arXiv subjects

Francis Halzen

Publications and source records attributed to Francis Halzen.

At least 19 recordsLinked to original sources

arXiv2026

On the Blueprint of Active Galaxies Producing Neutrinos

Based on the observation of the active galaxies NGC 1068 and TXS 0506+056, and on additional evidence for the sources NGC 4151, CGCG 420-015, NGC 7469, and the Circinus Galaxy emerging from IceCube data, we make the case for the production of high-energy neutrinos within a few gravitational radii of supermassive black holes surrounded by a dense plasma radiating X-rays. X-rays represents the target for the production of neutrinos by protons accelerated near the black hole; they also absorb the gamma rays from the decay of neutral pions produced in the same interactions. Neutrinos with energies of tens of TeV and above originate in photoproduction interactions with X-rays of $0.1 \sim 1$\,keV energy on the $\Delta$ resonance, $p + \gamma \rightarrow \Delta \rightarrow n + \pi^+$. Our analysis of the multimessenger data points to gamma-ray-obscured sources with a characteristic neutrino flux linearly proportional to the X-ray flux originating within $\sim 10$ gravitational radii of the black holes, with lower values preferred. We speculate on such sources producing the diffuse flux of neutrinos and cosmic rays of extragalactic origin.

astro-ph.HE↗

arXiv2026

The Delta Resonance in the Neutrino Sky

Recent measurements of the diffuse cosmic neutrino flux by IceCube show evidence for a spectral break at an energy near $E_\nu \sim 30$ TeV. In this letter, we suggest that this feature may be due to the $\Delta$-baryon resonance in $p\gamma$ interactions. We show that the measured spectrum, including the observed break, can be naturally accommodated by a flux of protons accelerated with a spectrum $dN_p /dE_p \propto E_p^{-3.1}$ interacting with X-rays of typical energy $E_{\gamma} \sim 0.3\,{\rm keV}$. We also point out that the presence of this spectral break significantly reduces the contribution of neutrino sources to the isotropic gamma-ray background, alleviating the longstanding tension between these measurements. In the $\Delta$-resonance scenario, the gamma rays accompanying neutrino production cascade down to MeV-GeV energies and contribute at the $\sim 10\%$ level to the isotropic gamma-ray background at $\sim 3$~GeV. If our proposal is realized, it may imply that we have identified the dominant sources that produce the extragalactic cosmic rays.

astro-ph.HE↗

arXiv2026

Correlation Between Hard X-Ray and Cosmic Neutrino Sources: From Obscured AGN to Blazars

The origin of high-energy astrophysical neutrinos remains a key open question in multimessenger astrophysics. A correlation between unabsorbed hard X-ray and high-energy neutrino luminosity has been reported in six active galactic nuclei with the highest individual IceCube significances, linking neutrino production to compact, photon-rich environments near supermassive black holes. We study whether the threshold-near IceCube excesses associated with seven NuSTAR-observed blazars are statistically consistent with that relation. Calibrating the $L_\mathrm{hX}$-$L_\nu$ relation on the six published sources via a Bayesian regression with errors on both axes, the slope is consistent with $\beta = 1$ and the intrinsic scatter is $\sim 0.6$\,dex. All seven new blazars are posterior-predictively consistent with this calibration ($\chi^2_7 = 1.58$, $p = 0.98$) under the working hypothesis that the published IceCube $\hat{n}_s$ values reflect the signal. A null-injection test confirms that, at the present calibration sample size, the consistency test does not by itself adjudicate between signal and selected-background origins. A distance-free $L_\mathrm{hX}/L_\nu$ ratio diagnostic places both populations within the photohadronic prediction band, statistically indistinguishable. Two diagnostics that control the common $d_L^{\,2}$ distance bias, a redshift-partial rank correlation ($\tau|z = 0.69$, $\sim\!2.7\,\sigma$) and a flux-space permutation test on the 13-source joint sample ($p = 6.3\times10^{-4}$, $3.23\,\sigma$), indicate a residual $L_\mathrm{hX}$-$L_\nu$ association beyond the distance-induced trend. We interpret these results as a conditional consistency check; a detection-level statement requires either an enlarged calibration set or an X-ray-weighted IceCube stacking likelihood with internal data.

