IceCube is gigaton neutrino detector mainly sensitive to TeV energies and above. But DeepCore, a multimegaton infill array, enhances IceCube’s performance by lowering the energy threshold to 10 GeV, allowing for neutrino oscillation studies and improved dark matter searches. And, as we are learning now, low-energy neutrinos also enlarge IceCube’s discovery potential for transient astrophysical neutrino sources.
In a paper submitted today to the Astrophysical Journal, the IceCube Collaboration presents results of a search for astrophysical sources of transient neutrino emission using a sample of low-energy—30 to 300 GeV—muon neutrino events from DeepCore. Although no source is singled out, the study sets limits on soft-spectra models, such as energetic or nearby choked GRBs.
Astrophysicists are getting more and more excited by a possible near-future discovery of new transient sources, especially those also emitting neutrinos. IceCube has already performed several searches for energies above 1 TeV that, so far, have not identified any source, but results are promising as data-taking continues and analysis techniques are improved. However, little is known about astrophysical sources at lower energies, to which Super-Kamiokande has a limited sensitivity due to it’s small size—50,000 tons is still small when talking about neutrino telescopes!—and that IceCube had barely explored since it performs better at higher energies.
Now, a new data sample from the Antarctic observatory allows exploring the low-energy regime using DeepCore data taken from May 2012 to April 2013. “By using these muon neutrinos from DeepCore, we are able to search for astrophysical sources in this underexplored energy range for the first time,” says Jacob Daughhetee, an IceCube researcher at Georgia Institute of Technology and one of the corresponding authors of this work. The searches looked for events at short emission timescales, i.e, from a few days to under a day, such as active galactic nuclei (AGN) flares or some GRB models, especially the so-called choked GRBs. This is a type of core-collapse supernova, where the associated relativistic jets do not have enough energy to break through the stellar envelope thus suppressing gamma-ray emission.
A point source scan of the Northern sky found the most significant flare at right ascension 268.75˚ and declination 54.25˚, but it was shown to be consistent with the background-only hypothesis.
Using this null result, IceCube researchers set an upper limit on the time-integrated neutrino flux produced by possible generic soft-spectrum neutrino flares and by emission from some choked GRB models.
“With the current search, we can place constraints on how often choked GRBs occur within a few mega-parsecs and therefore study the relationship between supernovae and GRBs,” explains Ignacio Taboada, who leads the IceCube team at Georgia Institute of Technology and is also a corresponding author of this paper. “Within 10 mega-parsecs, very close in cosmological scales, there are approximately 500 galaxies but, so far, we have not yet seen a choked GRB.”
+ Info: “Search for Transient Astrophysical Neutrino Emission with IceCube-DeepCore,” IceCube Collaboration: M. G. Aartsen et al. The Astrophysical Journal 816 (2016) 75, iopscience.iop.org arxiv.org/abs/1509.05029