IceCube Signal Research Articles

Upper limit on the coronal cosmic ray energy budget in Seyfert galaxies

Abstract The IceCube Collaboration has reported possible detections of high-energy neutrinos from nearby Seyfert galaxies. While central hot coronae are proposed as the primary neutrino production site, the exact coronal cosmic ray energy budget has been loosely constrained. In this study, we propose a new stringent upper bound on the coronal cosmic ray energy budget of Seyfert galaxies, considering both accretion dynamics and observed properties of radio-quiet Seyfert galaxies. Notably, even under the calorimetric condition where cosmic rays lose all their energy, our limit indicates that the coronal neutrino flux of NGC 1068 is about an order of magnitude fainter than the observed levels. This discrepancy suggests the need for further theoretical and observational investigations on the IceCube signals from Seyfert galaxies.

Correlating High-energy IceCube Neutrinos with 5BZCAT Blazars and RFC Sources

We investigate the possibility that blazars in the Roma-BZCAT Multifrequency Catalogue of Blazars (5BZCAT) are sources of the high-energy astrophysical neutrinos detected by the IceCube Neutrino Observatory, as recently suggested by Buson et al. Although we can reproduce their ∼4.5σ result, which applies to 7 yr of neutrino data in the southern sky, we find no significant correlation with 5BZCAT sources when extending the search to the northern sky, where IceCube is most sensitive to astrophysical signals. To further test this scenario, we use a larger sample consisting of 10 yr of neutrino data recently released by the IceCube Collaboration, this time finding no significant correlation in neither the southern nor the northern sky. These results suggest that the strong correlation reported by Buson et al. using 5BZCAT could be due to a statistical fluctuation and possibly the spatial and flux nonuniformities in the blazar sample. We perform some additional correlation tests using the more uniform, flux-limited, and blazar-dominated Radio Fundamental Catalogue and find a ∼3.2σ equivalent p-value when correlating it with the 7 yr southern neutrino sky. However, this correlation disappears completely when extending the analysis to the northern sky and when analyzing 10 yr of all-sky neutrino data. Our findings support a scenario where the contribution of the whole blazar class to the IceCube signal is relevant but not dominant, in agreement with most previous studies.

PeV IceCube signals and H_0 tension in the framework of Non-Local Gravity

We study possible effects of non-local gravity corrections on the recent discovery by the IceCube collaboration, reporting high-energy neutrino flux detected at energies of order PeV. Considering the 4-dimensional operator sim y_{alpha chi }overline{{L_{{alpha }}}}, H, chi, it is possible to explain both the IceCube neutrino rate and the abundance of Dark Matter, provided that non-local corrections are present in the cosmological background. Furthermore, the mechanism could constitute a natural way to address the H_0 tension issue.

STUDY OF BLAZAR ACTIVITY IN 10 YEAR FERMI-LAT DATA AND IMPLICATIONS FOR TEV NEUTRINO EXPECTATIONS

Blazars are the most active extragalactic gamma-ray sources. They show sporadic bursts of activity, lasting from hours to months. In this work we present a 10-year analysis of a sample of bright sources detected by Fermi-LAT (100 MeV - 300 GeV). Using 2-week binned light curves (LC) we estimate the duty cycle (DC): fraction of time that the source spends in an active state. The objects show different DC values, with an average of 22.74% and 23.08% when considering (or not) the extragalactic background light ( EBL). Additionally, we study the so-called “blazar sequence” trend for the sample of selected blazars in the ten years of data. This analysis constrains a possible counterpart of sub-PeV neutrino emission during the quiescent states, leaving open the possibility to explain the observed IceCube signal during the flaring states.

IceCube neutrinos from hadronically powered gamma-ray galaxies

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 γ-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 γ-ray background measured by the Fermi-LAT collaboration, we find that a hard power-law spectrum with spectral index α ⩽ 2.12 is compatible with all the estimations for the allowed contribution from non-blazar sources, within 1σ. Using such a spectrum we are able to interpret the IceCube results, showing that various classes of hadronically powered γ-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.

