High-energy Neutrino Production Research Articles

Establishing accretion flares from supermassive black holes as a source of high-energy neutrinos

ABSTRACT The origin of cosmic high-energy neutrinos remains largely unexplained. For high-energy neutrino alerts from IceCube, a coincidence with time-variable emission has been seen for three different types of accreting black holes: (1) a gamma-ray flare from a blazar (TXS 0506+056), (2) an optical transient following a stellar tidal disruption event (TDE; AT2019dsg), and (3) an optical outburst from an active galactic nucleus (AGN; AT2019fdr). For the latter two sources, infrared follow-up observations revealed a powerful reverberation signal due to dust heated by the flare. This discovery motivates a systematic study of neutrino emission from all supermassive black hole with similar dust echoes. Because dust reprocessing is agnostic to the origin of the outburst, our work unifies TDEs and high-amplitude flares from AGN into a population that we dub accretion flares. Besides the two known events, we uncover a third flare that is coincident with a PeV-scale neutrino (AT2019aalc). Based solely on the optical and infrared properties, we estimate a significance of 3.6σ for this association of high-energy neutrinos with three accretion flares. Our results imply that at least ∼10 per cent of the IceCube high-energy neutrino alerts could be due to accretion flares. This is surprising because the sum of the fluence of these flares is at least three orders of magnitude lower compared to the total fluence of normal AGN. It thus appears that the efficiency of high-energy neutrino production in accretion flares is increased compared to non-flaring AGN. We speculate that this can be explained by the high Eddington ratio of the flares.

Hadronic Processes at Work in 5BZB J0630−2406

Recent observations are shedding light on the important role that active galactic nuclei play in the production of high-energy neutrinos. In this study, we focus on one object, 5BZB J0630−2406, which is among the blazars recently proposed as associated with neutrino emission during the first 7 yr of IceCube observations. Modeling the quasi-simultaneous, broadband spectral energy distribution, we explore various scenarios from purely leptonic to leptohadronic models, testing the inclusion of external photon fields. This theoretical study provides a complementary testing ground for the proposed neutrino–blazar association. Despite being historically classified as a BL Lac, our study shows that 5BZB J0630−2406 belongs to the relatively rare subclass of high-power flat-spectrum radio quasars. Our results indicate that interactions between protons and external radiation fields can produce a neutrino flux that is within the reach of the IceCube detector. Furthermore, the spectral shape of the X-ray emission suggests the imprint of hadronic processes related to very energetic protons.

High-energy Neutrino Emission Associated with GWs from Binary Black Hole Mergers in AGN Accretion Disks

The search for multimessenger signals of binary black hole (BBH) mergers is crucial to understanding the merger process of BBH and the relative astrophysical environment. Considering BBH mergers occurring in the active galactic nuclei (AGN) accretion disks, we focus on the accompanying high-energy neutrino production from the interaction between the jet launched by the postmerger remnant BH and disk materials. Particles can be accelerated by the shocks generated from the jet–disk interaction and subsequently interact with the disk gas and radiations to produce high-energy neutrinos through hadronic processes. We demonstrate that the identification of the high-energy neutrino signal from BBH merger in AGN disks is feasible. In addition, the joint BBH gravitational wave (GW) and neutrino detection rate is derived, which can be used to constrain the BBH merger rate and the accretion rate of the remnant BH based on the future associated detections of GWs and neutrinos. To date, an upper limit of BBH merger rate density in AGN disks of R 0 ≲ 3 Gpc−3 yr−1 is derived for the fiducial parameter values based on the current null association of GWs and neutrinos.

