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

Abstract

The discovery of extraterrestrial very-high-energy neutrinos by the IceCube collaboration has launched a quest for the identification of their astrophysical sources. Gamma-ray blazars have been predicted to yield a cumulative neutrino signal exceeding the atmospheric background above energies of 100 TeV, assuming that both the neutrinos and the γ-ray photons 2 are produced by accelerated protons in relativistic jets. Since the background spectrum falls steeply with increasing energy, the individual events with the clearest signature of being of an extraterrestrial origin are those at PeV energies. Inside the large positional-uncertainty fields of the first two PeV neutrinos detected by IceCube, the integrated emission of the blazar population has a sufficiently high electromagnetic flux to explain the detected IceCube events, but fluences of individual objects are too low to make an unambiguous source association.Here, we report that a major outburst of the blazar PKS B1424−418 occurred in temporal and positional coincidence with the third PeV-energy neutrino event (IC 35) detected by IceCube. Based on an analysis of the full sample of γ-ray blazars in the IC 35 field and assuming a photo-hadronic emission model, we show that the long-term average γ-ray emission of blazars as a class is in agreement with both the measured all-sky flux of PeV neutrinos and the spectral slope of the IceCube signal. The outburst of PKS B1424−418 has provided an energy output high enough to explain the observed PeV event, indicative of a direct physical association.The neutrino excess detected by IceCube comprises 37 events with energies between 30 TeV and 2 PeV, rejecting a purely atmospheric origin at a significance of 5.7 standard deviations [1][2][3] .These events show a broad distribution across both hemispheres of the sky consistent with an extragalactic source population. Due to the very steep background of atmospheric neutrinos, events at PeV energies are best suited for attempting to establish associations with individual blazars. In the first two years of observations, IceCube detected two events with about 1 PeV of deposited energy 1, 2 (IC 14, and IC 20; dubbed 'Bert' and 'Ernie') the diffuse neutrino flux due to the integrated emission of AGN in a given large field at a given time, as well as the maximum possible neutrino flux associated with an individual object of the sample.Blazars are radio-loud AGN with jets oriented close to the line of sight. This substantially increases the apparent brightness of these objects owing to the Doppler boosting of the emission from the relativistically moving emission zones. A direct association of a PeV-neutrino with an * A different analysis of IceCube muon neutrinos finds an excess signal also from the northern sky 14 . † http://pulsar.sternwarte.uni-erlangen.de/tanami 4 individual γ-ray blazar would have the important implication that a sizeable fraction of their observed γ-ray emission must be due to hadronic decays, and that blazar jets are also sources of ultra-high-energy cos...

The e-ASTROGAM mission

“…Coincidences in time and space of astrophysical neutrino events with exceptional flares of blazars have been proposed by several authors (e.g. [80]). Among the speculated possible origins of the IceCube events are GRBs with jets shocked in surrounding matter.…”

Fermi Non-detections of Four X-Ray Jet Sources and Implications for the IC/CMB Mechanism

“…However, one distinguishing characteristic of hadronic models would be the expected neutrino emission from charged pion or neutron decay resulting from interactions between high energy protons and photons (Mannheim & Biermann 1989). It has been claimed that the bright gamma-ray flare observed in the blazar PKS B1424-418 is hadronic in nature due to the detection of a PeV-energy neutrino with a high degree of positional and temporal coincidence (Kadler et al 2016) by the IceCube neutrino detector. However, the detection rate of IceCube high energy neutrinos is so low that this is not expected to be a very robust means of distinguishing between hadronic and leptonic models for blazars.…”

Multiwavelength follow-up of a rare IceCube neutrino multiplet

“…It is however likely unrelated given the large implied neutrino luminosity. Padovani et al (2016) observe a correlation between extreme blazars and highenergy neutrino events and Kadler et al (2016) found a bright gamma-ray outburst of a blazar which was aligned with a multi PeV neutrino event. However, all of these associations have a chance-coincidence probability of a few percent and are hence not significant detections.…”