On the relevance of prompt neutrinos for the interpretation of the IceCube signals

The IceCube collaboration has discovered a new, cosmic component of high-energy neutrinos. Although neutrino oscillations suggest that the cosmic neutrino spectrum is almost the same for every neutrino flavor, the attempts to reconstruct it, based on different analyses, lead to different energy spectra below 100 TeV. In this work, we propose a phenomenological model that, assuming collisions between cosmic rays and hadrons as the production mechanism of high-energy neutrinos, yields quantitative expectations for each neutrino flavor. We discuss the detectability of the prompt component of the atmospheric neutrino spectrum, pointing out the most relevant dataset, which has to be muon-neutrino depleted and to cover the energy region 10–100 TeV. We argue that the prompt component can cause the spectral difference between the High-Energy-Starting-Event (HESE) and the through-going muon datasets. Finally, we point out the need for adopting a consistent model for the interpretation of the data, stressing that a separate treatment of the different datasets is, by converse, a suboptimal procedure.

PeV IceCube signals and Dark Matter relic abundance in modified cosmologies

The discovery by the IceCube experiment of a high-energy astrophysical neutrino flux with energies of the order of PeV, has opened new scenarios in astroparticles physics. A possibility to explain this phenomenon is to consider the minimal models of Dark Matter (DM) decay, the 4-dimensional operator sim y_{alpha chi }overline{{L_{L_{alpha }}}}, H, chi , which is also able to generate the correct abundance of DM in the Universe. Assuming that the cosmological background evolves according to the standard cosmological model, it follows that the rate of DM decay Gamma _chi sim |y_{alpha chi }|^2 needed to get the correct DM relic abundance (Gamma _chi sim 10^{-58}) differs by many orders of magnitude with respect that one needed to explain the IceCube data (Gamma _chi sim 10^{-25}), making the four-dimensional operator unsuitable. In this paper we show that assuming that the early Universe evolution is governed by a modified cosmology, the discrepancy between the two the DM decay rates can be reconciled, and both the IceCube neutrino rate and relic density can be explained in a minimal model.

TANAMI: Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry

Context. TANAMI is a multiwavelength program monitoring active galactic nuclei (AGN) south of − 30° declination including high-resolution very long baseline interferometry (VLBI) imaging, radio, optical/UV, X-ray, and γ-ray studies. We have previously published first-epoch8.4 GHz VLBI images of the parsec-scale structure of the initial sample. In this paper, we present images of 39 additional sources. The full sample comprises most of the radio- and γ-ray brightest AGN in the southern quarter of the sky, overlapping with the region from which high-energy (> 100 TeV) neutrino events have been found. Aims. We characterize the parsec-scale radio properties of the jets and compare them with the quasi-simultaneous Fermi/LAT γ-ray data. Furthermore, we study the jet properties of sources which are in positional coincidence with high-energy neutrino events compared to the full sample. We test the positional agreement of high-energy neutrino events with various AGN samples. Methods. TANAMI VLBI observations at 8.4 GHz are made with southern hemisphere radio telescopes located in Australia, Antarctica, Chile, New Zealand, and South Africa. Results. Our observations yield the first images of many jets below − 30° declination at milliarcsecond resolution. We find that γ-ray loud TANAMI sources tend to be more compact on parsec-scales and have higher core brightness temperatures than γ-ray faint jets, indicating higher Doppler factors. No significant structural difference is found between sources in positional coincidence with high-energy neutrino events and other TANAMI jets. The 22 γ-ray brightest AGN in the TANAMI sky show only a weak positional agreement with high-energy neutrinos demonstrating that the > 100 TeV IceCube signal is not simply dominated by a small number of the γ-ray brightest blazars. Instead, a larger number of sources have to contribute to the signal with each individual source having only a small Poisson probability for producing an event in multi-year integrations of current neutrino detectors.

Diffuse neutrino emissions from the Southern sky and Mediterranean neutrino telescopes

Astrophysical high-energy neutrinos offer an extremely interesting window of observation on our Universe. Cosmic neutrinos are probes for extreme events happening nearby the most powerful astrophysical objects. Direct information on the behaviour of cosmic ray sources is provided by neutrinos: since they are weakly interacting neutral particles, barely changing their information load over cosmic distance, the detection of neutrinos from the interaction of primary cosmic rays close to their acceleration site could allow the identification of their sources and of their production and acceleration mechanisms. Compelling evidence for the existence of an astrophysical flux of neutrinos above some tens of TeV has been reported by the IceCube Collaboration. Some features of the energy and declination distributions of the IceCube signal hint at a North/South asymmetry of the measured neutrino flux, which could be related to the presence of the bulk of our Galaxy in the Southern hemisphere. The ANTARES neutrino telescope, operating in the Mediterranean Sea since 2007, offers the best sensitivity to muon neutrinos below 100TeV in this part of the sky. This allows the detector to focus on the neutrino flux produced by galactic cosmic ray interactions in the bulk of the Milky Way. Studies on possible neutrino signals using ANTARES data collected are reported in this paper, as well as the prospects for the next-generation neutrino telescope, KM3NeT/ARCA, to be built in the Mediterranean Sea. In particular, ANTARES can already test the propagation mechanisms of cosmic rays in the Milky Way by constraining the contribution from the Galactic Plane to the total neutrino flux observed by IceCube. The KM3NeT/ARCA detector will then allow the detailed study of galactic neutrino fluxes.

