arXiv subjects

K. Fang

Publications and source records attributed to K. Fang.

At least 19 recordsLinked to original sources

arXiv2026

IceCube neutrino point-source searches in the direction of the KM3NeT ultra-high-energy event

While still under construction, the KM3NeT Astroparticle Research with Cosmics in the Abyss (ARCA) detector recorded a $\sim$200 PeV neutrino on February 13th, 2023. This event is the highest-energy neutrino reported. IceCube, a cubic kilometer neutrino detector located at the geographic South Pole, has previously detected neutrinos up to approximately 10 PeV. We search for high-energy neutrinos from the location of the KM3NeT event using 15 years of IceCube data and considering three temporal hypotheses: steady or flaring in time coincidence, or at an arbitrary time. We find no evidence for neutrino emission for any of the studies performed. Correspondingly, we set upper limits on the neutrino flux from a point source in the direction of KM3-230213A. We compare these limits to KM3NeT's estimated flux and show that an astrophysical explanation of this event is strongly constrained for a variety of spectral assumptions for a steady or transient point source with the flux inferred from the single KM3NeT ultra-high-energy event assuming a spectral index of 2.0.

astro-ph.HE↗

arXiv2026

LHAASO-WCDA observed a $\sim$ 5 days TeV-delayed flaring event in blazar 1ES 1959+650

We report a day-scale hard lag between GeV and TeV $\gamma$-ray emission from the HBL 1ES~1959+650 in early 2024. Since the LHAASO-WCDA real-time monitoring system began operation in late 2023, multiple TeV flares from this source have been triggered, including the 1st trigger flare on 2024 February 9. A Bayesian-block analysis of the WCDA light curve identifies three TeV flares in 2024. For the second triggered flare, a discrete cross-correlation analysis reveals a $>3\,\sigma$ correlation (relative to uncorrelated red-noise simulations) at a time delay of $\Delta t = 5.0_{-2.1}^{+2.1}$ days, with the TeV emission lagging the GeV. Time-resolved spectroscopy shows that this flare has the softest TeV spectrum among these flares (intrinsic spectral index $\Gamma=3.16\pm0.18$), while the 1st trigger flare is harder ($\Gamma=2.48\pm0.21$). The observed five-day hard lag is difficult to reconcile with a purely cooling-driven temporal ordering and is consistent with scenarios in which particle energization and/or transport may contribute to the evolution. However, the current data do not uniquely identify the underlying mechanism.

astro-ph.HE↗

arXiv2026

Search for Neutrinos from Tidal Disruption Events with IceCube

Tidal disruption events (TDEs) are theorized to produce high-energy neutrinos through photohadronic interactions between accelerated protons and multi-wavelength photons in the accretion disk and outflows. Detecting these neutrinos would provide insight into the dynamics of TDEs. Taking advantage of the recent increase in observed TDEs from wide field-of-view telescopes, we conduct a dedicated search for neutrinos coincident in optical/UV and X-ray wavelengths. We searched for neutrino emission from 89 TDEs selected based on X-ray and optical/UV observations using time-dependent likelihood analysis methods in two parts. First, we searched for emission from individual sources, where we fit the time window of expected neutrino emission. Second, we performed a study of jetted and non-jetted TDE subpopulations using a stacking search with a fixed one year time window. No significant neutrino excess was observed in either search. We set upper limits to the contribution of jetted and non-jetted TDEs detected in optical/UV and X-ray wavelengths to the diffuse astrophysical neutrino flux assuming TDEs are standard candles.

astro-ph.HE↗

arXiv2026

Searching for Extra Dimensions and Copies of the Standard Model with IceCube

The hierarchy problem remains an open question in particle physics. A number of theories that address this problem lower the fundamental scale of gravity, resulting in observable consequences in the neutrino sector. In this work, we place constraints on low-scale gravity scenarios using high-energy neutrinos observed with the IceCube Neutrino Observatory. The analysis is based on 10.7 years of upward-going muon neutrino data in the energy range from 0.5 to 100 TeV. In this energy range, the theories predict characteristic spectral distortions arising from matter effects when neutrinos propagate through Earth. In the context of large extra dimension models, we constrain the compactification radius of the largest extra dimension to $R \lesssim 0.17\,\mu\mathrm{m}$ at $90\%$ confidence level for both normal and inverted neutrino mass ordering. For scenarios with multiple Standard Model copies, we obtain lower limits of up to $N \gtrsim \mathcal{O}(400)$, depending on the value of the lightest neutrino mass. In parts of the parameter space, these results constitute the strongest constraints in the literature to our knowledge, while in other regions they probe previously unexplored parameter space.