Repeated patterns of gamma-ray flares suggest structured jets of blazars as likely neutrino sources

ABSTRACT Fermi-Large Area Telescope (LAT) observations provide continuous and regularly sampled measurements of gamma-ray photon flux for hundreds of blazars. Many of these light curves, spanning almost 15 yr, have been thoroughly examined for periodicity in multiple studies. However, the possibility that blazars may exhibit irregularly repeating flaring patterns in their gamma-ray light curves has not been systematically explored. In this study, we aim to find repeating episodes of flaring activity in the 100 brightest blazars using Fermi-LAT light curves with various integration times. We use a Bayesian Blocks representation to convert the time series into strings of symbols and search for repeating sub-strings using a fuzzy search algorithm. As a result, we identify 27 repeated episodes in the gamma-ray light curves of 10 blazars. We find that the patterns are most likely produced in structured jets composed of a fast spine and a slower sheath. When individual emission features propagate in the spine, they scatter seed photons produced in the non-uniform sheath through the inverse Compton mechanism, resulting in a set of gamma-ray flares with a similar profile every such passage. Additionally, we explore the theoretically predicted possibility that the spine-sheath structure facilitates the production of high-energy neutrinos in blazar jets. Using the catalogue of track-like events detected by the IceCube neutrino telescope, we find evidence supporting this hypothesis at a 2.8σ significance level.

High-energy Neutrino Production from AGN Disk Transients Impacted by the Circum-disk Medium

Various supernovae, compact object coalescences, and tidal disruption events are widely believed to occur embedded in active galactic nucleus (AGN) accretion disks and generate detectable electromagnetic signals. We collectively refer to them as AGN disk transients. The inelastic hadronuclear (pp) interactions between shock-accelerated cosmic rays and AGN disk materials shortly after the ejecta shock breaks out of the disk can produce high-energy neutrinos. However, the expected efficiency of neutrino production would decay rapidly by adopting a pure Gaussian density atmosphere profile applicable for stable gas-dominated disks. On the other hand, AGN outflows and disk winds are commonly found around AGN accretion disks. In this paper, we show that the circum-disk medium would further consume the shock kinetic energy to more efficiently produce high-energy neutrinos, especially for ∼ TeV−PeV neutrinos that IceCube detects. Thanks to the existence of the circum-disk medium, we find that the neutrino production will be enhanced significantly and make a much higher contribution to the diffuse neutrino background. Optimistically, ∼20% of the diffuse neutrino background can be contributed by AGN disk transients.

Production of high-energy neutrinos in binary-neutron-star merger events

High-energy neutral astrophysical messengers, such as neutrinos and photons, can be produced by the interaction of ultra-high-energy cosmic rays (UHECRs) with radiation fields, either during extragalactic propagation or within source environments. Neutrinos and gamma-rays can play a crucial role in the study of acceleration mechanisms of cosmic rays. In particular, after being produced, neutrinos leave the source environment and propagate to the Earth without further interactions. They are only subject to energy redshift and flavour oscillation, which makes them bearers of otherwise inaccessible information about their sources. We study high-energy environments of the type that are likely to be the end states of a binary-neutron-star (BNS) merger, and we model their local photon field as a black body at a given temperature. Using a modified version of the Monte Carlo code SimProp v2r4 we simulate the propagation and interaction of UHECRs through these environments. We consider several combinations for the spectral index and high-energy cutoff of the UHECR protons, in order to obtain the escaped neutrino flux. We propagate these fluxes to the Earth and compare to the astrophysical IceCube neutrino flux to obtain constraints on the BNS merger spectra properties, emissivity and density rate.

Assessing coincident neutrino detections using population models

Several tentative associations between high-energy neutrinos and astrophysical sources have been recently reported, but a conclusive identification of these potential neutrino emitters remains challenging. We explore the use of Monte Carlo simulations of source populations to gain deeper insight into the physical implications of proposed individual source–neutrino associations. In particular, we focus on the IC170922A–TXS 0506+056 observation. Assuming a null model, we find a 7.6% chance of mistakenly identifying coincidences between γ-ray flares from blazars and neutrino alerts in 10-year surveys. We confirm that a blazar–neutrino connection based on the γ-ray flux is required to find a low chance coincidence probability and, therefore, a significant IC170922A–TXS 0506+056 association. We then assume this blazar–neutrino connection for the whole population and find that the ratio of neutrino to γ-ray fluxes must be ≲10−2 in order not to overproduce the total number of neutrino alerts seen by IceCube. For the IC170922A–TXS 0506+056 association to make sense, we must either accept this low flux ratio or suppose that only some rare sub-population of blazars is capable of high-energy neutrino production. For example, if we consider neutrino production only in blazar flares, we expect the flux ratio of between 10−3 and 10−1 to be consistent with a single coincident observation of a neutrino alert and flaring γ-ray blazar. These constraints should be interpreted in the context of the likelihood models used to find the IC170922A–TXS 0506+056 association, which assumes a fixed power-law neutrino spectrum of E−2.13 for all blazars.