High energy neutrinos from the tidal disruption of stars

We study the production of high energy neutrinos in jets from the tidal disruption of stars by supermassive black holes. The diffuse neutrino flux expected from these tidal disruption events (TDEs) is calculated both analytically and numerically, taking account the dependence of the rate of TDEs on the redshift and black hole mass. We find that ~10% of the observed diffuse flux at IceCube at an energy of about 1 PeV can come from TDEs if the characteristics of known jetted tidal disruption events are assumed to apply to the whole population of these sources. If, however, plausible scalings of the jet Lorentz factor or variability timescale with the black hole mass are taken into account, the contribution of the lowest mass black holes to the neutrino flux is enhanced. In this case, TDEs can account for most of the neutrino flux detected at IceCube, describing both the neutrino flux normalization and spectral shape with moderate baryonic loadings. While the uncertainties on our assumptions are large, a possible signature of TDEs as the origin of the IceCube signal is the transition of the flux flavor composition from a pion beam to a muon damped source at the highest energies, which will also result in a suppression of Glashow resonance events.

A consistent model for leptogenesis, dark matter and the IceCube signal

We discuss a left-right symmetric extension of the Standard Model in which the three additional right-handed neutrinos play a central role in explaining the baryon asymmetry of the Universe, the dark matter abundance and the ultra energetic signal detected by the IceCube experiment. The energy spectrum and neutrino flux measured by IceCube are ascribed to the decays of the lightest right-handed neutrino $N_1$, thus fixing its mass and lifetime, while the production of $N_1$ in the primordial thermal bath occurs via a freeze-in mechanism driven by the additional $SU(2)_R$ interactions. The constraints imposed by IceCube and the dark matter abundance allow nonetheless the heavier right-handed neutrinos to realize a standard type-I seesaw leptogenesis, with the $B-L$ asymmetry dominantly produced by the next-to-lightest neutrino $N_2$. Further consequences and predictions of the model are that: the $N_1$ production implies a specific power-law relation between the reheating temperature of the Universe and the vacuum expectation value of the $SU(2)_R$ triplet; leptogenesis imposes a lower bound on the reheating temperature of the Universe at $7\times10^9\,\mbox{GeV}$. Additionally, the model requires a vanishing absolute neutrino mass scale $m_1\simeq0$.

Perspectives of the KM3NeT project

KM3NeT is a large distributed research infrastructure that comprises a network of deep-sea neutrino telescopes in the Mediterranean Sea with user ports for Earth and Sea sciences. The main objectives of KM3NeT are the discovery and subsequent observation of high-energy neutrino sources in the Universe (ARCA) and the determination of the mass hierarchy of neutrinos (ORCA). Technically, the network of telescopes will consist of building blocks of 115 vertical detection units anchored at the seabed and connected to shore via a deep sea electro-optical cable. Each detection unit carries 18 optical modules equipped with 31 3” photomultipliers. Two configurations for the building blocks are defined to optimally detect neutrinos in different ranges of energy. The modular technical design of the KM3NeT telescope allows for a progressive implementation and for data taking even with an incomplete detector. The first phase of implementation has started. The next phase foresees the installation of three building blocks: two building blocks, for a total instrumented volume of 1 km3 (ARCA), at the KM3NeT-It site, at a depth of 3500 m, about 100 km offshore Capo Passero, Sicily. The main scientific goals of the ARCA detector is the exploration of the neutrino sky with unprecedented resolution, searching for neutrinos coming from defined sources or sky regions, like the Galactic Plane. It will also look for diffuse high energy neutrino fluxes following the indication provided by the IceCube signal. The third building block, with a more compact distribution of the optical modules, will be deployed at the KM3NeT-Fr site, 40 km offshore Toulon at a depth of 2500 m (ORCA). The main objective of ORCA is studying the neutrino mass-hierarchy problem and exploring the low energy region of the spectrum. The status of the first phase of the KM3NeT implementation is described and a survey of the physics potentiality of the telescope is given in this contribution, with particular emphasis on the high energy studies.

