High-energy Neutrino Flux Research Articles

On the flux of high-energy cosmogenic neutrinos and the influence of the extragalactic magnetic field

ABSTRACT Cosmogenic neutrinos originate from interactions of cosmic rays propagating through the universe with cosmic background photons. Since both high-energy cosmic rays and cosmic background photons exist, the existence of high-energy cosmogenic neutrinos is certain. However, their flux has not been measured so far. Therefore, we calculated the flux of high-energy cosmogenic neutrinos arriving at the Earth on the basis of elaborate 4D simulations that take into account three spatial degrees of freedom and the cosmological time-evolution of the universe. Our predictions for this neutrino flux are consistent with the recent upper limits obtained from large-scale cosmic-ray experiments. We also show that the extragalactic magnetic field has a strong influence on the neutrino flux. The results of this work are important for the design of future neutrino observatories, since they allow to assess the detector volume and observation time that are necessary to detect high-energy cosmogenic neutrinos in the near future. An observation of such neutrinos would push multimessenger astronomy to hitherto unachieved energy scales.

Search for transient optical counterparts to high-energy IceCube neutrinos with Pan-STARRS1

In order to identify the sources of the observed diffuse high-energy neutrino flux, it is crucial to discover their electromagnetic counterparts. To increase the sensitivity of detecting counterparts of transient or variable sources by telescopes with a limited field of view, IceCube began releasing alerts for single high-energy (Eν > 60 TeV) neutrino detections with sky localisation regions of order 1° radius in 2016. We used Pan-STARRS1 to follow-up five of these alerts during 2016–2017 to search for any optical transients that may be related to the neutrinos. Typically 10–20 faint (miP1 ≲ 22.5 mag) extragalactic transients are found within the Pan-STARRS1 footprints and are generally consistent with being unrelated field supernovae (SNe) and AGN. We looked for unusual properties of the detected transients, such as temporal coincidence of explosion epoch with the IceCube timestamp, or other peculiar light curve and physical properties. We found only one transient that had properties worthy of a specific follow-up. In the Pan-STARRS1 imaging for IceCube-160427A (probability to be of astrophysical origin of ∼50%), we found a SN PS16cgx, located at 10.0′ from the nominal IceCube direction. Spectroscopic observations of PS16cgx showed that it was an H-poor SN at redshiftz = 0.2895 ± 0.0001. The spectra and light curve resemble some high-energy Type Ic SNe, raising the possibility of a jet driven SN with an explosion epoch temporally coincident with the neutrino detection. However, distinguishing Type Ia and Type Ic SNe at this redshift is notoriously difficult. Based on all available data we conclude that the transient is more likely to be a Type Ia with relatively weak Si IIabsorption and a fairly normal rest-framer-band light curve. If, as predicted, there is no high-energy neutrino emission from Type Ia SNe, then PS16cgx must be a random coincidence, and unrelated to the IceCube-160427A. We find no other plausible optical transient for any of the five IceCube events observed down to a 5σlimiting magnitude ofmiP1 ≈ 22 mag, between 1 day and 25 days after detection.

Cosmogenic neutrinos through the GRAND lens unveil the nature of cosmic accelerators

The sources of cosmic rays with energies above 55 EeV are still mysterious. A guaranteed associated flux of ultra high energy neutrinos known as the cosmogenic neutrino flux will be measured by next generation radio facilities, such as the proposed Giant Radio Array for Neutrino Detection (GRAND). By using the orthogonal information provided by the cosmogenic neutrino flux, we here determine the prospects of GRAND to constrain the source redshift evolution and the chemical composition of the cosmic ray sources. If the redshift evolution is known, independently on GRAND's energy resolution, GRAND with 200,000 antennas will constrain the proton/iron fraction to the ∼5–10% level after one year of data taking; on the other hand, if hints on the average source composition are given, GRAND will measure the redshift evolution of the sources to a ∼ 10% uncertainty. However, the foreseen configuration of GRAND alone will not be able to break the degeneracy between redshift evolution of the sources and their composition. Our findings underline the discriminating potential of next generation radio array detectors and motivate further efforts in this direction.