First constraints on axionlike particles from Galactic sub-PeV gamma rays

Experimental refinements and technical innovations in the field of extensive air shower telescopes have enabled measurements of Galactic cosmic-ray interactions in the sub-PeV range, providing new avenues for the search for new physics and dark matter. For the first time, we exploit sub-PeV (1 TeV -- 1 PeV) observations of Galactic diffuse gamma rays by HAWC and Tibet AS$\gamma$ to search for an axion-like-particle (ALP) induced gamma-ray signal directly linked to the origin of the IceCube extragalactic high-energy neutrino flux. Indeed, the production of high-energy neutrinos in extragalactic sources implies the concomitant production of gamma rays at comparable energies. Within the magnetic field of the neutrino emitting sources, gamma rays may efficiently convert into ALPs, escape their host galaxy un-attenuated, propagate through intergalactic space, and reconvert into gamma rays in the magnetic field of the Milky Way. Such a scenario creates an all-sky diffuse high-energy gamma-ray signal in the sub-PeV range. Accounting for the guaranteed Galactic astrophysical gamma-ray contributions from cosmic-ray interactions with gas and radiation and from sub-threshold sources, we set competitive upper limits on the photon-ALP coupling constant $g_{a\gamma\gamma}$. We find $g_{a\gamma\gamma} < 2.1\times10^{-11}$ GeV$^{-1}$ for ALP masses $m_a \leq 2\times10^{-7}$ eV at a 95\% confidence level. Our results are comparable to previous limits on ALPs derived from the TeV gamma-ray domain and progressively close the mass gap towards ADMX limits. The code and data to reproduce the results of this study are available on GitHub \url{https://github.com/ceckner/subPeVALPs}.

Search for neutrino emission from cores of active galactic nuclei

The sources of the majority of the high-energy astrophysical neutrinos observed with the IceCube neutrino telescope at the South Pole are unknown. So far, only a gamma-ray blazar was compellingly associated with the emission of high-energy neutrinos. In addition, several studies suggest that the neutrino emission from the gamma-ray blazar population only accounts for a small fraction of the total astrophysical neutrino flux. In this work we probe the production of high-energy neutrinos in the cores of Active Galactic Nuclei (AGN), induced by accelerated cosmic rays in the accretion disk region. We present a likelihood analysis based on eight years of IceCube data, searching for a cumulative neutrino signal from three AGN samples created for this work. The neutrino emission is assumed to be proportional to the accretion disk luminosity estimated from the soft X-ray flux. Next to the observed soft X-ray flux, the objects for the three samples have been selected based on their radio emission and infrared color properties. For the largest sample in this search, an excess of high-energy neutrino events with respect to an isotropic background of atmospheric and astrophysical neutrinos is found, corresponding to a post-trial significance of 2.60$\sigma$. Assuming a power-law spectrum, the best-fit spectral index is $2.03^{+0.14}_{-0.11}$, consistent with expectations from particle acceleration in astrophysical sources. If interpreted as a genuine signal with the assumptions of a proportionality of X-ray and neutrino fluxes and a model for the sub-threshold flux distribution, this observation implies that at 100 TeV, 27$\%$ - 100$\%$ of the observed neutrinos arise from particle acceleration in the core of AGN.

