenario requires a hard proton spectrum with a spectral index of $p \sim 1.6-1.7$ and a cutoff energy ranging from 30 to 90 TeV, as well as a relatively large ADAF radius. Such hard proton spectra suggest that the dominant acceleration mechanisms are likely magnetic reconnection and/or stochastic acceleration processes within ADAFs. Additionally, the emission from the cascaded electrons results in a bump in the keV--MeV band; however, it is overwhelmed by the jet emission. Although the hadronuclear ($pp$) process cannot be entirely ruled out, it would necessitate an even harder proton spectrum and a higher cutoff energy compared to the $p\gamma$ process, making it a less favorable explanation for the observed TeV excess.

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arXiv2025

On the origin of a possible hard VHE spectrum from M87 discovered by LHAASO

Recent LHAASO observations hint at potential spectral hardening around 20 TeV in M87's very high energy (VHE) emission, suggesting a possible new radiation component. In this work, we construct averaged multiwavelength SEDs by combining data from Chandra and Swift-UVOT/XRT covering the same period as the LHAASO detection to investigate the origin of this feature. We test several radiation mechanisms, including the pp interaction, proton synchrotron emission, photomeson process and two-zone leptonic model. We find that only the pion decay gamma rays in pp interactions can interpret this feature in the framework of the one-zone model. With analytical analysis, we prove that proton synchrotron emission cannot generate a hard spectrum above 0.17~TeV. For photomeson model, it requires an emission zone compressed near the Schwarzschild radius of the central supermassive black hole, incompatible with broadband optical-GeV spectral constraints. In addition, the two-zone leptonic model also emerges as a viable alternative.

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arXiv2025

Time-Dependent Leptohadronic Modeling of Markarian 421

Due to its proximity, Markarian 421 is one of the most extensively studied jetted active galactic nuclei. Its spectral energy distribution and light curve are widely studied, serving as primary means for understanding jet radiation mechanisms. Numerous intriguing observational results have been discovered, some of which, such as the hard X-ray excess, and the associated variability between X-ray and very-high-energy (VHE) emissions, challenge the commonly adopted one-zone leptonic model. In this work, by establishing a time-dependent leptohadronic model, we explore whether the hard X-ray excess and the associated variability between X-ray and VHE emissions could be interpreted by emission from hadronic interactions. Our modeling finds that for the hard X-ray excess found in 2013, both of the secondary emissions from photohadronic and hadronuclear interactions could be a possible explanation for the hard X-ray excess without introducing a super-Eddington jet power. The emission from the photohadronic interactions contributes only to the hard X-ray band, while the hadronuclear interactions also predict VHE emissions associated with the hard X-rays. While for the hard X-ray excess found in 2016, only the secondary emissions from photohadronic interactions provide an interpretation at the cost of introducing a super-Eddington jet power. For the associated variability between X-ray and VHE emissions in 2017, we find that hadronic interactions fail to provide a possible interpretation.

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arXiv2024

Detection of an Orphan X-ray Flare from a Blazar Candidate EP240709a with Einstein Probe

Blazars are often observed to flare across multiple wavelengths. Orphan flares from blazars have been only detected a few times, providing an opportunity to understand the structure of the jet in the accreting system. We report a remarkable orphan X-ray flare from a blazar candidate EP240709a, detected by Einstein Probe (EP) in July 2024. The multi-band spectral properties and variability support EP240709a as a high-energy peaked BL Lacertae-type object. The flux in 0.5-10 keV increases by at least 28 times to the value of low state in 2020, with non-detection of remarkable flaring in other bands during the same period. EP240709a exhibits the harder-when-brighter tendency in the X-ray band during the orphan flare, while its infrared-optical spectra are featureless. We employ one-zone and two-zone leptonic synchrotron self-Compton models to perform the spectral energy distribution fitting. Detecting this rare orphan flare shows the potential of EP in discovering peculiar activities from AGN in high-cadence X-ray sky surveys.