astro-ph.HE↗

arXiv2026

Upstream neutrino production and delayed jet emission in the blazar GB6 J1542+6129

We present a multimessenger case study of the blazar GB6 J1542+6129, examining whether its multiwavelength behavior is consistent with neutrino production in a compact region near the central black hole, or with the parsec-scale radio jet. We perform a multimessenger analysis combining ~17 years of Fermi-LAT gamma-ray data with ~14 years of VLBI/MOJAVE. These are compared to the temporal properties of a suspected IceCube neutrino flare with a duration of $147^{+110}_{-25}$ days, enabling a direct test of spatial and causal connections between neutrino and electromagnetic emission regions. We find that the suspected neutrino flare appears to precede both a $\gamma$-ray flare and a pronounced increase in the VLBI core Doppler factor by up to ~1 year. The duration of the post-flare $\gamma$-ray activity is comparable to that of the neutrino flare, which, in our framework, is consistent with both signatures originating from a single propagating disturbance whose temporal structure is preserved during the propagation. The gamma-ray spectral energy distribution remains consistent in shape across the full, flare, and post-flare intervals, indicating stable particle acceleration conditions. The temporal ordering, taken at face value, places the neutrino production site upstream of the VLBI core. The observations of GB6 J1542+6129 are consistent with a disturbance-driven, multi-zone scenario in which neutrinos are produced in a compact, photon-rich inner region upstream of the parsec-scale VLBI core, plausibly at the coronal region, while the same disturbance later enhances Doppler-boosted leptonic emission once it reaches the radio core. While the data alone do not strictly establish this scenario on a population level, they show how time-domain multimessenger observations of a single AGN can localize neutrino emission relative to the parsec-scale radio jet in a particular source.

astro-ph.HE↗

arXiv2026

Evaluating the Contribution of Active Galactic Nuclei to the Diffuse High-Energy Neutrino Flux

The detection of high-energy neutrinos from NGC 1068 and TXS-0506+56 suggests that active galactic nuclei (AGN) may contribute significantly to the the diffuse neutrino flux measured by IceCube. Using 10 years of publicly available IceCube data, we performed a systematic population analysis of X-ray-bright and gamma-ray-bright AGN to evaluate the extent to which this diffuse flux could originate from these sources. We find that gamma-ray-bright blazars can account for no more than 16\% of IceCube's total diffuse flux. Although we find no evidence of neutrino emission from gamma-ray-bright, non-blazar AGN, we cannot exclude the possibility that these sources contribute significantly to the diffuse flux. In contrast, we report (pre-trials) evidence of neutrino emission from several nearby, X-ray-bright, Seyfert-type AGN, including \mbox{NGC 1068} ($4.9\sigma$), SWIFT J1041.4-1740 ($2.6\sigma$), SWIFT J0202.4+6824A/B ($2.6\sigma$), SWIFT J0744.0+2914 (2.6$\sigma$), NGC 4151 ($2.5\sigma$), and NGC 3079 ($2.5\sigma$). Although not fully conclusive, these results suggest that IceCube may be detecting neutrinos from a larger population of Seyfert galaxies. The fact that these sources are not gamma-ray bright indicates that their neutrino production must be taking place in optically thick environments, such as in the coronae surrounding these galaxies' supermassive black holes. We also identify a $4.2\sigma$ correlation between the neutrinos detected by IceCube and members of the Swift-BAT catalog of X-ray-bright AGN, although this correlation is dominated by NGC 1068. We estimate that this class of sources contributes between 11.2\% and the entirety of IceCube's total diffuse neutrino flux. These results strengthen the emerging case for the prevalence of gamma-ray-obscured AGN as significant sources of high-energy neutrinos.

astro-ph.HE↗

arXiv2026

IceCube DeepCore's sensitivity to Non-Standard neutrino Interactions in the Earth

Neutrino oscillations continue to provide one of the most promising avenues for uncovering physics beyond the Standard Model. In particular, beyond-standard-model neutrino matter interactions may perturb neutrino oscillations in matter, leading to an observable signal in long baseline oscillation experiments. Moreover, such interactions can be a possible explanation of the rising tension between T2K and NOvA's $\delta_{\text{CP}}$ measurements. We examine IceCube DeepCore's sensitivity to these Non-Standard Interactions (NSI) by employing a model-independent NSI parameterization, and examine IceCube DeepCore's ability to comment on NSI being the cause of the T2K-NOvA $\delta_{\text{CP}}$ tension.

