Diffuse Flux Of Astrophysical Neutrinos Research Articles

Interpretation of the Diffuse Astrophysical Neutrino Flux in Terms of the Blazar Sequence

Abstract We study whether the diffuse astrophysical neutrino flux can come from blazar jets—a subclass of active galactic nuclei—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 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 γ-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 105—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 γ-ray-neutrino associations per year from the whole population, dominated by BL Lacs with and z ∼ 0.1.

Neutrino Sources from a Multi-Messenger Perspective

The field of high-energy neutrino astronomy is undergoing a rapid evolution. After the discovery of a diffuse flux of astrophysical TeV-PeV neutrinos in 2013, the Ice-Cube observatory has recently found first compelling evidence for neutrino emission from blazars. In this brief review, I will summarize the status of these neutrino observations and highlight the strong role of multi-messenger astronomy for their interpretation.

Analysis of High Energy Starting Events with the KM3NeT/ARCA detector

KM3NeT is a research infrastructure housing the next generation neutrino detectors in the depths of the Mediterranean Sea. The ARCA detector, which is currently under construction, is optimized for neutrino searches from astrophysical sources as well as measurements of the diffuse high energy astrophysical flux. The unambiguous detection of neutrinos of extraterrestrial origin by IceCube has led to the first measurement of a high energy astrophysical neutrino flux. The cutting-edge technology used for the design and construction of KM3NeT Digital Optical Modules along with the properties of sea water allow for a measurement of the neutrino direction with an unpresidented resolution for both track and cascade events. Taking advantage of this angular resolution a method to differentiate track and shower events and a method to reject the atmospheric muon background from track-like events were developed and combined to select a sample of high energy starting events. An analysis for the discovery potential of KM3NeT/ARCA for a diffuse astrophysical neutrino flux using these events is presented.

High Energy Neutrino Astronomy: Where Do We Stand, Where Do We Go?

With the identification of a diffuse flux of astrophysical ("cosmic") neutrinos in the TeV-PeV energy range, IceCube has opened a new window to the Universe. However, the corresponding cosmic landscape is still uncharted: so far, the observed flux does not show any clear association with known source classes. In the present talk, I sketch the way from Baikal-NT200 to IceCube and summarize IceCube's recent astrophysics results. Finally, I describe the present projects to build even larger detectors: GVD in Lake Baikal, KM3NeT in the Mediterranean Sea and IceCube-Gen2 at the South Pole. These detectors will allow studying the high-energy neutrino sky in much more detail than the present arrays permit.

Constraining high-energy neutrino emission from choked jets in stripped-envelope supernovae

There are indications that γ-ray dark objects such as supernovae (SNe) with choked jets, and the cores of active galactic nuclei may contribute to the diffuse flux of astrophysical neutrinos measured by the IceCube observatory. In particular, stripped-envelope SNe have received much attention since they are capable of producing relativistic jets and could explain the diversity in observations of collapsar explosions (e.g., gamma-ray bursts (GRBs), low-luminosity GRBs, and Type Ibc SNe). We use an unbinned maximum likelihood method to search for spatial and temporal coincidences between Type Ibc core-collapse SNe, which may harbor a choked jet, and muon neutrinos from a sample of IceCube up-going track-like events measured from May 2011–May 2012. In this stacking analysis, we find no significant deviation from a background-only hypothesis using one year of data, and are able to place upper limits on the total amount of isotropic equivalent energy that choked jet core-collapse SNe deposit in cosmic rays ℰcr and the fraction of core-collapse SNe which have a jet pointed towards Earth fjet. This analysis can be extended with yet to be made public IceCube data, and the increased amount of optically detected core-collapse SNe discovered by wide field-of-view surveys such as the Palomar Transient Factory and All-Sky Automated Survey for Supernovae. The choked jet SNe/high-energy cosmic neutrino connection can be more tightly constrained in the near future.

An algorithm for the reconstruction of high-energy neutrino-induced particle showers and its application to the ANTARES neutrino telescope

A novel algorithm to reconstruct neutrino-induced particle showers within the ANTARES neutrino telescope is presented. The method achieves a median angular resolution of 6^circ for shower energies below 100 TeV. Applying this algorithm to 6 years of data taken with the ANTARES detector, 8 events with reconstructed shower energies above 10 TeV are observed. This is consistent with the expectation of about 5 events from atmospheric backgrounds, but also compatible with diffuse astrophysical flux measurements by the IceCube collaboration, from which 2–4 additional events are expected. A 90% C.L. upper limit on the diffuse astrophysical neutrino flux with a value per neutrino flavour of text {E}^2cdot Phi ^{90%} = 4.9 cdot 10^{-8},mathrm {GeV} cdot mathrm {cm^{-2} cdot s^{-1}cdot sr^{-1}} is set, applicable to the energy range from 23 TeV to 7.8 PeV, assuming an unbroken text {E}^{-2} spectrum and neutrino flavour equipartition at Earth.

