Search

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 $\gamma \gamma$-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., Comment: Version accepted for publication in MNRAS

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 $5\sigma$ 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 $\rho_0 < 8.4 \cdot 10^{-2} \ \rm Mpc^{-3}$ at the 90\% confidence level., Comment: 10 pages, 6 figures, incl. appendices, accepted for publication in MNRAS

In this review paper, we present the main aspects of high-energy cosmic neutrino astrophysics. We begin by describing the generic expectations for cosmic neutrinos, including the effects of propagation from their sources to the detectors. Then we introduce the operating principles of current neutrino telescopes, and examine the main features (topologies) of the observable events. After a discussion of the main background processes, due to the concomitant presence of secondary particles produced in the terrestrial atmosphere by cosmic rays, we summarize the current status of the observations with astrophysical relevance that have been greatly contributed by IceCube detector. Then, we examine various interpretations of these findings, trying to assess the best candidate sources of cosmic neutrinos. We conclude with a brief perspective on how the field could evolve within a few years., Comment: 13 figures

In this paper, we present a model to predict the spread of the Covid-19 epidemic and apply it to the specific case of Italy. We started from a simple Susceptible, Infected, Recovered (SIR) model and we added the condition that, after a certain time, the basic reproduction number $R_0$ exponentially decays in time, as empirically suggested by world data. Using this model, we were able to reproduce the real behavior of the epidemic with an average error of 5\%. Moreover, we illustrate possible future scenarios, associated to different intervals of $R_0$. This model has been used since the beginning of March 2020, predicting the Italian peak of the epidemic in April 2020 with about 100.000 detected active cases. The real peak of the epidemic happened on the 20th of April 2020, with 108.000 active cases. This result shows that the model had predictive power for the italian case., Comment: The model presented in this paper has been adopted on Covstat.it. Errata corrige in the abstract

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., Comment: Manuscript accepted for publication in Phys. Rev. Lett

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., Comment: 11 pages, 7 figures, accepted for publication in MNRAS; Updated to match version to appear in MNRAS

We discuss possible interpretations of the neutrinos observed from the AGN blazar TXS 0506+056 in the multi-messenger and multi-wavelength context, including both the 2014-15 and 2017 neutrino flares. While the neutrino observed in September 2017 has to describe contemporary data in e.g. the X-ray and VHE gamma-ray ranges, data at the 2014-15 excess are much sparser. We demonstrate that in both cases the simplest possible one-zone AGN blazar models face challenges. While the 2017 flare can be well interpreted by considering more sophisticated source geometries, the 2014-15 flare is much harder to describe with conventional models. One challenge is the energy injected into the electromagnetic cascade coming together with the neutrino production, which cannot be reconciled with the 13 observed neutrino events. We also speculate if a common interpretation of both flares is feasible., Comment: Proceedings of the 36th International Cosmic Ray Conference (ICRC 2019), Madison, Wisconsin, USA

In this work we present an updated study of the flavor composition suggested by astrophysical neutrinos observed by IceCube. The main novelties compared to previous studies are the following: 1) we use the most recent measurements, namely 8 years of throughgoing muons and 7.5 years of High Energy Starting Events (HESE); 2) we consider a broken power law spectrum, in order to be consistent with the observations between 30 TeV and few PeV; 3) we use the throughgoing muon flux to predict the number of astrophysical HESE tracks. We show that accounting for the three previous elements, the result favors surprisingly the hypothesis of neutrinos produced by neutron decay, disfavoring the standard picture of neutrinos from pion decay at 2.0$\sigma$ and the damped muons regime at $2.6 \sigma$, once the atmospheric background is considered. Although the conventional scenario is not yet completely ruled out in the statistically and alternative interpretations are also plausible, such as an energy spectrum characterized by a non trivial shape, this intriguing result may suggest new directions for both theoretical interpretation and experimental search strategies., Comment: 8 pages, 5 figures. Published on EPJC