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arXiv2024

Hadronuclear interactions in AGN jets as the origin of the diffuse high-energy neutrino background

The origin of diffuse high-energy neutrinos from TeV to PeV energies detected by IceCube Observatory remains a mystery. In our previous work, we have shown that hadronuclear (p-p) interactions in AGN jets could be important and generate detectable very-high-energy emissions. Here, we further explore these interactions in the AGN jets based on their luminosity function. The diffuse neutrino flux and corresponding $\gamma$-ray flux have been calculated and compared with observational data. In our modeling, two beaming patterns are considered separately. To make sure that the corresponding $\gamma$-ray flux does not overshoot the diffuse $\gamma$-ray background, we find that if the neutrino production region in jet is opaque to $\gamma$ rays, p-p interactions in AGN jets with a small viewing angle (the blazar case) are able to interpret the PeV neutrino background. Similarly, AGN jets with a large viewing angle (the radio galaxy case) may interpret the TeV neutrino background. While, if the neutrino production region is transparent to $\gamma$ rays, only blazars have the potential to interpret the DNB around PeV band. Some caveats are also discussed.

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arXiv2024

Constraining the Physical Parameters of Blazars Using the Seed Factor Approach

The discovery that blazars dominate the extra-galactic {\gamma}-ray sky is a triumph in the Fermi era. However, the exact location of {\gamma}-ray emission region still remains in debate. Low-synchrotron-peaked blazars (LSPs) are estimated to produce high-energy radiation through the external Compton process, thus their emission regions are closely related to the external photon fields. We employed the seed factor approach proposed by Georganopoulos et al. It directly matches the observed seed factor of each LSP with the characteristic seed factors of external photon fields to locate the {\gamma}-ray emission region. A sample of 1138 LSPs with peak frequencies and peak luminosities was adopted to plot a histogram distribution of observed seed factors. We also collected some spectral energy distributions (SEDs) of historical flare states to investigate the variation of {\gamma}-ray emission region. Those SEDs were fitted by both quadratic and cubic functions using the Markov-chain Monte Carlo method. Furthermore, we derived some physical parameters of blazars and compared them with the constraint of internal {\gamma}{\gamma}-absorption. We find that dusty torus dominates the soft photon fields of LSPs and most {\gamma}-ray emission regions of LSPs are located at 1-10 pc. The soft photon fields could also transition from dusty torus to broad line region and cosmic microwave background in different flare states. Our results suggest that the cubic function is better than the quadratic function to fit the SEDs.

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arXiv2024

Cascade Radiations of $e^\pm$ from $\gamma\gamma$-annihilation process as an extra component of the Early Optical/X-Ray Afterglows of Gamma-Ray Bursts

Chromatic break and/or plateau observed in the early optical and X-ray afterglow lightcurves challenge the conventional external shock models of gamma-ray bursts (GRBs). Detection of TeV gamma-ray afterglows indicates strong gamma-ray production within the afterglow jets. We investigate the cascade radiations of the $e^\pm$ production via the $\gamma\gamma$ interaction in the jets. Our numerical calculations show that the cascade synchrotron emission can make a significant contribution to the early optical/X-ray afterglows. The combination of the primary and cascade emission fluxes can shape a chromatic break and/or plateau in the early optical/X-ray lightcurves, depending on the jet properties. Applying our model to GRBs 050801 and 080310, we found that their optical plateaus and the late X-ray/optical lightcurves can be explained with our model in reasonable parameter values. We suggest that such a chromatic optical plateau could be a signature of strong $e^\pm$ production in GRB afterglow jets. The TeV gamma-ray flux of such kind GRBs should be significantly reduced, hence tends to be detectable for those GRBs that have a single power-law decaying optical afterglow lightcurve.

