The origin of astrophysical high-energy neutrinos detected by the IceCube Neutrino Observatory remains a mystery to be solved. In this paper we search for neutrino source candidates within the $90$% containment area of $70$ track-type neutrino events recorded by the IceCube Neutrino Observatory. By employing the Fermi-LAT 4FGL-DR2, the Swift-XRT 2SXPS and the CRATES catalogs, we identify possible gamma, X-ray and flat-spectrum radio candidate sources of track-type neutrinos. We find that based on the brightness of sources and their spatial correlation with the track-type IceCube neutrinos, the constructed neutrino samples represent special populations of sources taken from the full Fermi-LAT 4FGL-DR2/Swift-XRT 2SXPS/CRATES catalogs with similar significance ($2.1σ$, $1.2σ$, $2σ$ at $4.8~\mathrm{GHz}$, $2.1σ$ at $8.4~\mathrm{GHz}$, respectively, assuming 50% astrophysical signalness). After collecting redshifts and deriving sub-samples of the CRATES catalog complete in the redshift--luminosity plane, we find that the 4.8 GHz ($8.4$~GHz) sub-sample can explain between 4% and 53% ($3$% and $42$%) of the neutrinos (90% C.L.), when the probability to detect a neutrino is proportional to the ($k$-corrected) radio flux. The overfluctuations indicate that a part of the sample is likely to contribute and that more sophisticated schemes in the source catalog selection are necessary to identify the neutrino sources at the $5σ$ level. Our selection serves as a starting point to further select the correct sources.
A time-dependent search for neutrino flares from pre-defined directions in the whole sky is presented. The analysis uses a time clustering algorithm combined with an unbinned likelihood method. This algorithm provides a search for significant neutrino flares over time-scales that are not fixed a-priori and that are not triggered by multiwavelength observations. The event selection is optimized to maximize the discovery potential, taking into account different time-scales of source activity and background rates. The method is applied to a pre-defined list of bright and variable astrophysical so…
The discovery of high-energy astrophysical neutrinos by IceCube kicked off a new line of research to identify the electromagnetic counterparts producing these neutrinos. Among the extragalactic sources, active galactic nuclei (AGN), and in particular Blazars, are promising candidate neutrino emitters. Their structure, with a relativistic jet pointing to the Earth, offers a natural accelerator of particles and for this reason a perfect birthplace of high energy neutrinos. A good characterisation of the spectral energy distribution (SED) of these sources can improve the understanding of the phys…
Extragalactic jets launched from the immediate vicinity of supermassive black holes in radio-loud active galactic nuclei (AGN) are key objects in modern astronomy and astroparticle physics. AGN jets carry a fraction of the total gravitational energy released during the accretion of matter onto supermassive black holes and are prime suspects as possible sources of ultrahigh-energy cosmic rays and the recently detected extraterrestrial neutrinos at PeV energies. TANAMI (Tracking Active galactic Nuclei with Austral Milliarcsecond Interferometry) is a multiwavelength program monitoring AGN jets of…
This contribution reviews recent advances in the possible identification of blazars as potential sources of at least some of the very-high-energy neutrinos detected by the IceCube neutrino detector at the South Pole. The basic physical requirements for neutrino production and physics constraints that may be drawn from neutrino - blazar associations are reviewed. Several individual cases of possible associations will be discussed in more detail. It is emphasized that due to $γγ$ opacity constraints in efficiently neutrino-producing blazars, an association between X-ray -- soft $γ$-ray activity …
MAGIC collaboration has recently analyzed data from a long-term multiwavelength campaign of the $γ$-ray blazar TXS 0506+056. In December 2018, it was flaring in the very-high-energy (VHE; $E>100$ GeV) $γ$-ray band, but no simultaneous neutrino event was detected. We model the observed spectral energy distribution (SED), using a one-zone leptohadronic emission. We estimate the neutrino flux through the restriction from observed X-ray flux on the secondary radiation due to hadronic cascade, initiated by protons with energy $E_p \lesssim 0.1$ EeV. We assume ultrahigh-energy cosmic rays (UHECRs; $…