hep-ex↗

arXiv2026

Astrophysical Sensitivity Projections for the IceCube Upgrade

Embedded in the South Pole's glacial ice, IceCube detects neutrino-induced Cherenkov light using an array of digital optical modules equipped with single photomultiplier tubes (PMTs). The new extension installed in 2025/2026, the IceCube Upgrade, introduces densely instrumented multi-PMT optical modules within the existing infill array known as IceCube DeepCore. It is expected to enhance sensitivity in the GeV regime, with commissioning of the detector expected to be complete by the end of 2026. We present the projected sensitivities of the IceCube Upgrade for three key analyses: neutrino transient searches, steady emission from point sources such as NGC 1068, and diffuse emission from the Milky Way. These case studies represent direct extensions of current IceCube analyses. Using new Monte Carlo datasets, we demonstrate that the IceCube Upgrade achieves order-of-magnitude improvement in sensitivity at low energies ($\lesssim 10$ GeV) for time-dependent sources across short timescales. Conversely, for time-independent searches, the relative impact of the IceCube Upgrade's low-energy data is diluted by the decade-long accumulation of high-energy archival data. Nevertheless, we project significant improvements for soft-spectrum sources especially across the southern sky, driven by the IceCube Upgrade's superior background rejection capabilities. The improved sensitivity at low energies for both transient and steady sources will open up an expanded discovery window for IceCube in the GeV band over the next decade.

astro-ph.HE↗

arXiv2026

Anisotropic Particle Transport from a Pulsar Wind Nebula Revealed by Einstein Probe and LHAASO

Pulsar wind nebulae (PWNe) are major cosmic ray accelerators, yet the mechanisms transporting high-energy particles into the interstellar medium remain elusive. Building on the LHAASO discovery of an ultra-high-energy (UHE) $\gamma$-ray source near the bow-shock PWN powered by the pulsar PSR J1740+1000, we present a joint Einstein Probe (EP) and LHAASO study of this system. EP observations reveal an extended X-ray tail far exceeding the structure previously seen by XMM-Newton. Updated LHAASO observations show that the $\gamma$-ray emission is elongated, with its major axis aligned with the extended X-ray tail revealed by EP. This is the first detection of an X-ray pulsar tail associated with a spatially coincident extended UHE $\gamma$-ray emission. The X-ray and $\gamma$-ray spectrum can be well explained with a single population of relativistic electrons via synchrotron and inverse Compton radiation, respectively, removing the need for particle re-acceleration during propagation. The results unambiguously show that electrons/positrons above 100 TeV are escaping from the PWN. Instead of the immediate, isotropic diffusion into ambient interstellar medium that is typically assumed, these particles are transported anisotropically over at least $\sim$10 pc, either guided by the background magnetic field or carried by an advective outflow.

astro-ph.HE↗

arXiv2026

Estimating the sensitivity of the IceCube Upgrade to probe the interior of the Earth using atmospheric neutrino oscillations

The IceCube Upgrade is a densely instrumented central region of the IceCube Neutrino Observatory, deployed during the 2025-26 polar season. It will reduce the detector's energy threshold and improve overall reconstruction capabilities for multi-GeV atmospheric neutrinos, which in turn enhance their sensitivity to Earth matter effects as they traverse through the deep Earth. In this study, we describe the potential of the IceCube Upgrade to observe Earth matter effects on atmospheric neutrinos and estimate the detector's sensitivity to probe key features of the Preliminary Reference Earth Model by utilizing these observations. We highlight the IceCube Upgrade's capability to estimate the mass of the Earth and verify the non-homogeneous distribution of matter density within the Earth. We also estimate the IceCube Upgrade sensitivity to measure the correlated densities of the Earth layers while incorporating constraints from the mass and moment of inertia of the Earth. Neutrino-based results would be independent and complementary to the seismic and gravitational measurements.