Exploring the Universe with Neutrinos: Recent Results from IceCube

In 2013, the IceCube Neutrino Observatory located at the geographic South Pole detected evidence for a diffuse astrophysical neutrino flux above ∼60 TeV. To this day, IceCube has operated with full detector configuration for more than 6 years. The observed astrophysical neutrino flux has been confirmed with >6σ significance with both events starting within the detector (all flavor) and events traversing through the Earth (νμ charged-current). Somewhat equal flavor ratio of astrophysical neutrinos is expected at Earth assuming standard thorough oscillation. A search for tau neutrinos has been carried out but yielded null result. No neutrino sources have been found to contribute significantly to the diffuse flux at this point. In this paper, we will review the current status of the astrophysical neutrino flux, discuss the quest for neutrino point sources and overview the proposed design and physics potentials of the future IceCube-Gen2.

Astrophysical interpretation of small-scale neutrino angular correlation searches with IceCube

The IceCube Neutrino Observatory has discovered a diffuse all-flavor flux of high-energy astrophysical neutrinos. However, the corresponding astrophysical sources have not yet been identified. Neither significant point sources nor significant angular correlations of event directions have been observed by IceCube or other instruments to date. We present a new method to interpret the non-observation of angular correlations in terms of exclusions on the strength and number of point-like neutrino sources in generic astrophysical scenarios. Additionally, we constrain the presence of these sources taking into account the measurement of the diffuse high-energy neutrino flux by IceCube. We apply the method to two types of astrophysically motivated source count distributions: The first type is obtained by considering the cosmological evolution of the co-moving density of active galaxies, while the second type is directly derived from the gamma ray source count distribution observed by Fermi-LAT. As a result, we constrain the possible parameter space for both types of source count distributions.

LOWERING ICECUBE'S ENERGY THRESHOLD FOR POINT SOURCE SEARCHES IN THE SOUTHERN SKY

ABSTRACT Observation of a point source of astrophysical neutrinos would be a “smoking gun” signature of a cosmic-ray accelerator. While IceCube has recently discovered a diffuse flux of astrophysical neutrinos, no localized point source has been observed. Previous IceCube searches for point sources in the southern sky were restricted by either an energy threshold above a few hundred TeV or poor neutrino angular resolution. Here we present a search for southern sky point sources with greatly improved sensitivities to neutrinos with energies below 100 TeV. By selecting charged-current ν μ interacting inside the detector, we reduce the atmospheric background while retaining efficiency for astrophysical neutrino-induced events reconstructed with sub-degree angular resolution. The new event sample covers three years of detector data and leads to a factor of 10 improvement in sensitivity to point sources emitting below 100 TeV in the southern sky. No statistically significant evidence of point sources was found, and upper limits are set on neutrino emission from individual sources. A posteriori analysis of the highest-energy (∼100 TeV) starting event in the sample found that this event alone represents a 2.8σ deviation from the hypothesis that the data consists only of atmospheric background.

Search for astrophysical tau neutrinos in three years of IceCube data

The IceCube Neutrino Observatory has observed a diffuse flux of TeV-PeV astrophysical neutrinos at 5.7{\sigma} significance from an all-flavor search. The direct detection of tau neutrinos in this flux has yet to occur. Tau neutrinos become distinguishable from other flavors in IceCube at energies above a few hundred TeV, when the cascade from the tau neutrino charged current interaction becomes resolvable from the cascade from the tau lepton decay. This paper presents results from a dedicated search for tau neutrinos with energies between 214 TeV and 72 PeV. The analysis searches for IceCube optical sensors that observe two separate pulses in a single event - one from the tau neutrino interaction, and a second from the tau decay. This is the first IceCube tau neutrino search to be more sensitive to tau neutrinos than to any other neutrino flavor. No candidate events were observed in three years of IceCube data. For the first time, a differential upper limit on astrophysical tau neutrinos is derived around the PeV energy region, which is nearly three orders of magnitude lower in energy than previous limits from dedicated tau neutrino searches.

Capabilities of IceCube's gamma-ray, optical and X-ray follow-up programs

The IceCube neutrino observatory is a 1 km3 detector for Cherenkov light in the ice at the South Pole. Although the presence of a diffuse astrophysical neutrino flux has been confirmed, its origin has yet to be resolved. Given the current constraints on continuous point source searches, transient and variable objects emerge as promising, detectable source candidates. IceCube boosts the sensitivity to these types of sources by alerting third-party observatories of neutrino events clustered in direction and time. This paper will showcase several neutrino-triggered multi-messenger programs in IceCube along with their results and prospects.