Blazars constitute the vast majority of extragalactic $γ$-ray sources. They can also contribute a sizable fraction of the diffuse astrophysical neutrinos detected by IceCube. In the past few years, the real-time alert system of IceCube has led to multiwavelength follow-up of very high-energy neutrino events of plausible astrophysical origin. Spatial and temporal coincidences of these neutrino events with the high-activity state of $γ$-ray blazars can provide a unique opportunity to decipher cosmic-ray interactions in the relativistic jets. Assuming that blazars accelerate cosmic rays up to ult…
We propose an explanation for the far-infrared/radio correlation of galaxies in terms of the energy balance of the interstellar medium and determine the flux from high-energy photons and neutrinos from starburst galaxies. We present a catalog of the 127 brightest starburst galaxies with redshifts of z<0.03. In order to investigate the correlation between radio- and far-infrared emission, we apply the leaky box approximation. Further, we derive photon- and neutrino spectra from proton-proton interactions in supernova remnants (SNRs). Here, we assume that a fraction of the SNR's energy is transf…
High-energy neutrinos are a promising tool for identifying astrophysical sources of high and ultra-high energy cosmic rays (UHECR). Prospects of detecting neutrinos at high energies ($\gtrsim$TeV) from blazars have been boosted after the recent association of IceCube-170922A and TXS 0506+056. We investigate the high-energy neutrino, IceCube-190331A, a high-energy starting event (HESE) with a high likelihood of being astrophysical in origin. We initiated a Swift/XRT and UVOT tiling mosaic of the neutrino localisation, and followed up with ATCA radio observations, compiling a multiwavelength SED…
The abundance of accelerators and the ambient conditions make Cygnus X a natural laboratory for studying the life cycle of cosmic-rays (CRs). This naturally makes the Cygnus X complex a highly interesting source in neutrino astronomy, in particular concerning a possible detection with the IceCube Neutrino Observatory, which has a good view of the northern hemisphere. In this paper, we model the multiwavelength spectrum of the Cygnus, for the first time using a broad data set from radio, MeV (COMPTEL), GeV (Fermi), TeV (Argo) and 10s of TeV (Milagro) energies. The modeling is performed assuming…
The completed IceCube Observatory, the first km^3 neutrino telescope, is already providing the most stringent limits on the flux of high energy cosmic neutrinos from point-like and diffuse galactic and extra-galactic sources. The non-detection of extra-terrestrial neutrinos has important consequences on the origin of the cosmic rays. Here the current status of astrophysical neutrino searches, and of the observation of a persistent cosmic ray anisotropy above 100 TeV, are reviewed.
Since the discovery of a neutrino flux in excess of the atmospheric background by the IceCube Collaboration, searches for the astrophysical sources have been ongoing. Due to the steeply falling background towards higher energies, the PeV events detected in three years of IceCube data are the most likely ones to be of extraterrestrial origin. Even excluding the PeV events detected so far, the neutrino flux is well above the atmospheric background, so it is likely that a number of sub-PeV events originate from the same astrophysical sources that produce the PeV events. We study the high-energy p…
The unidentified TeV source MGRO J1908+06, with emission extending from hundreds of GeV to beyond 100TeV, is one of the most intriguing sources in the Galactic plane. MGRO J1908+06 spatially associates with an IceCube hotspot of neutrino emission. Although the hotspot is not significant yet, this suggests a possible hadronic origin of the observed gamma-ray radiation. Here we describe a multiwavelength analysis on MGRO J1908+06 to determine its nature. We identify, for the first time, an extended GeV source as the counterpart of MGRO J1908+06, discovering possibly associated molecular clouds (…
The IceCube Collaboration has recently reported evidence for a high-energy extraterrestrial neutrino flux. During two years of operation 28 events with energies between 30 TeV and 1.2 PeV were observed while only 10.6 events were expected from conventional atmospheric backgrounds. The hadronic interactions responsible for this IceCube excess will also produce a flux of high-energy gamma-rays that can serve as a probe of source direction and distance. We show that existing TeV to PeV diffuse gamma-ray limits support the interpretation that the IceCube excess is mostly of extragalactic origin. H…
