arXiv subjects

Walter Winter

Publications and source records attributed to Walter Winter.

At least 37 records · Page 2Linked to original sources

arXiv2023

Electromagnetic Cascade Emission from Neutrino-Coincident Tidal Disruption Events

The potential association between Tidal Disruption Events (TDEs) and high-energy astrophysical neutrinos implies the acceleration of cosmic rays. These accelerated particles will initiate electromagnetic (EM) cascades spanning from keV to GeV energies by the processes related to neutrino production. We model the EM cascade and neutrino emissions by numerically solving the time-dependent transport equations and discuss the implications for AT2019dsg and AT2019fdr in the X-ray and $γ$-ray bands. We show that the $γ$-ray constraints from \emph{Fermi} can constrain the size of the radiation zone and the maximum energy of injected protons, and that the corresponding expected neutrino event numbers in follow-up searches are limited to be less than about 0.1. Depending on the efficiency of $pγ$ interactions, the X-ray and $γ$-ray signals can be expected closer to the peak of the optical-ultraviolet (OUV) luminosity, or to the time of the neutrino production.

astro-ph.HE↗

arXiv2023

Differences Between the Pierre Auger Observatory and Telescope Array Spectra: Systematic Effects or Indication of a Local Source of Ultra-High-Energy Cosmic Rays?

The Pierre Auger Observatory (PAO) and Telescope Array (TA) collaborations report significant differences in the observed energy spectra of ultra-high-energy cosmic rays (UHECRs) above 30~EeV. In this work we present a joint fit of TA and PAO data using the rigidity-dependent maximum energy model, including a full marginalization over all relevant parameters. We test two possible scenarios to explain these differences. One is that they are due to complex energy-dependent experimental systematics; the other is the presence of a local astrophysical source in the Northern Hemisphere, which is only visible by the TA experiment. We show that the astrophysical and systematic scenarios improve the explanation of the data equally well, compared to the scenario where both experiments observe the same UHECR flux from a cosmological source distribution and have energy-independent systematics. We test different mass compositions emitted from the local source and conclude that the data are best described by a source lying at a distance below 26~Mpc that emits cosmic rays dominated by the silicon mass group. We also discuss possible source candidates, and the possible role of the putative local UHECR source in the observed TA anisotropy and in the differences in TA spectral data from different declination bands.

astro-ph.HE↗

arXiv2023

Interpretation of the observed neutrino emission from three Tidal Disruption Events

Three Tidal Disruption Event (TDE) candidates (AT2019dsg, AT2019fdr, AT2019aalc) have been associated with high energy astrophysical neutrinos in multi-messenger follow-ups. In all cases, the neutrino observation occurred O(100) days after the maximum of the optical-ultraviolet (OUV) luminosity. We discuss unified fully time-dependent interpretations of the neutrino signals where the neutrino delays are not a statistical effect, but rather the consequence of a physical scale of the post-disruption system. Noting that X-rays flares and infrared (IR) dust echoes have been observed in all cases, we consider three models in which quasi-isotropic neutrino emission is due to the interactions of accelerated protons of moderate, medium, and ultra-high energies with X-rays, OUV, and IR photons, respectively. We find that the neutrino time delays can be well described in the X-ray model assuming magnetic confinement of protons in a calorimetric approach if the unobscured X-ray luminosity is roughly constant over time, and in the IR model, where the delay is directly correlated with the time evolution of the echo luminosity (for which a model is developed here). The OUV model exhibits the highest neutrino production efficiency. In all three models, the highest neutrino fluence is predicted for AT2019aalc, due to its high estimated supermassive black hole mass and low redshift. All models result in diffuse neutrino fluxes that are consistent with observations.