Phenomenology of atmospheric neutrinos

The detection of astrophysical neutrinos, certainly a break-through result, introduced new experimental challenges and fundamental questions about acceleration mechanisms of cosmic rays. On one hand IceCube succeeded in finding an unambiguous proof for the existence of a diffuse astrophysical neutrino flux, on the other hand the precise determination of its spectral index and normalization requires a better knowledge about the atmospheric background at hundreds of TeV and PeV energies. Atmospheric neutrinos in this energy range originate mostly from decays of heavy-flavor mesons, which production in the phase space relevant for prompt leptons is uncertain. Current accelerator- based experiments are limited by detector acceptance and not so much by the collision energy. This paper recaps phenomenological aspects of atmospheric leptons and calculation methods, linking recent progress in flux predictions with particle physics at colliders, in particular the Large Hadron Collider.

Flavor Ratio of Astrophysical Neutrinos above 35TeV in IceCube.

A diffuse flux of astrophysical neutrinos above 100 TeV has been observed at the IceCube Neutrino Observatory. Here we extend this analysis to probe the astrophysical flux down to 35 TeV and analyze its flavor composition by classifying events as showers or tracks. Taking advantage of lower atmospheric backgrounds for showerlike events, we obtain a shower-biased sample containing 129 showers and 8 tracks collected in three years from 2010 to 2013. We demonstrate consistency with the (fe:fμ:fτ)⊕≈(1:1:1)⊕ flavor ratio at Earth commonly expected from the averaged oscillations of neutrinos produced by pion decay in distant astrophysical sources. Limits are placed on nonstandard flavor compositions that cannot be produced by averaged neutrino oscillations but could arise in exotic physics scenarios. A maximally tracklike composition of (0:1:0)⊕ is excluded at 3.3σ, and a purely showerlike composition of (1:0:0)⊕ is excluded at 2.3σ.

Search for a Diffuse Flux of Cosmic Neutrinos with ANTARES

The ANTARES neutrino telescope, situated off the French coast at about 2.5km depth in the Mediterranean Sea, is optimized to detect charged leptons induced by neutrinos in the TeV range. Since its full deployment in 2008, modelling and reconstruction of neutrino-induced event signatures have been introduced and developed to obtain a high degree of accuracy. In this work, muon track directional and energy reconstruction have been applied to four years of ANTARES data in the search for a diffuse flux of astrophysical neutrinos from the charged-current interactions of νμ. Reaching a sensitivity which well surpasses the sensitivity of previous ANTARES analyses, a best upper limit of Φν+ν,90%C.L.E2 = 5.1 × 10−8 GeV sr−1 s−1 cm−2 can be set.

A new physics interpretation of the IceCube data

IceCube has recently observed 37 events of TeV–PeV energies. The angular distribution, with a strong preference for downgoing directions, the spectrum, and the small muon to shower ratio in the data cannot be accommodated assuming standard interactions of atmospheric neutrinos. We obtain an excellent fit, however, if a diffuse flux of ultrahigh energy (cosmogenic) neutrinos experiences collisions where only a small fraction of the energy is transferred to the target nucleon. We show that consistent models of TeV gravity or other non-Wilsonian completions of the standard model provide cross sections with these precise features. An increased statistics could clearly distinguish our scenario from the one assumed by IceCube (a diffuse flux of astrophysical neutrinos with a ∝E-2 spectrum) and establish the need for new physics in the interpretation of the data.

Search for a diffuse flux of astrophysical muon neutrinos with the IceCube 59-string configuration

A search for high-energy neutrinos was performed using data collected by the IceCube Neutrino Observatory from May 2009 to May 2010, when the array was running in its 59-string configuration. The data sample was optimized to contain muon neutrino induced events with a background contamination of atmospheric muons of less than 1%. These data, which are dominated by atmospheric neutrinos, are analyzed with a global likelihood fit to search for possible contributions of prompt atmospheric and astrophysical neutrinos, neither of which have yet been identified. Such signals are expected to follow a harder energy spectrum than conventional atmospheric neutrinos. In addition, the zenith angle distribution differs for astrophysical and atmospheric signals. A global fit of the reconstructed energies and directions of observed events is performed, including possible neutrino flux contributions for an astrophysical signal and atmospheric backgrounds as well as systematic uncertainties of the experiment and theoretical predictions. The best fit yields an astrophysical signal flux for $\nu_\mu + \bar\nu_\mu $ of $E^2 \cdot \Phi (E) = 0.25 \cdot 10^{-8} \textrm{GeV} \textrm{cm}^{-2} \textrm{s}^{-1} \textrm{sr}^{-1}$, and a zero prompt component. Although the sensitivity of this analysis for astrophysical neutrinos surpasses the Waxman and Bahcall upper bound, the experimental limit at 90% confidence level is a factor of 1.5 above at a flux of $E^2 \cdot \Phi (E) = 1.44 \cdot 10^{-8} \textrm{GeV} \textrm{cm}^{-2} \textrm{s}^{-1} \textrm{sr}^{-1}$.