Decays of long-lived relics and their signatures at IceCube

We consider long-lived relic particles as the source of the PeV-scale neutrinos detected at the IceCube observatory over the last six years. We derive the present day neutrino flux, including primary neutrinos from direct decays, secondary neutrinos from electroweak showering, and tertiary neutrinos from re-scatters off the relic neutrino background. We compare the high-energy neutrino flux prediction to the most recently available datasets and find qualitative differences to expected spectra from other astrophysical processes. We utilize electroweak corrections to constrain heavy decaying relic abundances, using measurements impacted by electromagnetic energy injection, such as light element abundances during Big Bang nucleosynthesis, cosmic microwave background anisotropies, and diffuse γ-ray spectra. We compare these abundances to those necessary to source the IceCube neutrinos and find two viable regions in parameter space, ultimately testable by future neutrino, γ-ray, and cosmic microwave background observatories.

On the Neutrino Flares from the Direction of TXS 0506+056

Abstract A multimessenger campaign has associated a high-energy cosmic neutrino with a distant gamma-ray blazar, TXS 0506+056. IceCube archival data subsequently revealed that the high-energy neutrino flux from the direction of this source, integrated over the last 10 yr, is dominated by a single bright neutrino flare in 2014, leaving the multimessenger flare as a subluminous second flare. The extraordinary brightness of the blazar despite its distance suggests that it may belong to a special class of sources that produce cosmic rays. We show that the diffuse IceCube flux discovered in 2013 can be accommodated by a subclass of blazars, on the order of 5%, that episodically produce neutrinos with the luminosity of the 2014 neutrino flare. Matching the cosmic-ray flux required to produce the neutrinos to the one observed implies highly efficient neutrino sources with large target photon densities that are not transparent to high-energy gamma-rays. The opacity of the source modifies the straightforward multimessenger connection in a way that is consistent with the gamma-ray observations coincident with the 2014 neutrino flare.

Search for steady point-like sources in the astrophysical muon neutrino flux with 8 years of IceCube data

The IceCube Collaboration has observed a high-energy astrophysical neutrino flux and recently found evidence for neutrino emission from the blazar TXS 0506+056. These results open a new window into the high-energy universe. However, the source or sources of most of the observed flux of astrophysical neutrinos remains uncertain. Here, a search for steady point-like neutrino sources is performed using an unbinned likelihood analysis. The method searches for a spatial accumulation of muon-neutrino events using the very high-statistics sample of about 497,000 neutrinos recorded by IceCube between 2009 and 2017. The median angular resolution is sim 1^circ at 1 TeV and improves to sim 0.3^circ for neutrinos with an energy of 1 PeV. Compared to previous analyses, this search is optimized for point-like neutrino emission with the same flux-characteristics as the observed astrophysical muon-neutrino flux and introduces an improved event-reconstruction and parametrization of the background. The result is an improvement in sensitivity to the muon-neutrino flux compared to the previous analysis of sim 35% assuming an E^{-2} spectrum. The sensitivity on the muon-neutrino flux is at a level of E^2 mathrm {d} N /mathrm {d} E = 3cdot 10^{-13},mathrm {TeV},mathrm {cm}^{-2},mathrm {s}^{-1}. No new evidence for neutrino sources is found in a full sky scan and in an a priori candidate source list that is motivated by gamma-ray observations. Furthermore, no significant excesses above background are found from populations of sub-threshold sources. The implications of the non-observation for potential source classes are discussed.

On the possibilities of high-energy neutrino production in the jets of microquasar SS433 in light of new observational data

Microquasar SS433 is composed by a supergiant star that feeds mass through a supercritical accretion disk to a ∼10 M⊙ black hole. The latter launches two oppositely directed jets that precess with a period of 162 days. The system has been detected at different spatial scales in frequencies ranging from radio to gamma rays, and has long been considered as a potential neutrino source which has been observed by AMANDA in the past, and later IceCube, leading to more restrictive upper bounds on the neutrino flux. In this work, we explore the possibilities that neutrinos could be produced in the jets of this source at levels consistent, or at least, not incompatible with any current data on electromagnetic emission available. In order to do so, we consider the injection of both electrons and protons at different positions in the jets, and we compute their broadband photon emission by synchrotron and interactions with ambient photons and matter. After correcting the high energy photon flux by the effect of γγ and γN absorption, we obtain the surviving flux that arrive on Earth and compare it with observational data by gamma-ray detectors. The flux of high energy neutrinos is consistently computed and we find that if they are eventually detected with IceCube, production must take place at the inner jets, where gamma-ray absorption is important, in order to avoid current VHE constraints form HESS and MAGIC. Additionally, we find that if the flux of 25 TeV gamma-rays recently detected with HAWC and which corresponds to the jet termination region were produced mainly by pp interactions, this would lead to a too faint neutrino flux that is beyond the reach of IceCube in its present configuration.