High-Energy Neutrinos from Starburst galaxies: implications for neutrino astronomy

Starburst and Starforming galaxies are astrophysical sources characterised by an intense star-forming activity. They are reservoirs of cosmic rays and host a high density target gas in the central region called Starburst Nucleus (SBN). In this contribution, we present the results of two model studies regarding the high-energy neutrino production. For the first one, we perform a diffuse analysis. In particular, we consider a blending of spectral indexes and obtain a multi-component description of extragalactic background light (EGB), high energy starting events (HESE) and high-energy cascade IceCube data. Remarkably, we find that, differently from recent prototype scenarios, the spectral index blending allows starburst galaxies to account for up to 40% of the HESE events at 95.4% CL and favors a maximal energy of the accelerated cosmic rays at teens of PeV. For the second one, we present a point-like analysis regarding nearby SFGs. We apply the calorimetric approach to the known SBGs within 100 Mpc, considering, were possible, a source-by-source description of the star formation rate. These results are then compared with what IceCube has observed at TeV energies as well as with what can be expected from the incoming KM3NeT and IceCube gen 2.

Simulating the Galactic multi-messenger emissions with HERMES

Context. The study of nonthermal processes such as synchrotron emission, inverse Compton scattering, bremsstrahlung, and pion production is crucial to understanding the properties of the Galactic cosmic-ray population, to shed light on their origin and confinement mechanisms, and to assess the significance of exotic signals possibly associated to new physics. Aims. We present a public code called HERMES which is designed generate sky maps associated to a variety of multi-messenger and multi-wavelength radiative processes, spanning from the radio domain all the way up to high-energy gamma-ray and neutrino production. Methods. We describe the physical processes under consideration, the code concept and structure, and the user interface, with particular focus on the python-based interactive mode. In particular, present the modular and flexible design that allows the user to easily extend the numerical package according to their needs. Results. In order to demonstrate the capabilities of the code, we describe the details of a comprehensive set of sky maps and spectra associated to all physical processes included in the code. We comment in particular on the radio, gamma-ray, and neutrino maps, and mention the possibility of studying signals stemming from dark matter annihilation. Conclusions. HERMES can be successfully applied to constrain the properties of the Galactic cosmic-ray population, improve our understanding of the diffuse Galactic radio, gamma-ray, and neutrino emission, and search for signals associated to particle dark matter annihilation or decay.

Observation of Photons above 300 TeV Associated with a High-energy Neutrino from the Cygnus Region

Abstract Galactic sites of acceleration of cosmic rays to energies of order 1015 eV and higher, dubbed PeVatrons, reveal themselves by recently discovered gamma radiation of energies above 100 TeV. However, joint gamma-ray and neutrino production, which marks unambiguously cosmic-ray interactions with ambient matter and radiation, was not observed until now. In 2020 November, the IceCube neutrino observatory reported an ∼150 TeV neutrino event from the direction of one of the most promising Galactic PeVatrons, the Cygnus Cocoon. Here we report on the observation of a 3.1σ (post-trial) excess of atmospheric air showers from the same direction, observed by the Carpet–2 experiment and consistent with a few months flare in photons above 300 TeV, in temporal coincidence with the neutrino event. The fluence of the gamma-ray flare is of the same order as that expected from the neutrino observation, assuming the standard mechanism of neutrino production. This is the first evidence for the joint production of high-energy neutrinos and gamma-rays in a Galactic source.

High-energy neutrino production in clusters of galaxies

ABSTRACT Clusters of galaxies can potentially produce cosmic rays (CRs) up to very high energies via large-scale shocks and turbulent acceleration. Due to their unique magnetic-field configuration, CRs with energy ≤1017 eV can be trapped within these structures over cosmological time-scales, and generate secondary particles, including neutrinos and gamma rays, through interactions with the background gas and photons. In this work, we compute the contribution from clusters of galaxies to the diffuse neutrino background. We employ 3D cosmological magnetohydrodynamical simulations of structure formation to model the turbulent intergalactic medium. We use the distribution of clusters within this cosmological volume to extract the properties of this population, including mass, magnetic field, temperature, and density. We propagate CRs in this environment using multidimensional Monte Carlo simulations across different redshifts (from z ∼ 5 to z = 0), considering all relevant photohadronic, photonuclear, and hadronuclear interaction processes. We find that, for CRs injected with a spectral index α = 1.5–2.7 and cutoff energy Emax = 1016–5 × 1017 eV, clusters contribute to a sizeable fraction to the diffuse flux observed by the IceCube Neutrino Observatory, but most of the contribution comes from clusters with M ≳ 1014 M⊙ and redshift z ≲ 0.3. If we include the cosmological evolution of the CR sources, this flux can be even higher.