astro-ph.HE↗

arXiv2023

Multi-collision internal shock lepto-hadronic models for energetic GRBs

For a sub-population of energetic Gamma-Ray Bursts (GRBs), a moderate baryonic loading may suffice to power Ultra-High-Energy Cosmic Rays (UHECRs). Motivated by this, we study the radiative signatures of cosmic-ray protons in the prompt phase of energetic GRBs. Our framework is the internal shock model with multi-collision descriptions of the relativistic ejecta (with different emission regions along the jet), plus time-dependent calculations of photon and neutrino spectra. Our GRB prototypes are motivated by {\em Fermi}-LAT detected GRBs (including GRB~221009A) for which further, owing to the large energy flux, neutrino non-observation of single events may pose a strong limit on the baryonic loading. We study the feedback of protons on electromagnetic spectra in synchrotron- and inverse Compton-dominated scenarios to identify the multi-wavelength signatures, to constrain the maximally allowed baryonic loading, and to point out the differences between hadronic and inverse Compton signatures. We find that hadronic signatures appear as correlated flux increases in the optical-UV to soft X-ray and GeV to TeV gamma-ray ranges in the synchrotron scenarios, whereas they are difficult to identify in inverse Compton-dominated scenarios. We demonstrate that baryonic loadings around 10, which satisfy the UHECR energetic requirements, do not distort the predicted photon spectra in the {\em Fermi}-GBM range and are consistent with constraints from neutrino data if the collision radii are large enough (i.e., the time variability is not too short). It therefore seems plausible that under the condition of large dissipation radii a population of energetic GRBs can be the origin of the UHECRs.

astro-ph.HE↗

arXiv2023

Multi-messenger model for the prompt emission from GRB 221009A

We present a multi-messenger model for the prompt emission from GRB 221009A within the internal shock scenario. We consider the time-dependent evolution of the outflow with its impact on the observed light curve from multiple collisions, and the self-consistent generation of the electromagnetic spectrum in synchrotron and inverse Compton-dominated scenarios. Our leptohadronic model includes UHE protons potentially accelerated in the outflow, and their feedback on spectral energy distribution and on the neutrino emission. We find that we can roughly reproduce the observed light curves with an engine with varying ejection velocity of ultra-relativistic material, which has an intermediate quiescent period of about 200 seconds and a variability timescale of $\sim1$~s. We consider baryonic loadings of 3 and 30 that are compatible with the hypothesis that the highest-energetic LHAASO photons might come from UHECR interactions with the extragalactic background light, and the paradigm that energetic GRBs may power the UHECR flux. For these values and the high dissipation radii considered we find consistency with the non-observation of neutrinos and no significant signatures on the electromagnetic spectrum. Inverse Compton-dominated scenarios from the prompt emission are demonstrated to lead to about an order of magnitude higher fluxes in the HE-range; this enhancement is testable by its spectral impact in the Fermi-GBM and LAT ranges.

astro-ph.HE↗

arXiv2022

Candidate Tidal Disruption Event AT2019fdr Coincident with a High-Energy Neutrino

The origins of the high-energy cosmic neutrino flux remain largely unknown. Recently, one high-energy neutrino was associated with a tidal disruption event (TDE). Here we present AT2019fdr, an exceptionally luminous TDE candidate, coincident with another high-energy neutrino. Our observations, including a bright dust echo and soft late-time X-ray emission, further support a TDE origin of this flare. The probability of finding two such bright events by chance is just 0.034%. We evaluate several models for neutrino production and show that AT2019fdr is capable of producing the observed high-energy neutrino, reinforcing the case for TDEs as neutrino sources.

astro-ph.HE↗

arXiv2022

Multi-wavelength radiation models for low-luminosity GRBs, and the implications for UHECRs

We study the prompt phase of low-luminosity Gamma-Ray Bursts (ll-GRBs) as potential source of very-high-energy (VHE) gamma rays and Ultra-High-Energy Cosmic Rays (UHECRs). Within the internal shock model we choose parameters for the relativistic outflow such that our representative events have observed properties similar to GRBs 980425, 100316D and 120714B and self-consistently calculate the full spectral and temporal properties in a leptonic synchrotron self-Compton scenario. To investigate the conditions under which inverse Compton radiation may lead to a peak in the GeV--TeV range, we vary the fraction of internal energy supplying the magnetic field. Further, we determine the maximal energies achievable for UHECR nuclei and derive constraints on the baryonic loading/typical duration by comparing to the extragalactic gamma-ray background. We find that ll-GRBs are potential targets for multiwavelength studies and in reach for Imaging Atmospheric Cherenkov Telescopes (IACTs) and optical/UV instruments. For comparable sub-MeV emission and similar dynamical evolution of the outflow, weak (strong) magnetic fields induce high (low) fluxes in the VHE regime and low (high) fluxes in the optical. VHE emission may be suppressed by $γγ$-absorption close to the engine or interactions with the extragalactic background light for redshifts $z > 0.1$. For UHECRs, the maximal energies of iron nuclei (protons) can be as high as $\simeq 10^{11}$~GeV ($10^{10}$~GeV) if the magnetic energy density is large (and the VHE component is correspondingly weak). These high energies are possible by decoupling the production regions of UHECR and gamma-rays in our multizone model. Finally, we find basic consistency with the energy budget needed to accommodate the UHECR origin from ll-GRBs.

