arXiv subjects

Bing Theodore Zhang

Publications and source records attributed to Bing Theodore Zhang.

8 recordsLinked to original sources

arXiv2026

Microquasar Remnants as Pevatrons Illuminating the Galactic Cosmic Ray Knee

Microquasars are primary candidates for Galactic PeVatrons, yet their collective contribution to the cosmic ray (CR) ``knee" remains poorly understood. We investigate this contribution by simulating anisotropic diffusive propagation through the Galactic magnetic field. Our results demonstrate that spatial alignment and magnetic connectivity between source locations and the solar neighborhood govern the local flux: sources aligned with local magnetic field lines yield pronounced flux enhancements, whereas magnetically disconnected locations are strongly suppressed. We find that the characteristic proton bump near PeV is robust across random realizations of the Galactic microquasar remnant population, with as few as four nearby remnants accounting for 50\% of the observed PeV flux. Our findings suggest that the integrated history of microquasar remnants, governed by source temporal and spatial distribution and magnetic transport, naturally populates the observed CR ``knee''.

astro-ph.HE↗

arXiv2026

Constraining Cosmic-Ray Acceleration and Escape in Middle-Aged Supernova Remnants with GeV-TeV Gamma-Ray Observations

In this work, we perform a systematic, time-dependent study of the gamma-ray emission from four representative middle-aged SNRs (W51C, IC~443, W44, W28), incorporating both CRs within the remnant shells and escaped CRs interacting with surrounding molecular clouds. We compare our results with GeV--TeV gamma-ray observations from Fermi-LAT, H.E.S.S., MAGIC, and LHAASO, including a dedicated analysis of the Fermi-LAT data for regions A and B associated with W28. We find that the observed spectra favor steeper CR injection spectra with indices of $α\sim4.2$--$4.3$, maximum proton energies of $\sim$ $100$--$300$ TeV, diffusion coefficients below the Galactic average, and CR acceleration efficiencies from a few to tens of percent. In particular, the VHE emission detected by LHAASO from W51C is more naturally explained by escaped CRs interacting with a nearby molecular cloud. We also investigate the contribution of escaped CRs to the VHE emission from IC~443, W44, and W28. We further demonstrate that escaped CRs can substantially enhance the TeV neutrino flux from middle-aged SNRs, improving their prospects as potential neutrino sources. These results provide new constraints on CR acceleration and escape in middle-aged SNRs and highlight the important role of escaped CRs in shaping their high-energy gamma-ray and neutrino emission.

astro-ph.HE↗

arXiv2026

Source models of ultrahigh-energy cosmic rays

We investigate potential sources of ultrahigh-energy cosmic rays (UHECRs) and their acceleration mechanisms, focusing on astrophysical phenomena associated with massive stellar deaths and supermassive black holes. These phenomena include gamma-ray bursts (GRBs), engine-driven supernovae/hypernovae, magnetars, newly born pulsars, binary neutron star mergers (BNS), tidal disruption events (TDEs), and active galactic nuclei (AGN). While high-luminosity GRBs (HL GRBs) are constrained as UHECR sources by high-energy neutrino observations, low-luminosity GRBs (LL GRBs) and engine-driven supernovae remain promising candidates, with intermediate-mass nuclei as the dominant components. Compact binary mergers and $r$-process nucleosynthesis in neutron-rich environments may also contribute to ultraheavy UHECRs. The composition of UHECRs from TDEs depends on the properties of the disrupted stars. AGN, particularly radio galaxies, remain promising sources, with acceleration occurring in their large-scale jets and lobes. Shear acceleration mechanisms have been proposed as a viable alternative for accelerating UHECRs, involving the re-acceleration of low-energy cosmic rays and being compatible with the observed spectrum and composition. Future multi-messenger observations, especially from upcoming observatories, are expected to provide critical data to refine our understanding of UHECR origins, test existing models, and explore new acceleration mechanisms.

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arXiv2026

Galactic Microquasar and Supernova Remnants Imprinting on Diffuse Neutrino and Gamma-Ray Sky

Recent detections of Galactic diffuse neutrinos by IceCube and $γ$-rays by LHAASO offer direct probes into the origin of Galactic cosmic rays. Conventional diffuse templates typically assume a single cosmic-ray injection spectrum across a wide energy range, without accounting for independent contributions from distinct accelerator populations. Here, we present a numerical framework that models Galactic diffuse neutrino and $γ$-ray emission that incorporates contributions from both microquasar and supernova remnant populations. By anchoring CR injection to local observations and utilizing high-resolution 3D target gas distributions, our model suggests multi-population contributions to the diffuse sky: escaped cosmic rays from supernova remnants dominate below $\sim10\text{ TeV}$, while those from microquasars become the primary driver at higher energies. Our predicted neutrino flux agrees well with the recent 12-year IceCube measurements, establishing a physically motivated baseline for the diffuse hadronic background while leaving room for unresolved point-like sources. This flexible framework provides testable predictions for current and future multi-messenger observatories, accommodating diverse accelerator populations and updated observational constraints.