A new parton fragmentation procedure for heavy hadron production in proton–proton collisions

We propose a new procedure for parton-to-hadron fragmentation in proton–proton collisions. The hadronization is considered in the parton-parton center-of-mass (CM) system and hadrons are assumed to be emitted in the direction of outgoing partons in that frame and then their four-momenta are transformed to the overall CM system using relevant Lorentz transformations and appropriate distributions are constructed. For heavy hadron production the energy–momentum condition is imposed. In many cases our procedure disagrees with the commonly used rule that rapidity of produced hadron is the same as rapidity of the parent parton. We illustrate the newly proposed scheme for production of D mesons, Λ c baryon and η c quarkonium in leading-order approach to parton production. The transverse momentum, rapidity and Feynman-x F distributions are shown. We consider c → D and g → D as well as c → η c and g → η c hadronization processes. The resulting x F distributions are much narrower than those obtained in the traditional approach with parton–hadron rapidity equivalence. This has consequences both for mid- and forward rapidities and could have consequences for high-energy neutrino production in the Earth’s atmosphere. We discuss also and fragmentation and find that the second mechanism could be partly responsible for the enhanced production of observed recently by the ALICE collaboration.

High-energy Neutrino and Gamma-Ray Emission from Tidal Disruption Events

Abstract Tidal disruption events (TDE) have been considered as cosmic-ray and neutrino sources for a decade. We suggest two classes of new scenarios for high-energy multi-messenger emission from TDEs that do not have to harbor powerful jets. First, we investigate high-energy neutrino and gamma-ray production in the core region of a supermassive black hole. In particular, we show that ∼1–100 TeV neutrinos and MeV gamma rays can efficiently be produced in hot coronae around an accretion disk. We also study the consequences of particle acceleration in radiatively inefficient accretion flows (RIAFs). Second, we consider possible cosmic-ray acceleration by sub-relativistic disk-driven winds or interactions between tidal streams, and show that subsequent hadronuclear and photohadronic interactions inside the TDE debris lead to GeV-PeV neutrinos and sub-GeV cascade gamma rays. We demonstrate that these models should be accompanied by soft gamma rays or hard X-rays as well as optical/UV emission, which can be used for future observational tests. Although this work aims to present models of non-jetted high-energy emission, we discuss the implications of the TDE AT2019dsg that might coincide with the high-energy neutrino IceCube-191001A, by considering the corona, RIAF, hidden sub-relativistic wind, and hidden jet models. It is not yet possible to be conclusive about their physical association and the expected number of neutrinos is typically much less than unity. We find that the most optimistic cases of the corona and hidden wind models could be consistent with the observation of IceCube-191001A, whereas jet models are unlikely to explain the multi-messenger observations.

Intrinsic charm in the nucleon and charm production at large rapidities in collinear, hybrid and kT-factorization approaches

We discuss the role of intrinsic charm (IC) in the nucleon for forward production of c-quark (or overline{c} -antiquark) in proton-proton collisions for low and high energies. The calculations are performed in collinear-factorization approach with on-shell partons, kT-factorization approach with off-shell partons as well as in a hybrid approach using collinear charm distributions and unintegrated (transverse momentum dependent) gluon distributions. For the collinear-factorization approach we use matrix elements for both massless and massive charm quarks/antiquarks. The distributions in rapidity and transverse momentum of charm quark/antiquark are shown for a few different models of IC. Forward charm production is dominated by gc-fusion processes. The IC contribution dominates over the standard pQCD (extrinsic) gg-fusion mechanism of coverline{c} -pair production at large rapidities or Feynman-xF. We perform similar calculations within leading-order and next-to-leading order kT-factorization approach. The kT-factorization approach leads to much larger cross sections than the LO collinear approach. At high energies and large rapidities of c-quark or overline{c} -antiquark one tests gluon distributions at extremely small x. The IC contribution has important consequences for high-energy neutrino production in the Ice-Cube experiment and can be, to some extent, tested at the LHC by the SHIP and FASER experiments by studies of the ντ neutrino production.