hep-ex↗

arXiv2026

High-energy neutrino emission from the Milky Way

The Milky Way hosts astrophysical objects that accelerate cosmic rays to energies beyond the reach of terrestrial particle accelerators. It remains a longstanding goal to locate the sites of these powerful Galactic engines and understand how cosmic rays propagate through the Galaxy, leading to the production of high-energy neutrinos. In this paper, we combine event morphologies characteristic of all three neutrino flavours and apply recent improvements in ice modelling, calibration and reconstruction to 12 years of IceCube data. With a predefined, global analysis we establish high-energy neutrino emission from the Galactic plane at 5.7 $\sigma$ significance. A further study shows that the inner region of the Galaxy is a prominent neutrino source, with 217 shower events with visible energy above 5 TeV compared with an expected background of 154.4 $\pm$ 4.1. These results herald a new era of Galactic multi-messenger astronomy, creating new opportunities to study cosmic-ray propagation and probe neutrino properties over kiloparsec distances.

astro-ph.HE↗

arXiv2026

The Extended Ultrahigh-energy Gamma-Ray Emission in the Vicinity of PSR J2238+5903

We present a comprehensive analysis of the recently discovered TeV gamma-ray source, LHAASO J2238+5900. Based on data collected from the LHAASO, our fitting results suggest that the source is significantly extended with an angular extension of 0.54{\deg} \pm 0.01{\deg} and is spatially coincident with the pulsar PSR J2238+5903. Its spectrum is characterized by a power-law with a cutoff at 41.0\pm 3.5 TeV. Additionally, the source exhibits a significant signal of 7.9\sigma above 100 TeV, implying that it is a PeVatron candidate. While the gamma-ray emission is consistent with a pulsar wind nebula (PWN) scenario, the relatively large extension size also allows for a halo interpretation, potentially caused by electron-positron pairs escaping from the PWN.

astro-ph.HE↗

arXiv2026

WavePID: Low-energy flavor identification using single-PMT time series in IceCube

The IceCube Neutrino Observatory, a cubic-kilometer detector at the South Pole, identifies neutrino flavor through event morphology. Sparse photon detection makes this classification particularly challenging in the 5--100~GeV regime, the energy range relevant for oscillation measurements and searches for physics beyond the Standard Model. We introduce WavePID, a template-based log-likelihood-ratio classifier that exploits nanosecond-scale timing on individual detector modules through three observables: the distance to the reconstructed vertex, the early-charge fraction, and the module-to-module time difference. Evaluated on a cascade-enriched sample selected by a state-of-the-art graph neural network, WavePID improves both cascade purity and classification performance over the neural network alone. This demonstrates that per-module pulse timing carries flavor-identification information complementary to morphology-based classifiers, opening a new physics-motivated observable for low-energy neutrino reconstruction. Geant4 simulations associate this signal with differences in Cherenkov emission geometry between muon tracks and electromagnetic showers. These results motivate exploiting nanosecond-scale pulse timing in future low-energy classifiers and in detector designs with improved per-module timing in next-generation neutrino telescopes.

physics.ins-det↗

arXiv2026

High-Energy Neutrino Tomography of the Earth's Interior with IceCube

The Earth's interior reflects its geological evolution, from accretion to present-day dynamics. Its structure drives the geodynamo in the outer core, generating the magnetic field that shields the surface from charged cosmic radiation. The primary observables of the Earth's interior are its radial density distribution and derived quantities such as its mass and moment of inertia. These have traditionally been inferred from gravity and seismic wave propagation, which probe the macroscopic response of matter to gravitational and elastic forces. Here we instead constrain the Earth's density profile using high-energy neutrinos observed by the IceCube Neutrino Observatory at the South Pole. We analyze 10.7 years of predominantly muon-neutrino data spanning 500 GeV--100 TeV, including atmospheric neutrinos produced by cosmic-ray interactions in the Earth's atmosphere and the diffuse astrophysical neutrino flux. Neutrino attenuation depends on both the traversed column density and neutrino energy. By measuring the zenith- and energy-dependent flux suppression, we infer the Earth's radial density profile by fitting a concentric uniform-density shell model that incorporates neutrino fluxes, interaction cross sections, detector response, and glacial-ice systematic uncertainties. From the resulting density posteriors, we derive the Earth's mass and polar moment of inertia as measured by neutrinos. These are the most precise weak-interaction measurements of these quantities to date and are consistent with the Preliminary Reference Earth Model and independent gravitational determinations. Our results demonstrate that neutrinos provide a novel probe of planetary interiors via a distinct physical interaction, complementing gravity and seismology. With improved detectors and precision, neutrinos will further contribute to a multifaceted understanding of the Earth's structure.