hep-ph↗

arXiv2026

The Cosmic-ray Knee as a Local Signature of Nearby PeVatrons

A "knee" in the cosmic-ray spectrum, characterized by a sudden steepening of the spectral shape at $\sim 4$ PeV, may be interpreted either as a global feature of Galactic cosmic rays or as a local signature. In the former scenario, cosmic-ray spectra throughout the Galaxy would be similar to that observed in the solar neighborhood, and the knee would be a common feature of the cosmic-ray sea. In the latter scenario, the PeV cosmic-ray flux varies across the Galactic disk, and the knee is dominantly contributed by a small number of nearby sources. By simulating cosmic-ray propagation in the Galactic magnetic field and interstellar medium, we show that the two scenarios correspond to different regimes of the birth rate of PeV proton accelerators and depend on the presence of powerful nearby sources. By comparison with both cosmic-ray and gamma-ray observations, we find that a local knee would be best explained by sources located at distances of order $\sim1$ kpc and with ages in the range 0.1-1 Myr, with the Cygnus Cocoon being a particularly promising candidate.

astro-ph.HE↗

arXiv2025

A Seyfert galaxy as a hidden counterpart to a neutrino-associated blazar

The origin and production mechanisms of high-energy astrophysical neutrinos remain open questions in multimessenger astronomy. Previous studies have hinted at a possible linear correlation between the hard X-ray and high-energy neutrino emission in active galactic nuclei. New \textit{NuSTAR} observations, first presented here, reveal that blazar PKS 1424+240, located within a prominent IceCube neutrino hotspot, is far fainter in hard X-rays than expected from this trend. Motivated by this apparent ambiguity, we identify the nearby Seyfert galaxy NGC 5610, also coincident with the hotspot, whose unabsorbed hard X-ray flux exceeds that of PKS 1424+240 by about an order of magnitude. When the local IceCube neutrino flux is apportioned between the two AGN in proportion to their hard X-ray emission, both align with the previously suggested X-ray-neutrino correlation. This suggests that certain IceCube hotspots may be unresolved blends of multiple AGN, and supports a multimessenger scenario in which high-energy neutrinos and hard X-rays originate from the same hadronic interactions, with the X-ray emission produced through cascade reprocessing.

astro-ph.HE↗

arXiv2025

Neutrinos from Primordial Black Holes in Theories with Extra Dimensions

The quantum gravity scale within the dark dimension scenario ($M_* \sim 10^{9}~{\rm GeV}$) roughly coincides with the energy scale of the KM3-230213A neutrino ($E_\nu \sim 10^{8}~{\rm GeV}$). We propose an interpretation for this intriguing coincidence in terms of Hawking evaporation of five-dimensional (5D) primordial black holes (PBHs). 5D PBHs are bigger, colder, and longer-lived than 4D PBHs of the same mass. For brane observers, PBHs residing in the higher-dimensional bulk decay essentially invisibly (only through gravitationally and sterile coupled modes). As a consequence, constraints on the density of PBHs relative to that of dark matter from null searches of Hawking evaporation can be avoided. We demonstrate that Hawking evaporation of 5D bulk PBHs can explain the KM3-230213A neutrino, evade constraints from upper limits on the gamma-ray flux, and remain consistent with IceCube upper limits on the partial decay width of superheavy dark matter particles into neutrinos.

hep-ph↗

arXiv2025

Cascaded Gamma-ray Emission Associated with the KM3NeT Ultra-High-Energy Event KM3-230213A

A neutrino-like event with an energy of $\sim 220 \,{\rm PeV}$ was recently detected by the KM3NeT/ARCA telescope. If this neutrino comes from an astrophysical source, or from the interaction of an ultra-high-energy cosmic ray in the intergalactic medium, the ultra-high-energy gamma rays that are co-produced with the neutrinos will scatter with the extragalactic background light, producing an electromagnetic cascade and resulting in emission at GeV-to-TeV energies. In this paper, we compute the gamma-ray flux from this neutrino source considering various source distances and strengths of the intergalactic magnetic field (IGMF). We find that the associated gamma-ray emission could be observed by existing imaging air cherenkov telescopes and air shower gamma-ray observatories, unless the strength of the IGMF is $B\gtrsim 3\times 10^{-13}$ G, or the ultra-high-energy gamma-rays are attenuated inside of the source itself. In the latter case, this source is expected to be radio-loud.