We investigate whether the emission of neutrinos observed in 2014-15 from the direction of the blazar TXS 0506+056 can be accommodated with leptohadronic multi-wavelength models of the source commonly adopted for the 2017 flare. While multi-wavelength data during the neutrino flare are sparse, the large number of neutrino events ($13\pm5$) challenges the missing activity in gamma rays. We illustrate that two to five neutrino events during the flare can be explained with leptohadronic models of different categories: a one-zone model, a compact core model, and an external radiation field model. If, however, significantly more events were to be accommodated, the predicted multi-wavelength emission levels would be in conflict with observational X-ray constraints, or with the high-energy gamma ray fluxes observed by the Fermi LAT, depending on the model. For example, while the external radiation field model can predict up to five neutrino events without violating X-ray constraints, the absorption of high-energy gamma rays is in minor tension with data. We therefore do not find any model that can simultaneously explain the high event number quoted by IceCube and the (sparse) electromagnetic data during the neutrino flare., Comment: Accepted for publication in ApJ Letters

In this work we use a multi-messenger approach to determine if the high energy diffuse neutrino flux observed by the IceCube Observatory can originate from $\gamma$-ray sources powered by Cosmic Rays interactions with gas. Typical representatives of such sources are Starburst and Ultra-Luminous Infrared Galaxies. Using the three most recent calculations of the non-blazar contribution to the extragalactic $\gamma$-ray background measured by the Fermi-LAT collaboration, we find that a hard power-law spectrum with spectral index $\alpha \leq 2.12$ is compatible with all the estimations for the allowed contribution from non-blazar sources, within 1$\sigma$. Using such a spectrum we are able to interpret the IceCube results, showing that various classes of hadronically powered $\gamma$-ray galaxies can provide the dominant contribution to the astrophysical signal above 100 TeV and about half of the contribution to the energy flux between 10-100 TeV. With the addition of neutrinos from the Galactic plane, it is possible to saturate the IceCube signal at high energy. Our result shows that these sources are still well motivated candidates., Comment: Accepted for publication on JCAP

This writeup is an introduction to neutrino astronomy, addressed to astronomers and written by astroparticle physicists. While the focus is on achievements and goals in neutrino astronomy, rather than on the aspects connected to particle physics, we will introduce the particle physics concepts needed to appreciate those aspects that depend on the peculiarity of the neutrinos. The detailed layout is as follows: In Sect.~1, we introduce the neutrinos, examine their interactions, and present neutrino detectors and telescopes. In Sect.~2, we discuss solar neutrinos, that have been detected and are matter of intense (theoretical and experimental) studies. In Sect.~3, we focus on supernova neutrinos, that inform us on a very dramatic astrophysical event and can tell us a lot on the phenomenon of gravitational collapse. In Sect.~4, we discuss the highest energy neutrinos, a very recent and lively research field. In Sect.~5, we review the phenomenon of neutrino oscillations and assess its relevance for neutrino astronomy. Finally, we offer a brief overall assessment and a summary in Sect.~6. The material is selected - i.e., not all achievements are reviewed - and furthermore it is kept to an introductory level, but efforts are made to highlight current research issues. In order to help the beginner, we prefer to limit the list of references, opting whenever possible for review works and books., Comment: 15 pages, 5 figures. Accepted for publication The European Physical Journal Plus. Based on the lecture given at the "4th Azarquiel School of Astronomy", June 2017, Porto Paolo di Capo Passero, Syracuse (Italy) https://agenda.infn.it/conferenceDisplay.py?confId=12080