In the past years, evidence has started piling up that some high-energy cosmic neutrinos can be associated with blazars in flaring states. On February 26, 2022, a new blazar-neutrino coincidence has been reported: the track-like neutrino event IC220225A detected by IceCube is spatially coincident with the flat-spectrum radio quasar PKS 0215+015. Like previous associations, this source was found to be in a high optical and $γ$-ray state. Moreover, the source showed a bright radio outburst, which substantially increases the probability of a true physical association. We have performed six observ…
The recent IceCube discovery of 0.1-1 PeV neutrinos of astrophysical origin opens up a new era for high-energy astrophysics. Although there are various astrophysical candidate sources, a firm association of the detected neutrinos with one (or more) of them is still lacking. A recent analysis of plausible astrophysical counterparts within the error circles of IceCube events showed that likely counterparts for nine of the IceCube neutrinos include mostly BL Lacs, among which Mrk 421. Motivated by this result and a previous independent analysis on the neutrino emission from Mrk 421, we test the B…
We discuss the status of the kilometer-scale neutrino detector IceCube and its low energy upgrade Deep Core and review its scientific potential for particle physics. We subsequently appraise IceCube's potential for revealing the enigmatic sources of cosmic rays. After all, this aspiration set the scale of the instrument. While only a smoking gun is missing for the case that the Galactic component of the cosmic ray spectrum originates in supernova remnants, the origin of the extragalactic component remains as inscrutable as ever. We speculate on the role of the nearby active galaxies Centaurus …
The IceCube (IC) collaboration recently reported the detection of TeV-PeV extraterrestrial neutrinos whose origin is yet unknown. By the photon-neutrino connection in $pp$ and $pγ$ interactions, we use the \fermi-LAT observations to constrain the origin of the IC detected neutrinos. We find that Galactic origins, i.e., the diffuse Galactic neutrinos due to cosmic ray (CR) propagation in the Milky Way, and the neutrinos from the Galactic point sources, may not produce the IC neutrino flux, thus these neutrinos should be of extragalactic origin. Moreover, the extragalactic gamma-ray bursts (GRBs…
IceCube has discovered an unresolved and isotropic flux of neutrinos between 10 TeV and 8 PeV. Extragalactic origin for this flux is usually assumed, as well as a correlation with the sources of cosmic rays. To date, no clear association with a class of objects has been made. HAWC is a very-high-energy (VHE, or $\gtrsim$100 GeV) gamma ray observatory in operation in central Mexico. HAWC has studied 2/3 of the entire sky ($\sim8~sr)$. We use this survey to search for optically thin sources of cosmic rays and neutrinos, responsible for IceCube's observations. We have written a simulation of the …
At a time when IceCube is nearing completion, we revisit the rationale for constructing kilometer-scale neutrino detectors. We focus on the prospect that such observatories reveal the still-enigmatic sources of cosmic rays. While only a "smoking gun" is missing for the case that the Galactic component of the cosmic-ray spectrum originates in supernova remnants, the origin of the extragalactic component remains a mystery. We speculate on neutrino emission from gamma-ray bursts and active galaxies.
Recent IceCube results suggest that the first detection of very high energy astrophysical neutrinos have been accomplished. We consider these results at face value in a Galactic origin context. Emission scenarios from both the Fermi bubble and broader halo region are considered. We motivate that such an intensity of diffuse neutrino emission could be Galactic in origin if it is produced from an outflow into the halo region. This scenario requires cosmic ray transport within the outflow environment to be different to that inferred locally within the disk and that activity in the central part of…
After a brief review of the status of the kilometer-scale neutrino observatory IceCube, we discuss the prospect that such detectors discover the still-enigmatic sources of cosmic rays. After all, this aspiration set the scale of the instrument. While only a smoking gun is missing for the case that the Galactic component of the cosmic-ray spectrum originates in supernova remnants, the origin of the extragalactic component remains as inscrutable as ever. We speculate on the role of the nearby active galaxies Centaurus A and M87.