astro-ph.HE↗

arXiv2021

Extragalactic magnetic field constraints from ultra-high-energy cosmic rays from local galaxies

We interpret the correlation between local star-forming galaxy positions and ultra-high-energy cosmic ray (UHECR) directions, recently detected by the Pierre Auger Observatory (PAO), in terms of physical parameters: the local density of sources and the magnetic fields governing the UHECR propagation. We include a Galactic magnetic field model on top of a random extragalactic magnetic field description to determine the level of UHECR deflections expected from an ensemble of source positions. Besides deflections in magnetic fields, we also take into account energy losses with background photon fields as well as spectrum and composition measurements by the PAO. We find consistency between the PAO anisotropy measurement and the local star-forming galaxy density for large extragalactic magnetic field strengths with $B > 0.2 \ \rm nG$ (for a coherence length of $1 \ \rm Mpc$) at the $5σ$ confidence level. Larger source densities lead to more isotropic background and consequently allow for weaker extragalactic magnetic fields. However, the acceleration of UHECR by such abundant sources is more challenging to motivate. Too large source densities and extragalactic magnetic field strengths, on the other hand, are also disfavored as that decreases the expected level of anisotropy. This leads to upper limits of $B < 22 \ \rm nG$ and $ρ_0 < 8.4 \cdot 10^{-2} \ \rm Mpc^{-3}$ at the 90\% confidence level.

astro-ph.HE↗

arXiv2021

Status and Perspectives of Neutrino Physics

This review demonstrates the unique role of the neutrino by discussing in detail the physics of and with neutrinos. We deal with neutrino sources, neutrino oscillations, absolute masses, interactions, the possible existence of sterile neutrinos, and theoretical implications. In addition, synergies of neutrino physics with other research fields are found, and requirements to continue successful neutrino physics in the future, in terms of technological developments and adequate infrastructures, are stressed.

hep-ph↗

arXiv2021

A concordance scenario for the observed neutrino from a Tidal Disruption Event

During a tidal disruption event, a star is torn apart by the tidal forces of a supermassive black hole, with about 50% of the star's mass eventually accreted by the black hole. The resulting flare can, in extreme cases of super-Eddington mass accretion, result in a relativistic jet. While tidal disruption events have been theoretically proposed as sources of high-energy cosmic rays and neutrinos, stacking searches indicate that their contribution to the diffuse extragalactic neutrino flux is very low. However, a recent association of a track-like astrophysical neutrino with a tidal disruption event (AT2019dsg) indicates that some tidal disruption events can accelerate cosmic rays to PeV energies. Here we introduce a phenomenological concordance scenario with a relativistic jet to explain this association: an expanding cocoon progressively obscures the X-rays emitted by the accretion disk, while at the same time providing a sufficiently intense external target of back-scattered X-rays for the production of neutrinos via proton-photon interactions. We also reproduce the delay (relative to the peak) of the neutrino emission by scaling the production radius with the black body radius. Our energetics and assumptions for the jet and the cocoon are compatible with expectations from numerical simulations of tidal disruption events.

astro-ph.HE↗

arXiv2021

Unified thermal model for photohadronic neutrino production in astrophysical sources