Multimessenger Implications of AT2018cow: High-energy Cosmic-Ray and Neutrino Emissions from Magnetar-powered Superluminous Transients

Abstract Newly born, rapidly spinning magnetars have been invoked as the power sources of superluminous transients, including the class of “fast blue optical transients” (FBOTs). The extensive multiwavelength analysis of AT2018cow, the first FBOT discovered in real time, is consistent with the magnetar scenario and offers an unprecedented opportunity to comprehend the nature of these sources and assess their broader implications. Using AT2018cow as a prototype, we investigate high-energy neutrino and cosmic-ray production from FBOTs and the more general class of superluminous supernovae (SLSNe). By calculating the interaction of cosmic rays and the time-evolving radiation field and baryon background, we find that particles accelerated in the magnetar wind may escape the ejecta at ultrahigh energies. The predicted high-energy neutrino fluence from AT2018cow is below the sensitivity of the IceCube Observatory, and estimates of the cosmically integrated neutrino flux from FBOTs are consistent with the extreme-high-energy upper limits posed by IceCube. High-energy γ rays exceeding GeV energies are obscured for the first months to years by thermal photons in the magnetar nebula, but are potentially observable at later times. Given their potentially higher volumetric rate compared to other engine-powered transients (e.g., SLSNe and gamma-ray bursts), we conclude that FBOTs are favorable targets for current and next-generation multimessenger observatories.

Describing correlated observations of neutrinos and gamma-ray flares from the blazar TXS 0506+056 with a proton blazar model

Recent detection of the neutrino event IceCube-170922A by the IceCube Observatory from the blazar TXS $0506+056$ in the state of enhanced gamma-ray emission indicates the acceleration of cosmic rays in the blazar jet. The nondetection of the broadline emission in the optical spectrum of TXS $0506+056$ and other BL Lac objects suggests that external photon emissions are weak, and hence, photo-meson ($p\ensuremath{\gamma}$) interaction may not be a favored mechanism for high-energy neutrino production. The lack of broadline signatures also creates doubt about the presence of a high density cloud in the vicinity of the supermassive black hole of TXS $0506+056$ and consequently raised question on hadronuclear ($pp$) interaction interpretation like relativistic jet meets with high density cloud. Here we demonstrate that nonrelativistic protons in the proton blazar model---those that come into existence under the charge neutrality condition of the blazar jet---offer sufficient target matter for $pp$ interaction with shock-accelerated protons, and consequently, the model can describe consistently the observed high-energy gamma rays and neutrino signal from the blazar TXS $0506+056$.

Cosmic ray transport and radiative processes in nuclei of starburst galaxies

AbstractThe high rate of star formation and supernova explosions of starburst galaxies make them interesting sources of high-energy radiation. Depending on the level of turbulence present in their interstellar medium, the bulk of cosmic rays produced inside starburst galaxies may lose most of their energy before escaping, thereby making these sources behave as calorimeters, at least up to some maximum energy. Contrary to previous studies, here we investigate in detail the conditions under which cosmic ray confinement may be effective for electrons and nuclei and we study the implications of cosmic ray confinement in terms of multifrequency emission from starburst nuclei and production of high-energy neutrinos. The general predictions are then specialized to three cases of active starbursts, namely, M82, NGC 253, and Arp220. Both primary and secondary electrons, as well as electron–positron pairs produced by gamma-ray absorption inside starburst galaxies are taken into account. Electrons and positrons produced as secondary products of hadronic interactions are found to be responsible for most of the emission of leptonic origin. In particular, synchrotron emission of very high energy secondary electrons produces an extended emission of hard X-rays that represents a very interesting signature of hadronic process in starburst galaxies, potentially accessible to current and future observations in the X-ray band. A careful understanding of both the production and absorption of gamma-rays in starburst galaxies is instrumental to the assessment of the role of these astrophysical sources as sources of high-energy astrophysical neutrinos.

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.