Escape of Cosmic Rays from the Galaxy and Effects on the Circumgalactic Medium.

The escape of cosmic rays from the Galaxy is expected to shape their spectrum inside the Galaxy. Yet, this phenomenon is very poorly understood and, in the absence of a physical description, it is usually modeled as a free escape from a given boundary, typically located at a few kiloparsec distance from the Galactic disc. We show that the assumption of free escape leads to the conclusion that the cosmic ray current propagating in the circumgalactic medium is responsible for a nonresonant cosmic ray induced instability that in turn leads to the generation of a magnetic field of strength ∼2×10^{-8} G on a scale ∼10 kpc around our Galaxy. The self-generated diffusion produces large gradients in the particle pressure that induce a displacement of the intergalactic medium with velocity ∼10-100 km/s. Cosmic rays are then carried away by advection. If the overdensity of the intergalactic gas in a region of size ∼10 kpc around our Galaxy is ≳100 with respect to the cosmological baryon density Ω_{b}ρ_{cr}, then the flux of high energy neutrinos due to pion production becomes comparable with the flux of astrophysical neutrinos recently measured by IceCube.

Active galactic nuclei and the origin of IceCube's diffuse neutrino flux

The excess of neutrino candidate events detected by IceCube from the direction of TXS 0506+056 has generated a great deal of interest in blazars as sources of high-energy neutrinos. In this study, we analyze the publicly available portion of the IceCube dataset, performing searches for neutrino point sources in spatial coincidence with the blazars and other active galactic nuclei contained in the Fermi 3LAC and the Roma BZCAT catalogs, as well as in spatial and temporal coincidence with flaring sources identified in the Fermi Collaboration's All-Sky Variability Analysis (FAVA). We find no evidence that blazars generate a significant flux of high-energy neutrinos, and conclude that no more than 5–15% of the diffuse flux measured by IceCube can originate from this class of objects. While we cannot rule out the possibility that TXS 0506+056 has at times generated significant neutrino emission, we find that such behavior cannot be common among blazars, requiring TXS 0506+056 to be a rather extreme outlier and not representative of the overall blazar population. The bulk of the diffuse high-energy neutrino flux must instead be generated by a significantly larger population of less-luminous sources, such as non-blazar active galactic nuclei.

A study on a minimally broken residual TBM-Klein symmetry with its implications on flavoured leptogenesis and ultra high energy neutrino flux ratios

We present a systematic study on minimally perturbed neutrino mass matrices which at the leading order give rise to Tri-BiMaximal (TBM) mixing due to a residual ℤ2× ℤ2μτ Klein symmetry in the neutrino mass term of the low energy effective seesaw Lagrangian. Considering only the breaking of ℤ2μτ with two relevant breaking parameters (ϵ4,6′), after a comprehensive numerical analysis, we show that the phenomenologically viable case in this scenario is a special case of TM1 mixing. For this class of models, from the phenomenological perspective, one always needs large breaking (more than 45%) in one of the breaking parameters. However, to be consistent the maximal mixing of θ23, while more than 35% breaking is needed in the other, a range 49.4̂−53̂ and 38̂−40̂ could be probed allowing breaking up to 25% in the same parameter. Thus though this model cannot distinguish the octant of θ23, non-maximal mixing is preferred from the viewpoint of small breaking. The full ℤ2× ℤ2μτ symmetry leads to a degeneracy in the eigenvalues of right handed (RH) neutrino mass matrices (MR). Nevertheless, breaking of ℤ2μτ which is also necessary to generate nonzero θ13 in this model, lifts that degeneracy. Thus unlike the standard N1-leptogenesis scenario where only the decays and other interactions due to the lightest RH neutrino N1 are relevant, here all the RH neutrinos contribute to the process of baryogenesis via leptogenesis due to the small mass splitting controlled by the ℤ2μτ breaking parameters. Including flavour coupling effects (In general, which have been partially included in all the leptogenesis studies in perturbed TBM framework), we solve flavour dependent coupled Boltzmann equations for heavy neutrino as well as the light leptonic number densities to compute the final baryon asymmetry YB. Thus our analysis and results pertaining to a successful leptogenesis are more accurate than any other studies in existing literature that deal with perturbed TBM scenario. Finally, in the context of recent discovery of the ultra high energy (UHE) neutrino events at IceCube, assuming UHE neutrinos originate from purely astrophysical sources, we obtain prediction on the neutrino flux ratios at neutrino telescopes.

