Walter Winter

Radio flares have been postulated as being associated with the production of astrophysical neutrinos. For example, TXS 0506+056 exhibits a 2–3 yr delay between the 2017 IceCube-170922A/ γ -ray flare and a GHz radio maximum. We quantitatively test if the delayed radio flare originates from the same compact region where neutrinos and γ rays are produced as it expands downstream and synchrotron self-absorption is reduced. Starting from the 2017 flare blob parameters, we model the expanding production region and its evolving radio emission with LeHaMoC in a fully time-dependent framework, and compare our 1.2–22 GHz light curves to RATAN-600 data. We study different scenarios with increasing levels of sophistication, including continuous injection and energy redissipation on parsec scales. While a simple expanding-blob scenario fails to reproduce the radio data, a downstream dissipation episode of particles in the optically thin regime, followed by jet deceleration, successfully describes the radio evolution. The radio flare is powered by leptonic synchrotron emission and is largely insensitive to the proton population relevant for neutrino production, implying that the delayed radio flare mainly probes downstream dissipation and beaming in certain jet configurations, instead of being a direct radiative counterpart of the compact neutrino-production zone.

We study the origin of the ultra−high-energy (UHE) neutrino event KM3-230213A detected by KM3NeT, focusing on MRC 0614−083, which has been pinpointed as the closest blazar to the neutrino localization exhibiting variable multiwavelength emission. A joint interpretation of the optical, infrared, and X-ray light curves suggests that MRC 0614−083 has undergone a super-Eddington accretion flare accompanied by efficient proton acceleration. That flare has initiated a delayed infrared echo within the surrounding dust torus, which serves as a target for photomeson ( pγ ) interactions such that a self-consistent picture emerges that complements the blazar jet scenario: the predicted UHE neutrino flux is at the level expected from a joint E ⁻² fit with the IceCube measurements at lower energies, the variable nature of the event alleviates the tension with IceCube limits, and the accompanying electromagnetic cascade describes the X-ray flare around the neutrino detection time. Since a key remaining uncertainty is the unknown redshift of the source, we strongly encourage optical/ultraviolet spectroscopic measurements to determine its redshift.

We perform the first dedicated comparison of five hadronic codes (AM ³ , ATHE ν A, B13, LeHa-Paris, and LeHaMoC) that have been extensively used in modeling the spectral energy distribution (SED) of jetted active galactic nuclei. The purpose of this comparison is to identify the sources of systematic errors (e.g., implementation method of proton–photon interactions) and to quantify the expected dispersion in numerical SED models computed with the five codes. The outputs from the codes are first tested in synchrotron self-Compton scenarios that are the simplest blazar emission models used in the literature. We then compare the injection rates and spectra of secondary particles produced in pure hadronic cases with monoenergetic and power-law protons interacting on blackbody and power-law photon fields. We finally compare the photon SEDs and the neutrino spectra for realistic proton-synchrotron and leptohadronic blazar models. We find that the codes are in excellent agreement with respect to the spectral shape of the photons and neutrinos. There is a remaining spread in the overall normalization that we quantify, at its maximum, at the level of ±40%. This value should be used as an additional, conservative, systematic uncertainty term when comparing numerical simulations and observations.

High-energy gamma rays can trigger electromagnetic cascades via pair production on ambient photons, reprocessing their energy to lower frequencies. A classic example is the cascade from the gamma rays produced by ultra-high-energy cosmic rays in extragalactic photon fields, whose universal spectral shape was first described by Berezinsky in the 1970s. Recently, internal cascades, developing within the gamma-ray sources themselves, have gained a prominent role, as the IceCube data suggest that most detected neutrinos originate in gamma-ray-opaque environments. We analyze under what conditions these internal cascades can approach a universal spectrum. Since the Berezinsky treatment breaks down if synchrotron losses dominate, we present a generalized theory incorporating synchrotron-dominated cascades. We show the emergence of universal cascade spectrum among various examples of high-energy sources containing non-thermal cosmic rays, and discuss the conditions for its appearance.

