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


The astrophysical sources of the extraterrestrial, very high-energy neutrinos detected by the IceCube collaboration remain to be identified. Gamma-ray (γ-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 are produced by accelerated protons in relativistic jets. As the background spectrum falls steeply with increasing energy, the individual events with the clearest signature of being of extraterrestrial origin are those at petaelectronvolt energies. Inside the large positional-uncertainty fields of the first two petaelectronvolt 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 a third petaelectronvolt-energy neutrino event (HESE-35) detected by IceCube. On the basis of an analysis of the full sample of γ-ray blazars in the HESE-35 field, 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 petaelectronvolt neutrinos and the spectral slope of the IceCube signal. The outburst of PKS B1424–418 provides an energy output high enough to explain the observed petaelectronvolt event, suggestive of a direct physical association.

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Figure 1: TANAMI γ-ray and radio monitoring of PKS B1424-418.
Figure 2: Dynamic SED of PKS B1424–418.


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The authors thank B. Lott, L. Baldini, P. Bruel, S. Digel, J. Finke, D. Gasparini, N. Omodei, J. S. Perkins and A. Reimer for discussions that have significantly improved this publication. We acknowledge support and partial funding by the Deutsche Forschungsgemeinschaft grant WI 1860-10/1 (TANAMI) and GRK 1147, Deutsches Zentrum für Luft- und Raumfahrt grant 50 OR 1311/50 OR 1303/50 OR 1401, the German Ministry for Education and Research (BMBF) grants 05A11WEA and 05A14WE3, the Helmholtz Alliance for Astroparticle Physics (HAP), the Spanish MINECO project AYA2012-38491-C02-01, the Generalitat Valenciana project PROMETEOII/2014/057, the COST MP0905 action ‘Black Holes in a Violent Universe’ and NASA through Fermi Guest Investigator grants NNH09ZDA001N, NNH10ZDA001N, NNH12ZDA001N and NNH13ZDA001N. This study made use of data collected by the Australian Long Baseline Array (LBA) and the AuScope initiative. The LBA is part of the Australia Telescope National Facility, which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. AuScope Ltd is funded under the National Collaborative Research Infrastructure Strategy (NCRIS), an Australian Commonwealth Government Programme. This paper made use of data from the ALMA calibrator database: ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This paper also made use of up-to-date SMARTS optical/near-infrared light curves that are available at The Fermi–LAT Collaboration acknowledges support for LAT development, operation and data analysis from NASA and DOE (United States), CEA/Irfu and IN2P3/CNRS (France), ASI and INFN (Italy), MEXT, KEK, and JAXA (Japan), and the K. A. Wallenberg Foundation, the Swedish Research Council and the National Space Board (Sweden). Science analysis support in the operations phase from INAF (Italy) and CNES (France) is also gratefully acknowledged. We thank J. E. Davis and T. Johnson for the development of the slxfig module and the SED scripts that have been used to prepare the figures in this work.

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Authors and Affiliations



The TANAMI programme is coordinated by R.O. and M.Kadler. F.K. led the multiwavelength data analysis and modelled the SED. K.M. led the theoretical interpretation of the SED data. C.M., R.S., J.T., B.C., A.Ka. (Univ. Würzburg), E.R., R.O. and M.Kadler analysed the LBA data. J.W. and N.G. were responsible for X-ray observations and data analysis. T.B., S.B., C.G., C.M., D.E.G., A.Kreikenbohm, K.L., E.L., F.L., T.S. and J.A.Z. contributed to the analysis and discussion of radio, optical/ultraviolet, X-ray and γ-ray data. LBA observations were conducted by P.G.E., S.G., H.H., S.H., J.E.J.L., T.N., C.Phillips, C.Plötz., J.Q., J.S., A.K.T. and S.W. Hard X-ray data were reduced and analysed by T.B., I.K., W.B. and M.L. G.A., T.E., D.E., C.W.J., A.Ka. (ECAP), U.K. and M.Kreter contributed to the discussion of neutrino astronomy aspects. M.Kadler, F.W.S. and J.S. led the neutrino-velocity discussion. D.J.T., R.O. and M.Kadler coordinated the TANAMI–LAT collaboration liaison. All authors discussed the results and commented on the manuscript.

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Correspondence to M. Kadler.

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Kadler, M., Krauß, F., Mannheim, K. et al. Coincidence of a high-fluence blazar outburst with a PeV-energy neutrino event. Nature Phys 12, 807–814 (2016).

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