A view of prompt atmospheric neutrinos with IceCube

Atmospheric neutrinos are produced in air showers, when cosmic-ray primaries hit the Earthʼs atmosphere and interact hadronically. The conventional neutrino flux, which dominates the neutrino data measured in the GeV to TeV range by neutrino telescopes, is produced by the decay of charged pions and kaons. Prompt atmospheric neutrinos are produced by the decay of heavier mesons typically containing a charm quark. Their production is strongly suppressed, but they are expected to exhibit a harder energy spectrum. Hence, they could dominate the atmospheric neutrino flux at energies above ∼100TeV. Such a prompt atmospheric flux component has not yet been observed. Therefore, it is an interesting signal in a diffuse neutrino search, but also a background in the search for a diffuse astrophysical neutrino flux. The sensitivity of diffuse neutrino searches with the IceCube Neutrino observatory has reached the level of theoretical expectations of prompt neutrino fluxes, and recent results are presented.

Search for a diffuse flux of astrophysical muon neutrinos with the IceCube 40-string detector

The IceCube Neutrino Observatory is a 1 km$^{3}$ detector currently taking data at the South Pole. One of the main strategies used to look for astrophysical neutrinos with IceCube is the search for a diffuse flux of high-energy neutrinos from unresolved sources. A hard energy spectrum of neutrinos from isotropically distributed astrophysical sources could manifest itself as a detectable signal that may be differentiated from the atmospheric neutrino background by spectral measurement. This analysis uses data from the IceCube detector collected in its half completed configuration which operated between April 2008 and May 2009 to search for a diffuse flux of astrophysical muon neutrinos. A total of 12,877 upward going candidate neutrino events have been selected for this analysis. No evidence for a diffuse flux of astrophysical muon neutrinos was found in the data set leading to a 90 percent C.L. upper limit on the normalization of an $E^{-2}$ astrophysical $\nu_{\mu}$ flux of $8.9 \times 10^{-9} \ \mathrm{GeV \ cm^{-2} \ s^{-1} \ sr^{-1}}$. The analysis is sensitive in the energy range between $35 \ \mathrm{TeV} - 7 \ \mathrm{PeV}$. The 12,877 candidate neutrino events are consistent with atmospheric muon neutrinos measured from 332 GeV to 84 TeV and no evidence for a prompt component to the atmospheric neutrino spectrum is found.

Upper limit on the diffuse flux of cosmic [formula omitted] with the ANTARES Neutrino Telescope

A search for a diffuse flux of astrophysical muon neutrinos, using data collected by the ANTARES neutrino telescope from December 2007 to December 2009 is presented. A ( 0.83 × 2 π ) sr sky was monitored for a total of 334 days of equivalent live time. The searched signal corresponds to an excess of events, produced by astrophysical sources, over the expected atmospheric neutrino background without any particular assumption on the source direction. the analysis are described. Since the number of detected events is compatible with the number of expected background events, a 90% c.l. upper limit on the diffuse ν μ flux with a E − 2 spectrum is set at E 2 Φ 90 % = 5.3 × 10 − 8 GeV cm − 2 s − 1 sr − 1 in the energy range 20 TeV – 2.5 PeV. Other signal models with different energy shape were also tested and some rejected.

Search for a diffuse flux of high-energy [formula omitted] with the ANTARES neutrino telescope

A search for a diffuse flux of astrophysical muon neutrinos, using data collected by the ANTARES neutrino telescope is presented. A (0.83×2π) sr sky was monitored for a total of 334 days of equivalent live time. The searched signal corresponds to an excess of events, produced by astrophysical sources, over the expected atmospheric neutrino background. The observed number of events is found compatible with the background expectation. Assuming an E−2 flux spectrum, a 90% c.l. upper limit on the diffuse νμ flux of E2Φ90%=5.3×10−8 GeVcm−2s−1sr−1 in the energy range 20 TeV–2.5 PeV is obtained. Other signal models with different energy spectra are also tested and some rejected.