astro-ph.HE↗

arXiv2024

Transforming Antarctic Ice into a Cherenkov Neutrino Detector

In this chapter, we describe how the IceCube Neutrino Observatory transformed a cubic kilometer of natural ice at the geographic South Pole into a neutrino telescope. The concept of using the neutrino as an astronomical messenger is as old as the neutrino itself, and the challenge to open this new window on the high-energy universe was technological in nature. We discuss how IceCube was constructed and how the detector operates, including some detail on the 5,484 optical sensors that comprise the array. We highlight some of the primary results of the experiment, including the discovery of a diffuse flux of high-energy neutrinos reaching us from the cosmos, the observation of the first high-energy neutrino sources in the sky, and the observation of our Galaxy in neutrinos.

astro-ph.HE↗

arXiv2024

An NGC 1068-Informed Understanding of Neutrino Emission of the Active Galactic Nucleus TXS 0506+056

We present arguments that the neutrinos observed by IceCube from the active galactic nucleus TXS 0506+056 may originate near its core and not in the blazar jet. The origin of the neutrinos is consistent with the mechanism that produces the neutrino flux observed from the active galaxies NGC 1068 and NGC 4151, but requires an Eddington luminosity cosmic ray flux to compensate for its larger distance. Like NGC 1068, the source is characterized by episodes of high X-ray emission and is gamma-ray-obscured during the 2014 burst, and there is evidence that this is also the case during the short burst in 2017 that produced IC-170922. The observations may be explained as a flux originating in an obscured core within $10 \sim 100$ Schwarzschild radii from the central black hole, which is not transparent to gamma rays from neutral pions accompanying the neutrinos.

astro-ph.HE↗

arXiv2024

Astroparticles from X-ray Binary Coronae

The recent observation of high-energy neutrinos from the Galactic plane implies an abundant population of hadronic cosmic-ray sources in the Milky Way. We explore the role of the coronae of accreting stellar-mass black holes as such astroparticle emitters. We show that the particle acceleration and interaction timescales in the coronal region are tied to the compactness of the X-ray source. Thus, neutrino emission processes may similarly happen in the cores of active galactic nuclei and black hole X-ray binaries (XRB), despite of their drastically different masses and physical sizes. We apply the model to the well-measured XRB Cygnus X-1 and find that the cascaded gamma rays accompanying the neutrino emission naturally explain the GeV emission that only presents during the source's hard state, while the state-averaged gamma-ray emission explains the LHAASO observation above 20 TeV. We show that XRB coronae could contribute significantly to the Galactic cosmic-ray and Galactic plane neutrino fluxes. Our model predicts variable high-energy neutrino emission from bright Galactic XRBs that may be observed by IceCube and future neutrino observatories.

astro-ph.HE↗

arXiv2024

IceCube Results and Perspective for Neutrinos from LHAASO Sources

We briefly review the main results of the IceCube Neutrino Observatory one decade after the discovery of cosmic neutrinos. We emphasize the importance of multimessenger observations, most prominently for the discovery of neutrinos from our own Galaxy. We model the flux from the Galactic plane produced by Galactic cosmic rays interacting with the interstellar medium and discuss the perspectives of understanding the TeV-PeV emission of the Galactic plane by combining neutrino and gamma-ray observations. We draw attention to the interesting fact that the neutrino flux from the Galaxy is not a dominant feature of the neutrino sky, unlike the case in any other wavelength of light. Finally, we review the attempts to identify PeVatrons by confronting the neutrino and gamma-ray emission of Galactic sources, including those observed by LHAASO. We end with a discussion of searches for neutrinos from LHAASO's extragalactic transient source gamma-ray burst 221009A.

astro-ph.HE↗

arXiv2024

Possible correlation between unabsorbed hard X-rays and neutrinos in radio-loud and radio-quiet AGN