astro-ph.HE↗

arXiv2026

Extreme PeV accelerator associated with GRS 1915+105

Microquasars, binary systems featuring relativistic jets, have emerged as sources for particle acceleration beyond PeV energies. We present a study of the broadband $\gamma$-ray emission from one of the most prominent Galactic microquasars GRS 1915+105 based on data accumulated by LHAASO and Fermi-LAT over 4 and 17 years, respectively. A joint analysis of LHAASO-WCDA and LHAASO-KM2A data reveals extended $\gamma$-ray emission whose centroid appears significantly shifted, by ~ 0.13{\deg}, from the binary system and its jets. The spectral energy distribution is well described by a curved spectrum with progressive steepening that can be described by a log-parabola function with no evidence for a sharp cutoff, consistent with parent particles reaching multi-PeV energies and an extreme acceleration efficiency approaching the limit set by the available potential drop across the source. Several features, most notably the shift of the emission and single-power-law spectrum down to GeV band, favor radiation by cosmic rays accelerated in the source interacting with the dense ambient medium. Our spectral modeling implies that at least a few percent of the jet mechanical power is transferred to protons, whose maximum energy reaches beyond 5 PeV. These results strengthen the case for microquasars as exceptionally efficient accelerators in our Galaxy.

astro-ph.HE↗

arXiv2026

IceCube Real-time Searches for High-energy Neutrinos Coincident with LIGO/Virgo/KAGRA Gravitational-Wave Alerts in O4a

Gravitational-wave events from mergers of compact objects are a predicted source of high-energy neutrinos. Using data from the IceCube Neutrino Observatory, we search for neutrinos coincident with 85 significant and 945 low-significance gravitational-wave candidate events from compact binary coalescences published in real-time by the LIGO-Virgo-KAGRA collaboration during the first part of its fourth observing run (O4a) and its preceding engineering run, within a time window of $\pm500$ seconds centered on the merger time. We report improvements to the online pipelines, including automatic sending of notices, which has decreased the IceCube real-time response time to gravitational-wave events. In addition, we search for long-duration neutrino emission (up to two weeks after the merger) from three candidate events: two neutron star-black hole mergers, and one low-significance gravitational-wave event with a possible subthreshold gamma-ray counterpart. We use two methods, both of which have been previously used to search for neutrino emission associated with gravitational-wave transients: an unbinned maximum likelihood analysis on significant alerts and a Bayesian analysis accounting for astrophysical priors on both significant and low-significance alerts. We find no statistically significant emission from any of the individual gravitational-wave events analyzed, and set upper limits on the time-integrated flux and energy emitted in high energy neutrinos assuming isotropic emission from each event.

astro-ph.HE↗

arXiv2026

IceCube Second Track Data Release IceTracks-DR2: Data from 2008-2022 for Neutrino Source Searches

We present IceCube's latest release of muon track data for neutrino point-source searches, extending the previously published 10-year dataset to cover 14 years of observations (April 6, 2008 - May 23, 2022). This release features an updated event selection and improved detector calibration for data recorded after June 1, 2010. The release also includes binned instrument response functions and effective areas, enabling the community to perform sensitive searches for steady and transient neutrino sources. We report on key science results obtained with this dataset using internal IceCube analysis tools and compare them to those derived from analyses based on the binned response functions included in this public release. To facilitate reproducible research, we provide benchmark results obtained using this data release and publicly available software. This release represents IceCube's most sensitive and comprehensive publicly available all-sky muon track dataset to date and should be preferred over previous releases.

astro-ph.HE↗

arXiv2026

Sensitivity Projections for Low-Mass Dark Matter Annihilation with the IceCube Upgrade

The IceCube Upgrade, an extension designed to enhance the IceCube Neutrino Observatory's detection of neutrinos with energies between 1 GeV and 500 GeV, will markedly improve IceCube's sensitivity to low-mass dark matter scenarios. In this study, we present sensitivity projections for the IceCube Upgrade to neutrino fluxes arising from dark matter annihilation. In particular, we consider dark matter with masses between 3 GeV to 500 GeV from both the core of the Sun and the Galactic Center. These projections indicate that the IceCube Upgrade will enable stringent limits on dark matter in this parameter space, achieving leading sensitivities to some dark matter models with only three years of data taking.