High-energy astrophysical neutrino fluxes are, for many applications, modeled as simple power laws as a function of energy. While this is reasonable in the case of neutrino production in hadronuclear $pp$ sources, it typically does not capture the behavior in photohadronic $pγ$ sources: in that case, the neutrino spectrum depends on the properties of the target photons the cosmic rays collide with and on possible magnetic-field effects on the secondary pions and muons. We show that the neutrino production from known photohadronic sources can be reproduced by a thermal (black-body) target-photon spectrum if one suitably adjusts the temperature, thanks to multi-pion production processes. This allows discussing neutrino production from most known $pγ$ sources, such as gamma-ray bursts, active galactic nuclei and tidal disruption events, in terms of a few parameters. We apply this thermal model to study the sensitivity of different classes of neutrino telescopes to photohadronic sources: we classify the model parameter space according to which experiment is most suitable for detection of a specific source class and demonstrate that different experiment classes, such as dense arrays, conventional neutrino telescopes, or radio-detection experiments, cover different parts of the parameter space. Since the model can also reproduce the flavor and neutrino-antineutrino composition, we study the impact on the track-to-shower ratio and the Glashow resonance.

astro-ph.HE↗

arXiv2021

Theoretical interpretation of the observed neutrino emission from Tidal Disruption Events

The observation of a neutrino at IceCube in association with the Tidal Disruption Event (TDE) AT2019dsg has suggested TDEs as a new class of sources of astrophysical neutrinos. We present a model of this multi-messenger observation in a jetted concordance scenario, where the neutrino production is directly linked to the observed X-rays, and the timing of the neutrino observation (about 150 days post peak) can be naturally described. We briefly discuss the implications of our model for future neutrino-TDE associations.

astro-ph.HE↗

arXiv2021

The Blazar Hadronic Code Comparison Project

Blazar hadronic models have been developed in the past decades as an alternative to leptonic ones. In hadronic models the gamma-ray emission is associated with synchrotron emission by protons, and/or secondary leptons produced in proton-photon interactions. Together with photons, hadronic emission models predict the emission of neutrinos that are therefore the smoking gun for acceleration of relativistic hadrons in blazar jets. The simulation of proton-photon interactions and all associated radiative processes is a complex numerical task, and different approaches to the problem have been adopted in the literature. So far, no systematic comparison between the different codes has been performed, preventing a clear understanding of the underlying uncertainties in the numerical simulations. To fill this gap, we have undertaken the first comprehensive comparison of blazar hadronic codes, and the results from this effort will be presented in this contribution.

astro-ph.HE↗

arXiv2021

AGN jets as the origin of UHECRs and perspectives for the detection of astrophysical source neutrinos at EeV energies

We demonstrate that a population of Active Galactic Nuclei (AGN) can describe the observed spectrum of ultra-high-energy cosmic rays (UHECRs) at and above the ankle, and that the dominant contribution comes from low-luminosity BL Lacs. An additional, subdominant contribution from high-luminosity AGN is needed to improve the description of the composition observables, leading to a substantial neutrino flux that peaks at EeV energies. We also find that different properties for the low- and high-luminosity AGN populations are required; a possibly similar baryonic loading can already be excluded from current IceCube observations. We also show that the flux of neutrinos emitted from within the sources should outshine the cosmogenic neutrinos produced during the propagation of UHECRs. This result has profound implications for the ultra-high-energy ($\sim$EeV) neutrino experiments, since additional search strategies can be used for source neutrinos compared to cosmogenic neutrinos, such as stacking searches, flare analyses, and multi-messenger follow-ups.