Neutrinos from type Ia supernovae: The deflagration-to-detonation transition scenario

It has long been recognized that the neutrinos detected from the next core-collapse supernova in the Galaxy have the potential to reveal important information about the dynamics of the explosion and the nucleosynthesis conditions as well as allowing us to probe the properties of the neutrino itself. The neutrinos emitted from thermonuclear - type Ia - supernovae also possess the same potential, although these supernovae are dimmer neutrino sources. For the first time, we calculate the time, energy, line of sight, and neutrino-flavor-dependent features of the neutrino signal expected from a three-dimensional delayed-detonation explosion simulation, where a deflagration-to-detonation transition triggers the complete disruption of a near-Chandrasekhar mass carbon-oxygen white dwarf. We also calculate the neutrino flavor evolution along eight lines of sight through the simulation as a function of time and energy using an exact three-flavor transformation code. We identify a characteristic spectral peak at $\sim 10$ MeV as a signature of electron captures on copper. This peak is a potentially distinguishing feature of explosion models since it reflects the nucleosynthesis conditions early in the explosion. We simulate the event rates in the Super-K, Hyper-K, JUNO, and DUNE neutrino detectors with the SNOwGLoBES event rate calculation software and also compute the IceCube signal. Hyper-K will be able to detect neutrinos from our model out to a distance of $\sim 10$ kpc. At 1 kpc, JUNO, Super-K, and DUNE would register a few events while IceCube and Hyper-K would register several tens of events.

Letter of intent for KM3NeT 2.0

The main objectives of the KM3NeT Collaboration are (i) the discovery and subsequent observation of high-energy neutrino sources in the Universe and (ii) the determination of the mass hierarchy of neutrinos. These objectives are strongly motivated by two recent important discoveries, namely: (1) the high-energy astrophysical neutrino signal reported by IceCube and (2) the sizable contribution of electron neutrinos to the third neutrino mass eigenstate as reported by Daya Bay, Reno and others. To meet these objectives, the KM3NeT Collaboration plans to build a new Research Infrastructure consisting of a network of deep-sea neutrino telescopes in the Mediterranean Sea. A phased and distributed implementation is pursued which maximises the access to regional funds, the availability of human resources and the synergistic opportunities for the Earth and sea sciences community. Three suitable deep-sea sites are selected, namely off-shore Toulon (France), Capo Passero (Sicily, Italy) and Pylos (Peloponnese, Greece). The infrastructure will consist of three so-called building blocks. A building block comprises 115 strings, each string comprises 18 optical modules and each optical module comprises 31 photo-multiplier tubes. Each building block thus constitutes a three-dimensional array of photo sensors that can be used to detect the Cherenkov light produced by relativistic particles emerging from neutrino interactions. Two building blocks will be sparsely configured to fully explore the IceCube signal with similar instrumented volume, different methodology, improved resolution and complementary field of view, including the galactic plane. One building block will be densely configured to precisely measure atmospheric neutrino oscillations.

Coincidence of a high-fluence blazar outburst with a PeV-energy neutrino event

The discovery of extraterrestrial very-high-energy neutrinos by the IceCube collaboration has launched a quest for the identification of their astrophysical sources. Gamma-ray blazars have been predicted to yield a cumulative neutrino signal exceeding the atmospheric background above energies of 100 TeV, assuming that both the neutrinos and the gamma-ray photons are produced by accelerated protons in relativistic jets. Since the background spectrum falls steeply with increasing energy, the individual events with the clearest signature of being of an extraterrestrial origin are those at PeV energies. Inside the large positional-uncertainty fields of the first two PeV neutrinos detected by IceCube, the integrated emission of the blazar population has a sufficiently high electromagnetic flux to explain the detected IceCube events, but fluences of individual objects are too low to make an unambiguous source association. Here, we report that a major outburst of the blazar PKS B1424-418 occurred in temporal and positional coincidence with the third PeV-energy neutrino event (IC35) detected by IceCube. Based on an analysis of the full sample of gamma-ray blazars in the IC35 field and assuming a photo-hadronic emission model, we show that the long-term average gamma-ray emission of blazars as a class is in agreement with both the measured all-sky flux of PeV neutrinos and the spectral slope of the IceCube signal. The outburst of PKS B1424-418 has provided an energy output high enough to explain the observed PeV event, indicative of a direct physical association.