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arXiv2024

The physical properties of Fermi-4LAC low-synchrotron-peaked BL Lac objects

Previous studies on the fitting of spectral energy distributions (SEDs) often apply the external-Compton process to interpret the high-energy peak of low-synchrotron-peaked (LSP) BL Lac objects (LBLs), despite the lack of strong broad emission lines observed for LBLs. In this work, we collect quasi-simultaneous multi-wavelength data of 15 LBLs from the Fermi fourth LAT AGN catalog (4LAC). We propose an analytical method to assess the necessity of external photon fields in the framework of one-zone scenario. Following derived analytical results, we fit the SEDs of these LBLs with the conventional one-zone leptonic model and study their jet physical properties. Our main results can be summarized as follows. (1)We find that most LBLs cannot be fitted by the one-zone synchrotron self-Compton (SSC) model. This indicates that external photons play a crucial role in the high-energy emission of LBLs, therefore we suggest that LBLs are masquerading BL Lacs. (2) We suggest that the $\gamma$-ray emitting regions of LBLs are located outside the broad-line region and within the dusty torus. (3) By extending the analytical method to all types of LSPs in Fermi-4LAC (using historical data), we find that the high-energy peaks of some flat spectrum radio quasars and blazar candidates of unknown types can be attributed to the SSC emission, implying that the importance of external photons could be minor. We suggest that the variability timescale may help distinguish the origin of the high-energy peak.

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arXiv2024

Understanding the phenomenological and intrinsic blazar sequence using a simple scaling model

The blazar sequence, including negative correlations between radiative luminosity $L_{\rm rad}$ and synchrotron peak frequency $\nu$, and between Compton dominance $Y$ and $\nu$, is widely adopted as a phenomenological description of spectral energy distributions (SEDs) of blazars, although its underlying cause is hotly debated. In particular, these correlations turn positive after correcting Doppler boosting effect. In this work, we revisit the phenomenological and intrinsic blazar sequence with three samples, which are historical sample (SEDs are built with historical data), quasi-simultaneous sample (SEDs are built with quasi-simultaneous data) and Doppler factor corrected sample (a sample with available Doppler factors), selected from literature. We find that phenomenological blazar sequence holds in historical sample, but does not exist in quasi-simultaneous sample, and intrinsic correlation between $L_{\rm rad}$ and $\nu$ becomes positive in Doppler factor corrected sample. We also analyze if the blazar sequence still exists in subclasses of blazars, i.e., flat-spectrum radio quasars and BL Lacertae objects, with different values of $Y$. To interpret these correlations, we apply a simple scaling model, in which physical parameters of the dissipation region are connected to the location of the dissipation region. We find that the model generated results are highly sensitive to the chosen ranges and distributions of physical parameters. Therefore, we suggest that even though the simple scaling model can reproduce the blazar sequence under specific conditions that have been fine-tuned, such results may not have universal applicability. Further consideration of a more realistic emission model is expected.

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arXiv2024

A Unified Model for Multi-epoch Neutrino Events and Broadband Spectral Energy Distribution of $\rm TXS~0506+056$

The blazar $TXS~0506+056$ has been proposed as a high-energy neutrino emitter. However, it has been shown that the standard one-zone model cannot produce sufficiently high neutrino flux due to constraints from the X-ray data, implying more complex properties of the radiation zones in the blazar than that described by the standard one-zone model. In this work we investigate multi-epoch high-energy muon neutrino events associated with the blazar $TXS~0506+056$ occured in 2014-2015, 2017-2018, 2021-2022 and 2022-2023, respectively. We applied the so-called ``stochastic dissipation model'' to account for the neutrino-blazar associations detected in the four epochs simultaenously. This model describes a scenario in which the emission of the blazar arise from the superimposition of two components: a persistent component related to the quasi-stable state of the blazar and a transient component responsible for the sudden enhancement of the blazar's flux, either in electromagnetic radiation or in neutrino emission. The latter component could form at a random distance along the jet by a strong energy dissipation event. Under such assumption, the multi-epoch broadband spectral energy distribution (SED) can be well explained and the expected number of high-energy neutrino events is statistically realistic. The expected number of neutrino events in half-year is around 8.2, 0.07, 0.73 and 0.41, corresponding to the epoch in 2014-2015, 2017-2018, 2021-2022 and 2022-2023, respectively. Hence, our model self-consistently explains the episodic neutrino emission from $TXS~0506+056$.

