Coincidence of a high-fluence blazar outburst with a PeV-energy neutrino event (1602.02012v2)

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 gamma-ray photons 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 (IC35) detected by IceCube. Based on an analysis of the full sample of gamma-ray blazars in the IC35 field and assuming a photo-hadronic emission model, we show that the long-term average gamma-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 paper demonstrates a spatial and temporal correlation between the PKS B1424-418 outburst and the IceCube PeV neutrino event IC-35, indicating a potential causal relation.
• The paper employs a multiwavelength approach using gamma-ray, X-ray, and radio observations to test the photo-hadronic model of neutrino production.
• The paper reinforces the role of gamma-ray blazars as plausible sources of ultra-high-energy cosmic neutrinos by aligning observational data with theoretical predictions.

Coincidence of High-Fluence Blazar Outburst with PeV-Energy Neutrino Event

The paper under discussion presents a significant observational paper that connects high-energy astrophysical phenomena, specifically the coincidence of a high-fluence blazar outburst with a PeV-energy neutrino event detected by the IceCube neutrino observatory. The authors investigate the temporal and spatial correlation between a major outburst in the blazar PKS B1424-418 and the detection of the IceCube event IC-35. This paper operates within the context of a potential origin for PeV neutrinos, supporting the hypothesis that gamma-ray blazars could serve as sources for such high-energy neutrinos.

Overview and Methodology

The discovery of very-high-energy neutrinos by IceCube marks an advancement in neutrino astronomy, with blazars posited as potential sources. The paper employs a multiwavelength approach, utilizing data from gamma-ray, X-ray, and radio observations to identify significant blazar activities coinciding with IceCube's neutrino detections. Specifically, the analysis centers on the blazar PKS B1424-418, which underwent a notable outburst phase in mid-2012 that persisted into 2013, aligned both temporally and spatially with the detection of the neutrino event IC-35.

Key Findings

1. Temporal and Spatial Coincidence: The PKS B1424-418 outburst not only matches the positional uncertainty region of the IC-35 event but also aligns within the timeframe during which the neutrino event was detected. This suggests a potential physical link between the two phenomena.
2. Multiwavelength Emission Analysis: The authors conducted an extensive analysis of gamma-ray data from Fermi/LAT, supplemented by X-ray and radio data. The gamma-ray emission was recorded to be among the highest compared to other blazars during the studied period, suggesting significant hadronic activity likely responsible for the detected neutrinos.
3. Photo-Hadronic Model Compatibility: The findings support the hypothesis that the observed gamma-ray and neutrino emissions are consistent with a photo-hadronic model where interactions between accelerated protons and seed photons in the blazar's jet produce neutrinos.
4. Predictive Analysis: By analyzing the integrated emission of blazars in the relevant sky field, the research corroborates the observed PeV neutrino all-sky flux, aligning the neutrino count with model predictions based on blazar emissions.

Theoretical and Practical Implications

The paper underscores the role of blazars as plausible astrophysical sources of ultra-high-energy cosmic rays and neutrinos, reinforcing the significance of gamma-ray observations in neutrino astrophysics. It also suggests that significant individual blazar outbursts can be indicators of neutrino production. Future directions may include enhancing angular resolution of neutrino detections and accumulating longer-term observations to strengthen the link between specific blazar outbursts and neutrino events.

Concluding Remarks

While the statistical correlation alone does not definitively establish a causal connection, the methodology employed in this paper provides a robust framework for identifying potential extragalactic neutrino sources. As data techniques and observational capabilities progress, such studies will be critical in elucidating the energetic processes underlying cosmic neutrino emissions, further bridging the gap between electromagnetic observations and neutrino astronomy.