We study if the diffuse astrophysical neutrino flux can come from blazar jets -- a subclass of Active Galactic Nuclei (AGNs) -- while it, at the same time, respects the blazar stacking limit based on source catalogs and is consistent with the observation from TXS 0506+056. We compute the neutrino flux from resolved and unresolved sources using an averaged, empirical relationship between electromagnetic spectrum and luminosity, known as the {\em blazar sequence}, for two populations of blazars (BL Lacs and FSRQs). Using a source model with realistic neutrino flux computations, we demonstrate that blazars can indeed power the diffuse neutrino flux at the highest energies and obey the stacking limit at the same time, and we derive the conditions for the baryonic loading (proton versus $\gamma$-ray luminosity) evolving over the blazar sequence. Under the hypothesis that low-luminosity blazars power the diffuse astrophysical neutrino flux, we find that the dominant contribution of the diffuse flux up to PeV energies must come from unresolved BL Lacs with baryonic loadings larger than about $10^5$ -- while only a very small contribution may come from resolved high-luminosity BL Lacs or FSRQs, which can be directly tested by the stacking limit. We find that the blazar TXS 0506+056 is on the verge of these populations in our baseline scenario, at a relatively high luminosity and redshift; as a consequence we predict about 0.3 $\gamma$-ray-neutrino associations per year from the whole population, dominated by BL Lacs with $L_\gamma \simeq 10^{45} \, \mathrm{erg/s}$ and $z \sim 0.1$., Comment: Accepted for publication in APJ

The evidence of a new population of diffuse high-energy neutrinos, obtained by IceCube, has opened a new era in the field of neutrino physics. Up to now the events detected are still without any source counterpart. The detected events are compatible with the standard picture of cosmic neutrinos undergoing 3-flavor neutrino oscillations. We analyze the implications of neutrino oscillations for the present and future experiments, focusing particularly on tau neutrinos. In fact tau neutrinos are very important: even if they are not produced in astrophysical sites, they have to exist due to oscillations and their observation should be regarded as a basic proof in support of this scenario. Moreover, IceCube's measurement of the flux of muon neutrinos implies that the flux of tau neutrinos is measured within $20\%$, just assuming standard neutrino oscillations. On this basis, after discussing the experimental signatures of tau neutrinos, we predict the rates for $\nu_\tau$ detection in the present and future neutrino telescopes. We show that the present IceCube detector is close to observe the first tau neutrinos, with a probability of about 90\%. Moreover the next generation of IceCube can identify about 2 neutrinos per year, reaching an evidence of 5$\sigma$ in about 10 years, despite the present uncertainty on the spectrum and on the production mechanism. The non observation of these neutrino events would have dramatic implications, such as the questioning of cosmic neutrino observations or the violation of neutrino oscillations over cosmological scales., Comment: 12 pages, 8 figures, 2 tables. Accepted for publication in JCAP

We propose a multi-component model for the observed diffuse neutrino flux, including the residual atmospheric backgrounds, a Galactic contribution (such as from cosmic ray interactions with gas), an extra-galactic contribution from pp interactions (such as from starburst galaxies) and a hard extragalatic contribution from photo-hadronic interactions at the highest energies (such as from Tidal Disruption Events or Active Galactic Nuclei). We demonstrate that this model can address the key problems of astrophysical neutrino data, such as the different observed spectral indices in the high-energy starting and through-going muon samples, a possible anisotropy due to Galactic events, the non-observation of point sources, and the constraint from the extragalatic diffuse gamma-ray background. Furthermore, the recently observed muon track with a deposited energy of 4.5 PeV might be interpreted as evidence for the extragalactic photo-hadronic contribution. We perform the analysis based on the observed events instead of the unfolded fluxes by computing the probability distributions for the event type and reconstructed neutrino energy. As a consequence, we give the probability to belong to each of these astrophysical components on an event-to-event basis., Comment: 20 pages, 12 figures. Accepted for publication in A&A

