Abstract
On 2017 September 22, IceCube released a public alert announcing the detection of a well-reconstructed, high-energy neutrino event. Such alerts issued through the Extremely High Energy (EHE) alert stream occur roughly four times a year. Subsequent multimessenger follow-up identified that the event was coincident in direction and time with a gamma-ray flare from the blazar TXS 0506+056. This association prompted an archival analysis searching for additional neutrinos from the direction of TXS 0506+056 using 9.5 years of IceCube neutrino observations. An excess of neutrino events with respect to atmospheric backgrounds was found between September 2014 and March 2015. The analysis yields 3:5σ evidence for neutrino emission from the direction of TXS 0506+056, independent of and prior to the 2017 emission.
Highlights
The IceCube Neutrino Observatory [1] instruments one cubic kilometer of deep glacial ice at the South Pole in order to detect the Cherenkov light from relativistic charged particles created by neutrino interactions in the ice and bedrock
The alert led to a substantial multi-messenger follow-up campaign when it was determined by the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope[5] that the direction of IceCube-170922A coincided with the location of the known blazar TXS 0506+056 during a time while it was in a state of enhanced gamma-ray activity [6]
The time window is incorporated into the likelihood as a probability distribution function (PDF), effectively weighting the events according to their arrival times with respect to the PDF
Summary
The IceCube Neutrino Observatory [1] instruments one cubic kilometer of deep glacial ice at the South Pole in order to detect the Cherenkov light from relativistic charged particles created by neutrino interactions in the ice and bedrock. On 2017 September 22, a high-energy neutrino event, IceCube-170922A, was selected by the Extremely High Energy (EHE) online event filter [2], and reported as a public alert [3]. At the time, such EHE alerts were sent at a rate of about four per year, with the selection threshold set so that approximately half of the events are estimated to be astrophysical neutrinos, the rest being atmospheric background events.
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