The IceCube evidence for cosmic neutrinos has inspired a large number of hypothesis on their origin, mainly due to the poor precision on the measurement of the direction of showering events. A North/South asymmetry in the present data set suggests the presence of a possible Galactic component. This could be originated either by single point-like sources or from an extended Galactic region. Expected fluxes derived from these hypotheses are presented. Some values have been constrained from the present available upper limits from the ANTARES neutrino telescope.
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.
The detection of a high energy muon neutrino on 22 September 2017 by IceCube neutrino detector coincides with the multiwavelength flaring from the BL Lac object TXS 0506+056, most likely confirming AGN as a source of high energy cosmic rays and neutrinos. Using the photohadronic model, we have explained the very high energy $γ$-rays observed by MAGIC telescopes few days after the neutrino event and extend the model to calculate the neutrino flux at different windows consistent with the flaring period of TXS 0506+056 and compared with the IceCube and MAGIC estimates. We also use this model to e…
Searches for spatial associations between high-energy neutrinos observed at the IceCube Neutrino Observatory and known astronomical objects may hold the key to establishing the neutrinos' origins and the origins of hadronic cosmic rays. While extragalactic sources like the blazar TXS 0506+056 merit significant attention, Galactic sources may also represent part of the puzzle. Here, we explore whether open clusters and supernova remnants in the Milky Way contribute measurably to the IceCube track-like neutrino events above 200 TeV. By searching for positional coincidences with catalogs of known…
The Antarctic Impulsive Transient Antenna (ANITA) collaboration has reported a total of three neutrino candidates from the experiment's first three flights. One of these was the lone candidate in a search for Askaryan radio emission, and the others can be interpreted as tau-neutrinos, with important caveats. Among a variety of explanations for these events, they may be produced by astrophysical transients with various characteristic timescales. We test the hypothesis that these events are astrophysical in origin by searching for IceCube counterparts. Using seven years of IceCube data from 2011…
Located at the South Pole, the IceCube Neutrino Observatory is the world largest neutrino telescope, instrumenting one cubic kilometre of Antarctic ice at a depth between 1450m to 2450m. In 2013 IceCube reported the first observations of a diffuse astrophysical high-energy neutrino flux. Although the IceCube Collaboration has identified more than 100 high-energy neutrino events, the origin of this neutrino flux is still not known. Blazars, a subclass of Active Galactic Nuclei and one of the most powerful classes of objects in the Universe, have long been considered promising sources of high en…
We study general implications of the IceCube observations in the energy range from $10^{6}$ GeV to $10^{10}$ GeV for the origin of extragalactic ultrahigh energy cosmic rays assuming that high energy neutrinos are generated by the photomeson production of protons in the extragalactic universe. The PeV-energy neutrino flux observed by IceCube gives strong bounds on the photomeson-production optical depth of protons in their sources and the intensity of the proton component of extragalactic cosmic rays. The neutrino flux implies that extragalactic cosmic-ray sources should have the optical depth…
Instrumenting a gigaton of ice at the geographic South Pole, the IceCube Neutrino Observatory has been at the forefront of groundbreaking scientific discoveries over the past decade. These include the observation of a flux of TeV-PeV astrophysical neutrinos, detection of the first astrophysical neutrino on the Glashow resonance and evidence of the blazar TXS 0506+056 as the first known astronomical source of high-energy neutrinos. Several questions, however, remain, pertaining to the precise origins of astrophysical neutrinos, their production mechanisms at the source and in Earth's atmosphere…
The evaluation of mass composition of cosmic rays in the knee region ($\sim 3$ PeV) is critical to understanding the transition in the origin of cosmic rays from galactic to extragalactic sources. The IceCube Neutrino Observatory at the South Pole is a multi-component detector consisting of the surface IceTop array and the deep in-ice IceCube detector. By applying modern machine-learning techniques to cosmic-ray air showers reconstructed coincidentally in both detector components of IceCube observatory, the energy and the mass of primary cosmic rays in this transition region can be measured. I…