The first high-energy neutrino source identified by IceCube was a blazar -- an active galactic nucleus driving a relativistic jet towards Earth. Jets driven by accreting black holes are commonly assumed to be needed for high-energy neutrino production. Recently, IceCube discovered neutrinos from Seyfert galaxies, which appears unrelated to jet activity. Here, we show that the observed luminosity ratios of neutrinos and hard X-rays from blazars TXS 0506+056 and GB6 J1542+6129 are consistent with neutrino production in a $\gamma$-obscured region near a central supermassive black hole, with the X-ray flux corresponding to reprocessed $\gamma$-ray emission with flux comparable to that of neutrinos. Similar neutrino - hard X-ray flux ratios are found for four Seyfert galaxies, NGC 1068, NGC 4151, CGCG 420-015 and NGC 3079, raising the possibility of a common neutrino production mechanism that may not involve a strong jet.

astro-ph.HE↗

arXiv2024

Thomas K. Gaisser, a Pioneer of Particle Astrophysics

We describe the pioneering contributions of Thomas K. Gaisser to the birth and development of particle astrophysics, a new field of research at the intersection of cosmic ray physics, astronomy, astrophysics, and particle physics that has emerged in the last few decades. We will especially focus on his studies of natural beams of neutrinos: those generated by the interactions of cosmic rays in the Earth's atmosphere and those emitted by astrophysical sources. Tom actively participated in the discovery of these cosmic neutrinos as well. His contributions also extend to gamma-ray astronomy, the study of the cosmic ray spectra and composition, and the modeling of cosmic ray interactions in the atmosphere and in astrophysical environments. Tom invariably focused his research on the theoretical and phenomenological problems of greatest interest at the time, producing frameworks that transparently interpreted often complex data. These studies have been very influential and have shaped the development of the field.

hep-ex↗

arXiv2023

High-energy Neutrinos from the Inner Circumnuclear Region of NGC 1068

High-energy neutrinos are detected by the IceCube Observatory in the direction of NGC 1068, the archetypical type II Seyfert galaxy. The neutrino flux, surprisingly, is more than an order of magnitude higher than the $\gamma$-ray upper limits at measured TeV energy, posing tight constraints on the physical conditions of a neutrino production site. We report an analysis of the sub-millimeter, mid-infrared, and ultraviolet observations of the central $50$ pc of NGC 1068 and suggest that the inner dusty torus and the region where the jet interacts with the surrounding interstellar medium (ISM) may be a potential neutrino production site. Based on radiation and magnetic field properties derived from observations, we calculate the electromagnetic cascade of the $\gamma$-rays accompanying the neutrinos. When injecting protons with a hard spectrum, our model may explain the observed neutrino flux above $\sim 10$ TeV. It predicts a unique sub-TeV $\gamma$-ray component, which could be identified by a future observation. Jet-ISM interactions are commonly observed in the proximity of jets of both supermassive and stellar-mass black holes. Our results imply that such interaction regions could be $\gamma$-ray obscured neutrino production sites, which are needed to explain the IceCube diffuse neutrino flux.

astro-ph.HE↗

arXiv2023

Milky Way as a Neutrino Desert Revealed by IceCube Galactic Plane Observation

The Galactic diffuse emission (GDE) is formed when cosmic rays leave the sources where they were accelerated, diffusively propagate in the Galactic magnetic field, and interact with the interstellar medium and interstellar radiation field. GDE in $\gamma$-ray (GDE-$\gamma$) has been observed up to sub-PeV energies, though its origin may be explained by either cosmic-ray nuclei or electrons. We show that the $\gamma$-rays accompanying the high-energy neutrinos recently observed by the IceCube Observatory from the Galactic plane have a flux that is consistent with the GDE-$\gamma$ observed by the {\it Fermi}-LAT and Tibet AS$\gamma$ experiments around 1 TeV and 0.5 PeV, respectively. The consistency suggests that the diffuse $\gamma$-ray emission above $\sim$1TeV could be dominated by hadronuclear interactions, though partial leptonic contribution cannot be excluded. Moreover, by comparing the fluxes of the Galactic and extragalactic diffuse emission backgrounds, we find that the neutrino luminosity of the Milky Way is one to two orders of magnitude lower than the average of distant galaxies. This implies that our Galaxy has not hosted the type of neutrino emitters that dominates the isotropic neutrino background at least in the past few tens of kiloyears.

astro-ph.HE↗