hep-ex↗

arXiv2026

Ultra-high-energy $\gamma$-ray imprints from PeV particles accelerated by supernova remnants

The quest for the origin of cosmic ray (CRs) is a fundamental issue in astrophysics. Shocks of supernova remnants (SNRs) have been considered as the dominant contributors to Galactic CRs below the spectral knee near $\sim 3$ petaelectronvolt (PeV). Whether SNRs are efficient accelerators of particles beyond PeV energies has long been debated. Here we report observations of very-high-energy $\gamma$-ray emission up to hundreds of TeV from two middle age shell-type SNRs, G150.3$+$4.5 and $\gamma$-Cygni, with the Large High Altitude Air Shower Observatory (LHAASO). Two (or three) distinct morphological/spectral components with convex spectral shapes are observed in both sources, with the low-energy one being more extended than the high-energy one. %Although it is possible that these high-energy components may be driven by powerful pulsars, The likely association of the high-energy component with molecular clouds at similar distances, and the weakness/absence of pulsar wind nebulae (PWNe) inside these SNRs clearly indicate for the first time that the highest energy emission is produced by collision of hadronic CRs up to PeV energies with the clouds. These results are compatible with the classic model prediction that PeV particles accelerated near the end of the free expansion phase of SNR evolution can illuminate nearby molecular clouds (MCs) to produce strong $\gamma$-ray emission.

astro-ph.HE↗

arXiv2026

Neural posterior estimation of the neutrino direction in IceCube using transformer-encoded normalizing flows on the sphere

IceCube is a cubic-kilometer-scale neutrino detector located at the geographic South Pole. A precise directional reconstruction of IceCube neutrinos is vital for associations with astronomical objects. In this context, we discuss neural posterior estimation of the neutrino direction via a transformer encoder that maps to a normalizing flow on the 2-sphere. It achieves a new state-of-the-art angular resolution for the two main event morphologies in IceCube - tracks and showers - while being significantly faster than traditional B-spline-based likelihood reconstructions. All-sky scans can be performed within seconds rather than hours, and take constant computation time, regardless of whether the posterior extent is arc-minutes or spans the whole sky. We utilize a combination of $C^2$-smooth rational-quadratic splines, scale transformations and rotations to define a novel spherical normalizing-flow distribution whose parameters are predicted as a whole as the output of the transformer encoder. We test several structural choices diverting from the vanilla transformer architecture. In particular, we find dual residual streams, nonlinear QKV projection and a separate class token with its own cross-attention processing to boost test-time performance. The angular resolution for both showers and tracks improves substantially over the whole trained energy range from 100 GeV to 100 PeV. At 100 TeV deposited energy, for example, the median angular resolution improves by a factor of $1.3$ for throughgoing tracks, by a factor of $1.7$ for showers and by a factor of $2.5$ for starting tracks compared to state-of-the art likelihood reconstructions based on B-splines. While previous machine-learning (ML) efforts have managed to obtain competitive shower resolutions, this is the first time an ML-based method outperforms likelihood-based muon reconstructions above 100 GeV.

hep-ex↗

arXiv2026

Search for TeV emission from spider millisecond pulsars with HAWC

Millisecond pulsars (MSPs) are observed to emit multi-wavelength radiation, from radio to GeV. Spider MSPs, which interact with their low-mass companion in close orbit (orbital periods $< 1$ day), may lead to strong intrabinary shocks that can further accelerate electron and positron pairs produced in the magnetosphere, possibly emitting very-high-energy (0.1--100 TeV; VHE) photons through inverse Compton scattering. Using 2565 days of HAWC Pass 5 data, we search for VHE emission from spider MSPs and present upper limits on individual sources. We also perform a stacking analysis to examine whether the two sets of spider systems, classified as redbacks and black widows depending on the companion mass, exhibit different spectral properties. Our study places constraints on TeV emission from MSPs and suggests that they are unlikely to contribute significantly to the Galactic diffuse emission at TeV and higher energies.

astro-ph.HE↗