astro-ph.HE↗

arXiv2020

Multi-wavelength and neutrino emission from blazar PKS 1502+106

In July of 2019, the IceCube experiment detected a high-energy neutrino from the direction of the powerful blazar PKS 1502+106. We perform multi-wavelength and multi-messenger modeling of this source, using a fully self-consistent one-zone model that includes the contribution of external radiation fields typical of flat-spectrum radio quasars (FSRQs). We identify three different activity states of the blazar: one quiescent state and two flaring states with hard and soft gamma-ray spectra. We find two hadronic models that can describe the multi-wavelength emission during all three states: a leptohadronic model with a contribution from photo-hadronic processes to X-rays and gamma rays, and a proton synchrotron model, where the emission from keV to 10 GeV comes from proton synchrotron radiation. Both models predict a substantial neutrino flux that is correlated with the gamma-ray and soft X-ray fluxes. Our results are compatible with the detection of a neutrino during the quiescent state, based on event rate statistics. We conclude that the soft X-ray spectra observed during bright flares strongly suggest a hadronic contribution, which can be interpreted as additional evidence for cosmic ray acceleration in the source independently of neutrino observations. We find that more arguments can be made in favor of the leptohadronic model vis-a-vis the proton synchrotron scenario, such as a lower energetic demand during the quiescent state. However, the same leptohadronic model would be disfavored for flaring states of PKS 1502+106 if no IceCube events were found from the direction of the source before 2010, which would require an archival search.

astro-ph.HE↗

arXiv2020

Systematic parameter space study for the UHECR origin from GRBs in models with multiple internal shocks

We scrutinize the paradigm that conventional long-duration Gamma-Ray Bursts (GRBs) are the dominant source of the ultra-high energy cosmic rays (UHECRs) within the internal shock scenario by describing UHECR spectrum and composition and by studying the predicted (source and cosmogenic) neutrino fluxes. Since it has been demonstrated that the stacking searches for astrophysical GRB neutrinos strongly constrain the parameter space in single-zone models, we focus on the dynamics of multiple collisions for which different messengers are expected to come from different regions of the same object. We propose a model which can describe both stochastic and deterministic engines, which we study in a systematic way. We find that GRBs can indeed describe the UHECRs for a wide range of different model assumptions with comparable quality albeit with the previously known problematic energy requirements; the heavy mass fraction at injection is found to be larger than 70% (95% CL). We demonstrate that the post-dicted (from UHECR data) neutrino fluxes from sources and UHECR propagation are indeed below the current sensitivities but will be reached by the next generation of experiments. We finally critically review the required source energetics with the specific examples found in this study.

astro-ph.HE↗

arXiv2020

Can astrophysical neutrinos trace the origin of the detected ultra-high energy cosmic rays?

Since astrophysical neutrinos are produced in the interactions of cosmic rays, identifying the origin of cosmic rays using directional correlations with neutrinos is one of the most interesting possibilities of the field. For that purpose, especially the Ultra-High Energy Cosmic Rays (UHECRs) are promising, as they are deflected less by extragalactic and Galactic magnetic fields than cosmic rays at lower energies. However, photo-hadronic interactions of the UHECRs limit their horizon, while neutrinos do not interact over cosmological distances. We study the possibility to search for anisotropies by investigating neutrino-UHECR correlations from the theoretical perspective, taking into account the UHECR horizon, magnetic-field deflections, and the cosmological source evolution. Under the assumption that the neutrinos and UHECRs all come from the same source class, we demonstrate that the non-observation of neutrino multiplets strongly constrains the possibility to find neutrino-UHECR correlations.

astro-ph.HE↗

arXiv2020

Impact of the collision model on the multi-messenger emission from Gamma-Ray Burst internal shocks

We discuss the production of multiple astrophysical messengers (neutrinos, cosmic rays, gamma-rays) in the Gamma-Ray Burst (GRB) internal shock scenario, focusing on the impact of the collision dynamics between two shells on the fireball evolution. In addition to the inelastic case, in which plasma shells merge when they collide, we study the Ultra Efficient Shock scenario, in which a fraction of the internal energy is re-converted into kinetic energy and, consequently, the two shells survive and remain in the system. We find that in all cases a quasi-diffuse neutrino flux from GRBs at the level of $10^{-11}$ to $10^{-10} \, \mathrm{GeV \, cm^{-2} \, s^{-1} \, sr^{-1}}$ (per flavor) is expected for protons and a baryonic loading of ten, which is potentially within the reach of IceCube-Gen2. The highest impact of the collision model for multi-messenger production is observed for the Ultra Efficient Shock scenario, that promises high conversion efficiencies from kinetic to radiated energy. However, the assumption that the plasma shells separate after a collision and survive as separate shells within the fireball is found to be justified too rarely in a multi-collision model that uses hydrodynamical simulations with the PLUTO code for individual shell collisions.

astro-ph.HE↗