There is mounting evidence that the IceCube findings cannot be described simply invoking a single power-law spectrum for cosmic neutrinos. We discuss which are the minimal modifications of the spectrum that are required by the existing observations and we obtain a universal cosmic neutrino spectrum, i.e. valid for all neutrino flavors. Our approach to such task can be outlined in three points: 1) we rely on the throughgoing muon analysis above 200 TeV and on the high-energy starting events (HESE) analysis below this energy, requiring the continuity of the spectrum; 2) we assume that cosmic neutrinos are subject to three-flavor neutrino oscillations in vacuum; 3) we make no assumption on the astrophysical mechanism of production, except for no tau neutrino component at the source. We test our model using the information provided by HESE shower-like events and by the lack of double pulses and resonant events. We find that a two power-law spectrum is compatible with all observations. The model agrees with the standard picture of pion decay as a source of neutrinos, and indicates a slight preference for a pgamma mechanism of production. We discuss the tension between the HESE and the throughgoing muons datasets around few tens TeV, focussing on the angular distributions of the spectra. The expected number of smoking-gun signatures of nu-tau induced events (referred to as double pulses) is quantified: in the baseline model we predict 0.65 double pulse events in 5.7 years. Uncertainties in the predictions are quantified., Comment: 14 pages, 5 figures. Prepared for submission to EPJC

The IceCube neutrino telescope has observed for the first time a diffuse flux of high energy neutrinos, with a possible astrophysical origin. Up to now there are no evidence of sources and many hypothesis are still plausible in order to explain the measured flux. In this proceeding we analyze an alternative way to interpret the IceCube neutrinos, in terms of sum of contributions from different sources., Comment: Prepared for the proceeding of the conference IFAE 2017

Multi-messenger astronomy can help to investigate the sources of the high-energy neutrinos observed by the high-energy neutrino telescope IceCube. We consider the hypothesis that the highest energy neutrinos are produced by BL Lacs, arguing that this is not contradicted severely by any known fact. We check the BL Lac hypothesis by searching for correlations between the through-going muon events of IceCube and the BL Lacs of the second catalog of Fermi-LAT (2FHL). We expect 10.2 $\pm$ 2.4 correlated events but we find that just 1 event has a BL Lac as counterpart. We also assess the probability of observing one multiplet from the same source, finding that the present null result is not yet of critical significance. We conclude that the hypothesis that the BL Lacs are the main emitters of the highest-energy neutrinos observed by IceCube is disfavored at 3.7{\sigma}. We discuss implications and possible ways out; for example, this could work if the angular resolution was 4{\deg}, which is much more than expected., Comment: 10 pages, 6 figures. Accepted for publication in Astronomy & Astrophysics

We study the Glashow resonance $\bar{\nu}_e + e^- \rightarrow W^- \rightarrow$ hadrons at 6.3 PeV as diagnostic of the production processes of ultra-high energy neutrinos. The focus lies on describing the physics of neutrino production from pion decay as accurate as possible by including the kinematics of weak decays and Monte Carlo simulations of pp and p$\gamma$ interactions. We discuss optically thick (to photohadronic interactions) sources, sources of cosmic ray nuclei and muon damped sources. Even in the proposed upgrade IceCube-Gen2, a discrimination of scenarios such as pp versus p$\gamma$ is extremely challenging under realistic assumptions. Nonetheless, the Glashow resonance can serve as a smoking gun signature of neutrino production from photohadronic (A$\gamma$) interactions of heavier nuclei, as the expected Glashow event rate exceeds that of pp interactions. We finally quantify the exposures for which the non-observation of Glashow events exerts pressure on certain scenarios.

Recently it was noted that different IceCube datasets are not consistent with the same power law spectrum of the cosmic neutrinos: this is the IceCube spectral anomaly, that suggests that they observe a multicomponent spectrum. In this work, the main possibilities to enhance the description in terms of a single extragalactic neutrino component are examined. The hypothesis of a sizable contribution of Galactic high-energy neutrino events distributed as $E^{-2.7}$ [ApJ 826, 185 (2016)] is critically analyzed and its natural generalization is considered. The stability of the expectations is studied by introducing free parameters, motivated by theoretical considerations and observational facts. The upgraded model here examined has 1)~a Galactic component with different normalization and shape $E^{- 2.4}$; 2)~an extragalactic neutrino spectrum based on new data; 3)~a non-zero prompt component of atmospheric neutrinos. The two key predictions of the model concern the `high-energy starting events' collected from the Southern sky. The Galactic component produces a softer spectrum and a testable angular anisotropy. A second, radically different class of models, where the second component is instead isotropic, plausibly extragalactic and with a relatively soft spectrum, is disfavored instead by existing observations of muon neutrinos from the Northern sky and below few 100 TeV., Comment: 10 pages, 4 figures