Latest results of the ANTARES neutrino telescope

Neutrino astronomy is in an exciting moment. The discovery of a cosmic flux of high energy neutrinos by IceCube heralds a new era in which neutrinos have finally joined the multi-messenger study of the Universe. This new important window complements more “traditional” probes (as cosmic rays or photons), given the particular combination of characteristics of neutrinos (neutral, stable and weakly interacting). The ANTARES detector, built in the Mediterranean Sea, has succeeded in two key points. First, it has shown the feasibility of the technique of underwater neutrino telescopes, which offers important advantages in terms of performance (better angular resolution, better visibility of the Galaxy if built in the Northern Hemisphere). This has paved the way for the next step, KM3NeT, already in construction. Second, the physics harvest of ANTARES is very rich, including many results that show the particular advantages of being in the Mediterranean, as mentioned above. The analyses performed include the search for point-like sources, diffuse fluxes, transient phenomena, dark matter, etc. In this talk we will review this long list of achievements.

KM3NeT Readout and Triggering

The KM3NeT collaboration is constructing new-generation neutrino detectors in the Mediterranean Sea. The main goals are the study of the high-energy neutrino flux (KM3NeT/ARCA, off-shore Capo-Passero, Italy) and the determination of the neutrino mass ordering (KM3NeT/ORCA, off-shore Toulon, France). This contribution describes the system to collect, transfer and process the data, which consists of time and intensity measurements of photons recorded by photo-multiplier tubes resulting from charged products from neutrino interactions, together with data from the acoustic position system. The trigger and data-acquisition system faces the challenge of transporting up to 30 Gbps of data to shore and filtering the neutrino signal from an overwhelming (O(107)) background due to photons from ambient 40K and bioluminescence.

The KM3NeT Digital Optical Module and Detection Unit

The KM3NeT Collaboration is constructing new-generation neutrino detectors in the Mediterranean Sea. The main goals are the study of the highenergy neutrino flux and the determination of the neutrino mass ordering. The basic detection element, the Digital Optical Module (DOM), houses 31 3-inch PMTs inside a 17-inch glass sphere. The aim is to measure photons emitted by products of neutrino interactions in the sea-water with nanosecond precision. The multi-PMT concept has many advantages over a single-PMT design. We will discus these aspects and the enabling technologies. Additionally, the required mechanical and optical/electrical system on which DOMs are deployed as vertical strings, called Detection Units will be discussed.

The Italian Site for KM3NeT ARCA

The Italian site for KM3NeT, located in the Ionian Sea about 100 km offshore Capo Passero, South East of Sicily, is dedicated to host (at least) two building blocks of the ARCA (Astronomy Research with Cosmics in the Abysses), comprising 230 Detection Units aiming at measurement of high-energy neutrino fluxes and discovery of their sources. The existing infrastructure is under upgrade within the framework of the IDMAR project jointly funded by Regione Siciliana and INFN. IDMAR at Capo Passero will be run in connection with the other abyssal infrastructure built by INFN 25 km offshore the town of Catania at 2100 m depth, hosting the Western Ionian Sea node of EMSO-ERIC.

The multi-PMT optical module for the IceCube-Upgrade

Following the first observation of an astrophysical high-energy neutrino flux with the IceCube Neutrino Observatory in 2013, planning for an upgrade of the detector is progressing, which will expand the capabilities of the detector both at low and high neutrino energies. A substantial contribution to the improved performance is anticipated to be achieved by the application of advanced optical module technology. The multi-PMT optical module, mDOM, consists of 24 3-inch PMTs which provide, amongst others, a large, homogeneous photosensitive area and sensitivity to the incident direction of photons. After an introduction, the current status of the mDOM development is presented with emphasis on the characterization of the photomultipliers under consideration.