IceCube is a cubic-kilometer scale neutrino detector instrumenting a gigaton of ice at the geographic South Pole in Antarctica. On average, 8 track-like high-energy neutrino events with a high probability of being astrophysical are detected and published as alerts per year. The bright appearance of these events in the detector allow a precise pointing to their origins. This work presents a search for cosmic neutrino sources. The analysis uses high statistics archival IceCube neutrino-induced through-going muon samples to search for these sources in the vicinity of the incoming directions of th…
The IceCube Collaboration has announced the discovery of a neutrino flux in excess of the atmospheric background. Due to the steeply falling atmospheric background spectrum, events at PeV energies are most likely of extraterrestrial origin. We present the multiwavelength properties of the six radio brightest blazars positionally coincident with these events using contemporaneous data of the TANAMI blazar sample, including high-resolution images and spectral energy distributions. Assuming the X-ray to γ-ray emission originates in the photoproduction of pions by accelerated protons, the integrat…
The IceCube DeepCore is a dense infill array of the IceCube Neutrino Observatory at the South Pole. While IceCube is best suited for detecting neutrinos with energies of several 100 GeV and above, DeepCore allows to probe neutrinos with lower energies. We focus on a sample of neutrinos with energies above approximately 10 GeV, which was originally optimised for oscillation experiments. Recently, it has been adapted to enable searches for transient sources of astrophysical neutrinos in the sky. In particular, this low-energy dataset can be used to conduct follow-up searches of gravitational wav…
We present a range of unbroken power-law fits to the astrophysical-neutrino spectrum consistent with the most recent published IceCube data at the 68\% confidence level. Assuming that the neutrinos originate in decays of pi mesons, we estimate accompanying gamma-ray fluxes for various distributions of sources, taking propagation effects into account. We then briefly discuss existing experimental results constraining PeV to EeV diffuse gamma-ray flux and their systematic uncertainties. Several scenarios are marginally consistent both with the KASKADE and CASA-MIA upper limits at (10^15-10^16) e…
The detection of an astrophysical flux of high-energy neutrinos by IceCube is a major step forward in the search for the origin of cosmic rays, as this emission is expected to originate in hadronic interactions taking place in or near cosmic-ray accelerators. No neutrino point sources, or a significant correlation with known astrophysical objects, have been identified in the IceCube data so far that could reveal the location of the neutrino emission sites. The hadronic interactions responsible for the neutrino emission should also lead to the production of high-energy gamma rays from neutral p…
A comprehensive study of the nucleus and western hotspot of Pictor A is carried out using AstroSat observations, 13 years of Fermi, and archival Swift observations, along with other published data. We report the first detection of the western hotspot of Pictor A in the far-UV band using observations from AstroSat-UVIT. The broad-band SED of the western hotspot is explained by a multizone emission scenario where X-ray emission is caused by synchrotron emission process in the substructures embedded in the diffuse region, while the emission in radio to optical is caused by synchrotron emission pr…
The IceCube Neutrino Observatory, located at the geographic South Pole, is a Cherenkov detector that continuously monitors a cubic kilometer of instrumented glacial ice for neutrino interactions in the sub-TeV to EeV energy range. Its primary design goal is the study of powerful astrophysical objects that could act as natural particle accelerators and thus as sources of (ultra) high energy cosmic rays - in short: to do neutrino astronomy. IceCube has discovered a diffuse flux of high energy astrophysical neutrinos consistent with an extra-galactic origin. In addition the IceCube Collaboration …