The hypothesis of a PeVatron in the Galactic Center, emerged with the recent {\gamma}-ray measurements of H.E.S.S., motivates the search for neutrinos from this source. The effect of {\gamma}-ray absorption is studied: at the energies currently probed, the known background radiation fields lead to small effects, whereas it is not possible to exclude large effects due to new IR radiation fields near the very Center. Precise upper limits on neutrino fluxes are derived and the underlying hypotheses are discussed. The expected number of events for ANTARES, IceCube and KM3NeT, based on the H.E.S.S. measurements, are calculated. It is shown that km3-class telescopes in the Northern hemisphere have the potential of observing high-energy neutrinos from this important astronomical object and can check the existence of a hadronic PeV galactic accelerator., Comment: 11 pages, 4 figures, 2 tables. Accepted for publication in EPJC

The hypothesis that high energy cosmic neutrinos are power law distributed is critically analyzed. We propose a model with two components that explains better the observations. The extragalactic component of the high energy neutrino flux has a canonical $E_\nu^{-2}$ spectrum while the galactic component has a $E_\nu^{-2.7}$ spectrum; both of them are significant. This model has several implications, that can be tested by IceCube and ANTARES in the next years. Moreover, the existence of a diffuse component close to the Galactic plane and that yields (20-30)\% of IceCube's events, is interesting for the future $\mbox{km}^3$ neutrino telescopes located in the Northern Hemisphere and for gamma ray telescopes aiming at measuring events till few 100 TeV from the Southern sky., Comment: 22 pages, 8 figures. Existence of galactic component supported also by the interpretation of passing-muons and angular distribution. Submitted to APJ

IceCube collaboration has seen an unexpected population of high energy neutrinos compatible with an astrophysical origin. We consider two categories of events that can help to diagnose cosmic neutrinos: double pulse, that may allow us to clearly discriminate the cosmic component of tau neutrinos; cascades with deposited energy above 2 PeV, including events produced by electron antineutrinos at the Glashow resonance, that can be used to investigate the neutrino production mechanisms. We show that one half of the double pulse signal is due to the neutrinos spectral region already probed by IceCube. By normalizing to HESE data, we find that 10 more years are required to obtain 90% probability to observe a double pulse. The cascades above 2 PeV provide us a sensitive probe of the high energy tail of the neutrino spectrum and are potentially observable, but even in this case, the dependence on type of the source is mild. In fact we find that pp or p{\gamma} mechanisms give a difference in the number of cascades above 2 PeV of about 25 % that can be discriminated at 2{\sigma} in about 50 years of data taking., Comment: 20 pages, 7 figures, accepted for publication in EPJC

The natural parameterization of vacuum oscillations in three neutrino flavors is studied. Compact and exact relations of its three parameters with the ordinary three mixing angles and CP violating phase are obtained. Its usefulness is illustrated by considering various applications: the study of the flavor ratio and of its uncertainties, the comparison of expectations and observations in the flavor triangle, the intensity of the signal due to Glashow resonance. The results in the literature are easily reproduced and in particular the recently obtained agreement of the observations of IceCube with the hypothesis of cosmic neutrino oscillations is confirmed. It is argued that a Gaussian treatment of the errors appropriately describes the effects of the uncertainties on neutrino oscillation parameters., Comment: 19 pages, 5 figures. Text and bibliography enhanced, results unchanged. New analysis on flavor triangle included. Accepted for publication in EPJC