Over the past decade, the burgeoning field of neutrino astronomy has made huge strides, from the first indication of very high energy neutrinos coming from outside our solar system announced in 2013 to the first observed extragalactic source of high-energy neutrinos in 2017. But there is still a lot to learn about these mysterious, lightweight subatomic particles and what they can teach us about the universe.
The IceCube Neutrino Observatory, an array of over 5,000 light detectors embedded in a cubic kilometer of ice at the South Pole, is the largest neutrino telescope on Earth. In the past, most IceCube searches for astrophysical neutrinos focused on high-energy neutrinos, specifically, neutrinos with energies between 1012 and 1015 electronvolts (TeV to PeV). Other neutrino detectors around the world, such as Super-Kamiokande in Japan, carry out searches for neutrinos with lower energies.
In a paper submitted recently to the Journal of Cosmology and Astroparticle Physics, the IceCube Collaboration describes a search for sub-TeV neutrino emission from astrophysical “transient” sources, which are sources that emit neutrinos primarily within a relatively short window of time. This is the first transient result from IceCube to use all neutrino flavors in the 1-100 GeV energy region. In the absence of any observed sources in three years of archival IceCube data, the researchers established new limits on the number of transients in a volume of space per year, known as the volumetric rate of transients.
Gamma-ray bursts (GRBs) are quick, extremely energetic explosions of gamma ray light that are frequently explored as possible transient sources of neutrinos. Some scenarios predict that GRBs emit neutrinos of low energies (~10-100 GeV) without the usual gamma ray counterpart. This could happen, for example, if relativistic GRB jets are “choked off” by a dense envelope of matter before they become visible via their bright gamma-ray display; even if gamma rays cannot make it out, neutrinos (especially low-energy neutrinos) can get through and reach Earth.
So, using neutrino data collected with the IceCube-DeepCore subarray between April 2012 and May 2015, IceCube collaborators searched for any low-energy neutrinos that were coincident in time and direction in a way that indicated a neutrino emission from a transient astrophysical phenomenon. Their analysis consisted of two parts: First, the researchers selected time windows in which there were particularly large densities of neutrino events. Next, they looked within each time window for neutrino events that came from points in the sky that were spatially close.
Among the three years of archival data, the researchers found no transient neutrino emission. Consequently, they were able to place an upper limit on the volumetric rate of astrophysical transient sources.
When the researchers compared their current upper limit to theoretical estimates, they found that they would need a significantly larger detector or improvements in analysis techniques in order to detect a transient point source by chance. Fortunately, IceCube’s low-energy neutrino triggering, simulations, and angular location algorithms have improved over the years and continue to improve.
“Although we didn’t find any sources, it was exciting to explore the intersection between astro- and particle physics by utilizing all three neutrino flavors in the search for astrophysical phenomena that to this day remain unobserved,” says Mia-Louise Nielsen, a graduate student at the Niels Bohr Institute and one of the main analyzers.
Along with IceCube collaborators, the researchers are now adapting the method used here to a real-time analysis to complement the existing high-energy real-time alert systems. Currently, the alert system notifies observatories around the world whenever IceCube sees a high-energy neutrino candidate event that meets certain criteria; adapting this analysis would extend the alerts’ energy range down to 10 GeV, opening up a new, unexplored energy regime for real-time follow-up.
“Multimessenger astronomy has entered a new era when it comes to using high-energy neutrinos to view the wider universe,” says Jason Koskinen, an associate professor at the Niels Bohr Institute. “With this analysis, we are taking some of the first steps in using lower energy neutrinos to probe an otherwise unexplored energy region.”
The authors point out that, while this analysis was optimized for a specific class of GRBs, it is also sensitive to many other transient neutrino emitters that may exist in the sub-TeV region but have not yet been predicted by theorists. With the forthcoming IceCube Upgrade in the early 2020s and the possibility of making correlations with other astrophysical messengers, a new multimessenger discovery is perhaps just around the corner.
info “Search for sub–TeV neutrino emission from transient sources with three years of IceCube data,” IceCube Collaboration: R. Abbasi et al., Journal of Cosmology and Astroparticle Physics 01 (2022) 027, iopscience.iop.org, arxiv.org/abs/2011.05096