Although many high-energy neutrinos detected by the IceCube telescope are believed to have anextraterrestrial origin, their astrophysical sources remain a mystery. Recently, an unprecedenteddiscovery of a high-energy muon neutrino event coincident with a multiwavelength flare from ablazar, TXS 0506+056, shed some light on the origin of the neutrinos. It is usually believed that ablazar is produced by a relativistic jet launched from an accreting supermassive black hole (SMBH).Here we show that the high-energy neutrino event can be interpreted by the inelastic hadronuclearinteractions between t…
The last IceCube catalog of High Energy Starting Events (HESE) obtained with a livetime of 1347 days comprises 54 neutrino events equally-distributed between the three families with energies between 25 TeV and few PeVs. Considering the homogeneous flavors distribution (1:1:1) and the spectral features of these neutrinos the IceCube collaboration claims the astrophysical origin of these events with more than $5σ$. The spatial distribution of cited events does not show a clear correlation with known astrophysical accelerators leaving opened both the Galactic and the extra-Galactic origin interpr…
Motivated by the indications of a possible deficit of muon tracks in the first three-year equivalent dataset of IceCube we investigate the possibility that the astrophysical (anti)neutrino flux (in the PeV energy range) could originate from β-decay of relativistic neutrons. We show that to accommodate IceCube observations it is necessary that only about 1% to 10% of the emitted cosmic rays in the energy decade 10^{8.5} \alt E_{CR}/GeV \alt 10^{9.5}$, yielding antineutrinos on Earth (10^{5.5} \alt E_{\bar ν}/GeV \alt 10^{6.5}), are observed. Such a strong suppression can be explained assuming m…
HESS J1534-571 is a very high energy gamma-ray source that was discovered by the H.E.S.S. observatory and reported as one of several new sources with a shell-like morphology at TeV energies, matching in size and location with the supernova remnant G323.7-1.0 discovered in radio observations by the Molonglo Galactic Plane Survey. Many known TeV shells also show X-ray emission, however, no X-ray counterpart has been seen for HESS J1534-571. The detection of a new GeV source using data from the \emph{Fermi} satellite that is compatible in extension with the radio supernova remnant and shows a ver…
In this review I discuss the ultra-high energy neutrinos (UHEN) originated from Cosmic-Rays propogation (GZK neutrinos) and from Gamma Ray Bursts (GRBs), and discuss their detectability in kilometers scale detectors like ARA and IceCube. While GZK neutrinos are expected from cosmic ray interactions on the CMB, the GRB neutrinos depend on the physics inside the sources. GRBs are predicted to emit UHEN in the prompt and in the later 'after-glow' phase. I discuss the constraints on the hadronic component of GRBs derived from the search of four years of IceCube data for a prompt neutrino fux from …
Star-forming galaxies, due to their high star-formation rates and hence large number of supernova remnants therein, are huge reservoirs of cosmic rays (CRs). These CRs collide with gases in the galaxies and produce high-energy neutrinos through $pp$ collisions. In this paper, we calculate the neutrino production efficiency in star-forming galaxies by considering realistic galaxy properties, such as the gas density and galactic wind in star-forming galaxies. To calculate the accumulated neutrino flux, we use the infrared luminosity function of star-forming galaxies obtained by {\em Herschel} PE…
Recently ANTARES collaboration presented a time dependent analysis to a selected number of flaring blazars to look for upward going muon events produced from the charge current interaction of the muon neutrinos. We use the same list of flaring blazars to look for possible positional correlation with the IceCube neutrino events. In the context of photohadronic model we propose that the neutrinos are produced within the nuclear region of the blazar where Fermi accelerated high energy protons interact with the background synchrotron/SSC photons. Although we found that some objects from the ANTARE…
We present a search in IceCube data for neutrino emission from Galactic TeV gamma-ray sources detected by the HAWC gamma-ray observatory. HAWC serves as the excellent instrument to complement IceCube with its energy range extending to very high energies. Assuming that the highest energy photons originate from the decay of pions, rather than from accelerated leptons, the very high energy gamma-rays observed by HAWC are expected to be correlated with neutrinos. Using eight years of IceCube data, we report on two analyses that investigate a possible neutrino--gamma ray correlation. The first is a…