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arXiv2023

A multi-zone view on the multi-wavelength emission of blazars

In this work, a time-dependent modeling is developed to study the emission properties of blazars in the low state. Motivated by various observations, we speculate and assume that numerous discrete radiation zones throughout the jet of a blazar contribute to the broadband emission. We model the temporal evolution of the electron spectrum in each emission zone taking into account the injection, cooling and escape of relativistic electrons. By doing so, we are able to calculate the multi-wavelength emission of each radiation zone. The observed emission of a blazar is then the superposition of the emission from all discrete radiation zones. We revisit the multi-wavelength spectral energy distributions, light curves and polarisation under the model, and discuss its potential to reproduce the flat radio spectra, the core-shift phenomena, the minute-scale gamma-ray variability, and the large polarisation-angle swings, which are difficult to explain under the conventional one-zone models simultaneously.

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arXiv2023

Broadband multi-wavelength study of LHAASO detected AGN

Recently, the Large High Altitude Air Shower Observatory (LHAASO) collaboration presented the first catalog of $\gamma$-ray sources using 508 days of LHAASO data, from March 2021 to September 2022. This catalog contains four blazars and a possible liner-type AGN counterpart. In this work, we establish averaged multi-wavelength SEDs by combining data from the \textsl{Fermi}-Large Area Telescope, \textsl{Swift}, ZTF, and WISE covering the same period as the LHAASO detection. In general, these five AGNs are found in low states at all wavelengths. To study the multi-wavelength properties of these AGNs, several jet emission models, including the one-zone leptonic model, the one-zone leptonic and hadronuclear ($pp$) model, the one-zone proton-synchrotron model, and the spine-layer model are applied to reproduce their averaged SEDs, respectively. We find that the one-zone leptonic model can reproduce most of the SEDs, except for the high-energy tail of the LHAASO spectra of Mrk 421 and Mrk 501. To improve the fitting, emission from $pp$ interactions is favoured in the framework of a one-zone model. The spine-layer model, which can be treated as a multi-zone scenario, can also provide good spectral fits. The influence of different extragalactic background light models on fitting LHAASO energy spectrum is also discussed.

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arXiv2023

Revisiting the proton synchrotron radiation in blazar jets: Possible contributions from X-ray to $\gamma$-ray bands

The proton synchrotron radiation is considered as the origin of high-energy emission of blazars at times. However, extreme physical parameters are often required. In this work, we propose an analytical method to study the parameter space when applying the proton synchrotron radiation to fit the keV, GeV, and very-high-energy emission of blazar jets. We find that proton synchrotron radiation can fit the high-energy hump when it peaks beyond tens GeV without violating basic observations and theories. For the high-energy hump peaked around GeV band, extreme parameters, such as a super-Eddington jet power and a very strong magnetic field, are required. For the high-energy hump peaked around keV band, if an acceptable parameter space can be found depends on the object's keV luminosity.

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arXiv2022

On the Hadronic Origin of High Energy Emission of $\gamma$-ray Loud Narrow-Line Seyfert 1 PKS 1502+036

The radiation mechanism of Radio-Loud Narrow-Line Seyfert 1 (RL-NLS1) from X-ray to $\gamma$-ray bands remains an open question. While the leptonic model has been employed to explain the spectral energy distribution (SED), the hadronic process may potentially account for the high energy radiation of some $\gamma$-ray loud Narrow-Line Seyfert 1 (NLS1) as well. We study one of such RL-NLS1, PKS 1502+036, comparing the theoretical SEDs predicted by the leptonic model and the lepto-hadronic model to the observed one. For the hadronic processes, we take into account the proton synchrotron radiation and proton-photon interactions (including the Bethe-Heitler process and the photopion process) including the emission of pairs generated in the electromagnetic cascade initiated by these processes. Our results show that the leptonic model can reproduce the SED of this source, in which the X-ray to $\gamma$-ray radiation can be interpreted as the inverse Compton (IC) scattering. On the other hand, the proton synchrotron radiation can also explain the high energy component of SED although extreme parameters are needed. We also demonstrate that the $p\gamma$ interactions as well as the cascade process cannot explain SED. Our results imply that a leptonic origin is favored for the multi-wavelength emission of PKS 1502+036.

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