Extreme blazars as counterparts of IceCube astrophysical neutrinos

We explore the correlation of $\gamma$-ray emitting blazars with IceCube neutrinos by using three very recently completed, and independently built, catalogues and the latest neutrino lists. We introduce a new observable, namely the number of neutrino events with at least one $\gamma$-ray counterpart, $N_{\nu}$. In all three catalogues we consistently observe a positive fluctuation of $N_{\nu}$ with respect to the mean random expectation at a significance level of $0.4 - 1.3$ per cent. This applies only to extreme blazars, namely strong, very high energy $\gamma$-ray sources of the high energy peaked type, and implies a model-independent fraction of the current IceCube signal $\sim 10 - 20$ per cent. An investigation of the hybrid photon -- neutrino spectral energy distributions of the most likely candidates reveals a set of $\approx 5$ such sources, which could be linked to the corresponding IceCube neutrinos. Other types of blazars, when testable, give null correlation results. Although we could not perform a similar correlation study for Galactic sources, we have also identified two (further) strong Galactic $\gamma$-ray sources as most probable counterparts of IceCube neutrinos through their hybrid spectral energy distributions. We have reasons to believe that our blazar results are not constrained by the $\gamma$-ray samples but by the neutrino statistics, which means that the detection of more astrophysical neutrinos could turn this first hint into a discovery.

Predicted sensitivity of the KM3NeT/ARCA detector to a diffuse flux of cosmic neutrinos

The KM3NeT Collaboration has started the construction of a research infrastructure hosting a network of underwater neutrino detectors in the Mediterranean Sea. Two instruments based on the same technology are being built: KM3NeT/ORCA to measure the neutrino mass hierarchy and to study atmospheric neutrino oscillations and KM3NeT/ARCA to detect high-energy cosmic neutrinos both in diffuse and point source mode. The excellent angular resolution of the ARCA detector, with an instrumented volume of about one Gton, will allow for an unprecedented exploration of the neutrino sky searching for neutrinos coming from defined sources of sky regions, like the Galactic Plane and the Fermi Bubbles. It will also look for diffuse high energy neutrino fluxes following the indication provided by the IceCube signal. This contribution will report on the sensitivity of the KM3NeT/ARCA telescope with particular attention to the region of the Galactic Plane. Comparisons with theoretical expectations are also discussed.

Diffuse neutrinos from extragalactic supernova remnants: Dominating the 100 TeV IceCube flux

IceCube has measured a diffuse astrophysical flux of TeV–PeV neutrinos. The most plausible sources are unique high energy cosmic ray accelerators like hypernova remnants (HNRs) and remnants from gamma ray bursts in star-burst galaxies, which can produce primary cosmic rays with the required energies and abundance. In this case, however, ordinary supernova remnants (SNRs), which are far more abundant than HNRs, produce a comparable or larger neutrino flux in the ranges up to 100–150 TeV energies, implying a spectral break in the IceCube signal around these energies. The SNRs contribution in the diffuse flux up to these hundred TeV energies provides a natural baseline and then constrains the expected PeV flux.

Restricting the LSND and MiniBooNE sterile neutrinos with the IceCube atmospheric neutrino data

We study oscillations of the high energy atmospheric neutrinos in the Earth into sterile neutrinos with the eV-scale mass. The MSW resonance and parametric enhancement of the $\bar{\nu}_\mu\to\bar{\nu}_s$ oscillations lead to distortion of the zenith angle distribution of the muon-track events which can be observed by IceCube. Due to matter effect, the IceCube signal depends not only on the mixing element $U_{\mu 4}$ relevant for LSND and MiniBooNE but also on $U_{\tau 4}$ and the CP-violating phase $\delta_{24}$. We show that the case with $U_{\tau 4} = \delta_{24} = 0$ leads to the weakest IceCube signal and therefore should be used to bound $U_{\mu 4}$. We compute the zenith angle distributions of the $\nu_\mu-$events for different energy intervals in the range (0.1 - 10) TeV and find that inclusion of the energy information (binning in energy) improves the sensitivity to $\nu_s$ drastically. We estimate that with already collected (during 3 - 4 years) IceCube statistics the bound $|U_{\mu 4}|^2 < 0.01$ ($99\%$ C.L.) can be established and the mixing required by LSND and MiniBooNE can be excluded at $(4 - 6) \sigma$ confidence level.

Atmospheric Muons as IceCube Signal

Muons of energies above 1 TeV produced in cosmic ray induced air showers account for the vast majority of events in IceCube. Its enormous size compared to previous volume detectors translates into an unprecedented amount of statistics for high-energy atmospheric muons. This offers a wide range of opportunities for original cosmic ray and particle physics.