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arXiv2026

A Minimal Interpretation of the Galactic Cosmic-Ray Proton and Helium Spectra from GeV to PeV Energies

High-precision measurements of the cosmic-ray (CR) proton and helium spectra have revealed significant deviations from a simple power law, characterized by multiple spectral features, including a hardening above $\sim$100~GeV, a broad excess in the multi-TeV range, and a pronounced structure at PeV energies. We propose a minimal phenomenological two-cosmic-ray-population framework that consistently accounts for the observed spectra of protons and helium across six decades in energy, with agreement at the level of $\sim 10 \%$ or better over most of the explored energy range. In this scenario, the spectral complexity arises from a transition between two Galactic CR populations in the 10~TeV-1~PeV energy range. The low-energy proton population exhibits a sharp cutoff at tens of TeV, while a second, higher-energy population emerges and dominates above 100~TeV, terminating with a smooth exponential cutoff at $\sim$6.5~PeV. The same two-component model applied to CR helium, with a slightly harder first component extending effectively to several hundred TeV and a second component that scales with the proton spectrum in magnetic rigidity, provides a consistent description of both the helium spectrum and the p/He ratio. This framework reproduces the main observed spectral features of CR protons and helium without invoking contributions from nearby sources or non-standard assumptions about CR acceleration or propagation. Recent gamma-ray observations of supernova remnants, star-forming regions, and microquasars offer plausible astrophysical sites for these two CR components.

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arXiv2026

Predicting Multiwavelength Emission Associated with X-Ray Flares and Extended Emission of Gamma-Ray Bursts

Gamma-ray bursts (GRBs) are one of the most extreme transients in the universe, but their explosion and emission mechanism remains unclear. To investigate the nature of GRB jets, here we focus on X-ray flares (XFs) and extended emissions (EEs), which are X-ray emissions that occur 100 to 1000 seconds after the main burst. They can be observed by recently developed multi-wavelength facilities. In this paper, we calculate emissions across multi-wavelengths associated with XFs and EEs under the hypothesis that XFs and EEs are optically-thin synchrotron emissions from nonthermal electrons in relativistic jets. Considering ranges of the dissipation radius $r_{\rm diss}$ and the Lorentz factor $Γ$ of the jet, we determine the parameter space in which a detectable emission can be produced at each wavelength. We found that simultaneous ultraviolet and very-high-energy gamma-ray emission associated with XFs or EEs can be detected by Swift/UVOT, SVOM/VT, and CTAO approximately every three years. The detection and non-detection rates for each detector are key to determining the uncertain yet essential values necessary for understanding the physics of GRB jets.

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arXiv2026

A Unified Framework for 10 TeV to EeV Diffuse Neutrino Sky and KM3-230213A

Establishing a unified framework that simultaneously accounts for the wideband diffuse neutrino flux and the physical origin of individual ultra-high-energy (UHE) neutrino detections, including KM3-230213A, remains a pressing challenge in multi-messenger astrophysics. In intrinsically low-luminosity gamma-ray bursts (LL~GRBs) driven by shock breakouts (SBOs), the evolving physical conditions naturally produce a multicomponent neutrino flux extending from 10 TeV to the EeV scale. By integrating prompt and afterglow phases within a unified framework grounded in multiwavelength observations of representative events, we show that LL GRB population accounts for this broadband neutrino emission through a characteristic two-hump spectrum. In this framework, the prompt emission from GRB~060218-like events accounts for $\gtrsim 10\%$ of the diffuse flux at 100~TeV, while GRB~100316D-like afterglow configuration predicts a distinct flux peak near $10^{-9}\rm~GeV~cm^{-2}~s^{-1}~sr^{-1}$ at 100~PeV. This two-hump spectrum provides a high-energy component flux consistent with the 220 PeV KM3-230213A event, while the low-energy component contributes non-trivially to the observed diffuse neutrinos and supports the lack of individual low-energy counterparts. Furthermore, we utilize Fermi-LAT gamma-ray upper limits to place constraints on the source distance and luminosity of the event, assuming a GRB 100316D-like afterglow configuration. Ultimately, this framework identifies SBO-like LL~GRBs as a unifying origin for these phenomena, providing a physical link across the 10 TeV to EeV neutrino sky that is testable by next-generation observatories, including GRAND, IceCube-Gen2, and RNO-G.

astro-ph.HE↗

arXiv2026

Multi-Messenger Modeling of Low-Luminosity Gamma-Ray Bursts

Low-luminosity gamma-ray bursts (LL GRBs), a subclass of the most powerful transients in the Universe, remain promising sources of high-energy astrophysical neutrinos, despite strong IceCube constraints on typical long GRBs. In this work, a novel approach is introduced to study a sample of seven LL~GRBs with their multi-wavelength observations to investigate leptohadronic processes during their prompt emission phases. The relative energy densities in magnetic fields, non-thermal electrons, and protons are constrained, with the latter defining the cosmic-ray (CR) loading factor. Our results suggest that LL~GRBs exhibit diverse emission processes, as confirmed by a machine-learning analysis of the fitted parameters. Across the seven LL~GRBs, we find the posterior medians of the CR loading factor in the range of $ξ_p \sim 0.2$--$1.6$. GRB~060218 and GRB~100316D, the lowest-luminosity bursts ($L_{γ, \rm iso} \sim 10^{46}$-$10^{47}\rm~erg~s^{-1}$) consistent with the shock-breakout (SBO) scenario, yield the highest CR loading factor and therefore are expected to produce neutrinos more efficiently. Our model predicts the expected number of neutrino signals that are consistent with current limits but would be detectable with next-generation neutrino observatories. These results strengthen the case for LL~GRBs as promising sources of high-energy astrophysical neutrinos and motivate real-time searches for coincident LL~GRB and neutrino events. Next-generation X-ray and MeV facilities will be critical for identifying more LL~GRBs and strengthening their role in multi-messenger astrophysics.

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