We revisit ultrahigh-energy cosmic-ray (UHECR) production in tidal disruption events (TDEs) in light of recent evidence of neutrino-TDE associations. We use an isotropically emitting source-propagation model, which has been developed to describe the neutrino production in AT2019dsg, AT2019fdr, and AT2019aalc. These TDEs have strong dust echoes in the infrared (IR) range, which are potentially linked to the neutrino production. A mechanism where neutrinos originate from cosmic-ray (CR) scattering on IR photons implies CRs in the ultrahigh-energy range, thus suggesting a natural connection with the observed UHECR. We extrapolate the three TDE associations to a population of neutrino- and UHECR-emitting TDEs, and postulate that these TDEs power the UHECRs. We then infer the source composition, population parameters, and local rates that are needed to describe UHECR data. We find that UHECR data point toward a mix of light to mid-heavy injection isotopes, which could be found, e.g., in oxygen-neon-magnesium white dwarfs, and to a contribution of at least two groups of TDEs with different characteristics, dominated by AT2019aalc-type events. The required local TDE rates of O ( 1 0 2 ) Gpc − 3 yr − 1 , however, are more indicative of the disruption of main-sequence stars. We propose an enhanced efficiency in the acceleration of heavier nuclei that could address this discrepancy. The predicted diffuse neutrino fluxes suggest a population of astrophysical neutrino sources that can be observed by future radio neutrino detection experiments. The derived source parameters are consistent with those expected from the individual neutrino observations.

The blazar TXS 0506+056 has been the first astrophysical source associated with high-energy astrophysical neutrinos, and it has emerged as the second-most-prominent hotspot in the neutrino sky over 10 yr of observations. Although neutrino production in blazars has traditionally been attributed to processes in the powerful relativistic jet, the observation of a significant neutrino flux from NGC 1068—presumably coming from the active galactic nucleus (AGN) corona—suggests that neutrinos can also be produced in the cores of AGN. This raises the question whether neutrino production in TXS 0506+056 is also associated with the core region. We study this scenario, focusing on the hypothesis that this blazar is a masquerading BL Lac object, a high-excitation quasar with hidden broad emission lines and a standard accretion disk. We show that magnetic reconnection is an acceleration process necessary to reach tens of PeV proton energies, and we use observationally motivated estimates of the X-ray luminosity of the coronal region to predict the emission of secondaries and compare them to the observed multiwavelength and neutrino spectra of the source. We find that the coronal neutrino emission from TXS 0506+056 is too low to describe the IceCube observed neutrinos from this AGN, which in turn suggests that the blazar jet remains the preferred location for neutrino production.

We investigate the external reverse shock (RS) region of relativistic jets as the origin of X-ray afterglows of jetted tidal disruption events (TDEs) that exhibit luminous jets accompanied by fast-declining nonthermal X-ray emissions. We model the dynamics of jet propagating within an external density medium, accounting for continuous energy injection driven by accretion activities. We compute the time-dependent synchrotron and inverse Compton emissions from the RS region. Our analysis demonstrates that the RS scenario can potentially explain the X-ray light curves and spectra of four jetted TDEs, namely, AT 2022cmc, Swift J1644, Swift J2058, and Swift J1112. Notably, the rapid steepening of the late-stage X-ray light curves can be attributed jointly to the jet break and cessation of the central engine as the accretion rate drops below the Eddington limit. Using parameters obtained from X-ray data fitting, we also discuss the prospects for γ -ray and neutrino detection.

Context. The origin of the astrophysical neutrino flux discovered by IceCube remains largely unknown. Several individual neutrino source candidates were observed. Among them is the gamma-ray flaring blazar TXS 0506+056. A similar coincidence of a high-energy neutrino and a gamma-ray flare was found in blazar PKS 0735+178. Aims. By modeling the spectral energy distributions of PKS 0735+178, we aimed to explore the physical conditions that lead to neutrino production at different stages of the source's activity. Methods. We analyzed the multiwavelength data from the selected time periods. Using numerical simulations of radiation processes in the source, we explored the parameter space of one-zone leptonic and leptohadronic models to find the best-fit solutions that explain the observed photon fluxes. Results. We demonstrated how model parameter degeneracy affected the prediction of neutrino spectra. We showed that the available multiwavelength data were insufficient to predict the neutrino spectrum unambiguously. However, under the condition of neutrino rates maximization, we proposed a scenario in which 0.1 neutrino events were produced during the 50-day flare.

Recent multiwavelength observations of gamma-ray burst afterglows observed in the TeV energy range challenge the simplest synchrotron self-Compton (SSC) interpretation of this emission and are consistent with a single power-law component spanning over 8 orders of magnitude in energy. To interpret this generic behavior in the single-zone approximation without adding further free parameters, we perform an exhaustive parameter space study using the public, time-dependent, multimessenger transport software AM ³ . This description accounts for the radiation from nonthermal protons and the leptohadronic cascade induced by pp and p γ interactions. We summarise the main scenarios that we have found (SSC, Extended-syn, Proton-syn, pp-cascade, and pγ-cascade) and discuss their advantages and limitations. We find that possible high-density environments, as may be typical for surrounding molecular cloud material, offer an alternative explanation for producing flat hard (source) spectra up to and beyond energies of 10 TeV.