The search for high-energy neutrinos coincident with fast radio bursts with the ANTARES neutrino telescope

In the past decade, a new class of bright transient radio sources with millisecond duration has been discovered. The origin of these so-called Fast Radio Bursts (FRBs) is still a great mystery despite the growing observational efforts made by various multi-wavelength and multi-messenger facilities. So far, many models have been proposed to explain FRBs but neither the progenitors nor the radiative and the particle acceleration processes at work have been clearly identified. In this paper, the question whether some hadronic processes may occur in the vicinity of the FRB source is assessed. If so, FRBs may contribute to the high energy cosmic-ray and neutrino fluxes. A search for these hadronic signatures has been done using the ANTARES neutrino telescope. The analysis consists in looking for high-energy neutrinos, in the TeV-PeV regime, spatially and temporally coincident with the detected FRBs. Most of the FRBs discovered in the period 2013-2017 were in the field of view of the ANTARES detector, which is sensitive mostly to events originating from the Southern hemisphere. From this period, 12 FRBs have been selected and no coincident neutrino candidate was observed. Upper limits on the per burst neutrino fluence have been derived using a power law spectrum, $\rm{\frac{dN}{dE_\nu}\propto E_\nu^{-\gamma}}$, for the incoming neutrino flux, assuming spectral indexes $\gamma$ = 1.0, 2.0, 2.5. Finally, the neutrino energy has been constrained by computing the total energy radiated in neutrinos assuming different distances for the FRBs. Constraints on the neutrino fluence and on the energy released are derived from the associated null results.

Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A.

Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain unidentified. On 22 September 2017, we detected a high-energy neutrino, IceCube-170922A, with an energy of ~290 tera-electron volts. Its arrival direction was consistent with the location of a known γ-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multiwavelength campaign followed, ranging from radio frequencies to γ-rays. These observations characterize the variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy γ-rays. This observation of a neutrino in spatial coincidence with a γ-ray-emitting blazar during an active phase suggests that blazars may be a source of high-energy neutrinos.

Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert

A high-energy neutrino event detected by IceCube on 22 September 2017 was coincident in direction and time with a gamma-ray flare from the blazar TXS 0506+056. Prompted by this association, we investigated 9.5 years of IceCube neutrino observations to search for excess emission at the position of the blazar. We found an excess of high-energy neutrino events, with respect to atmospheric backgrounds, at that position between September 2014 and March 2015. Allowing for time-variable flux, this constitutes 3.5σ evidence for neutrino emission from the direction of TXS 0506+056, independent of and prior to the 2017 flaring episode. This suggests that blazars are identifiable sources of the high-energy astrophysical neutrino flux.

High-energy Neutrinos from Galactic Superbubbles

Abstract We study the propagation of cosmic rays (CRs) generated by sources residing inside superbubbles. We show that the enhanced magnetic field in the bubble wall leads to an increase in the interior CR density. Because of the large matter density in the wall, the probability for CR interactions on gas peaks there. As a result, the walls of superbubbles located near young CR sources efficiently emit neutrinos. We apply this scenario to the Loop I and Local Superbubble: these bubbles are sufficiently near such that CRs from a young source such as Vela interacting in the bubble wall can generate a substantial fraction of the observed astrophysical high-energy neutrino flux below ∼few × 100 TeV.

Constraints on the diffuse high-energy neutrino flux from the third flight of ANITA

The Antarctic Impulsive Transient Antenna (ANITA), a NASA long-duration balloon payload, searches for radio emission from interactions of ultra-high-energy neutrinos in polar ice. The third flight of ANITA (ANITA-III) was launched in December 2014 and completed a 22-day flight. We present the results of three analyses searching for Askaryan radio emission of neutrino origin. In the most sensitive of the analyses, we find one event in the signal region on an expected a priori background of $0.7^{+0.5}_{-0.3}$. Though consistent with the background estimate, the candidate event remains compatible with a neutrino hypothesis even after additional post-unblinding scrutiny.