Astrophysical neutrinos are revealing a high-energy universe fairly different from the one we had originally envisioned. And that’s intriguing and fascinating at the same time. What will neutrinos tell us about the most powerful cosmic environments? Are there only a few of them, or are there many? Are they near or far in terms of astronomical distances?
We have learned that many high-energy neutrino sources are extragalactic, like active galaxies, which can create neutrinos but likely cannot account for most of the astrophysical neutrinos seen by IceCube. Another proposed source candidate, gamma-ray bursts, has been shown to emit fewer neutrinos than scientists had speculated.
However, physicists still think that these types of phenomena—the explosion and the merging of stars, their gravitational collapse into black holes, and sources powered by black holes or neutron stars—are what accelerate cosmic rays to the highest energies and, thus, also produce astrophysical neutrinos.
In a new search for neutrino sources, the IceCube Collaboration and other collaborators have looked for short-lived transient sources, including gamma-ray bursts, core-collapse supernovae, or neutron star mergers. The search, which looked for two or more neutrinos detected within 100 seconds from the same location, included transients that might not emit gamma rays and might be pointing to uncharted objects in the universe. The results submitted this week to Physical Review Letters did not identify any individual source but did show that the number of bright short-lived transient neutrino sources must be small or they must be fairly faint.
As soon as IceCube was large enough to detect astrophysical neutrinos—even before it was completed—a search for neutrinos from gamma-ray bursts brought the first no-detection result. Scientists had speculated that these powerful transient sources would likely emit neutrinos with energies and at rates that could be detected with a telescope such as IceCube.
This was the first unexpected result, from many to come, proving the limitations of our understanding of the extreme universe, which at that time was only accessible through electromagnetic radiation, from radio to high-energy gamma rays.
The search for short-lived transients in IceCube has since then continued, and with every step we have slowly uncovered a surprising and still mysterious extreme universe.
The results made public this week use a set of IceCube neutrino alerts that look for the temporal coincidence of two or more neutrinos, which may have been produced by the same source. These alerts are tuned to identify short-lived transients and thus look for neutrino multiplets detected within a 100-second window. An example of a cosmic phenomenon that could produce such a signature would be a bright gamma-ray burst, which would be followed by an X-ray or optical glow that could last for hours or even days. Most gamma-ray telescopes cannot point fast enough to check for gamma-ray emission from a potential source like this—which is also expected within the same 100-second window—but X-ray and optical telescopes have more time to perform follow-up observations.
Alerts from September 2011 to May 2016 triggered X-ray and optical follow-up observations, but they did not provide evidence for a neutrino source. In that period of time, IceCube detected 338 neutrino doublets and one neutrino triplet, which could be explained by chance coincidences of background events—i.e., muon neutrinos produced by the interaction of cosmic rays in the Earth’s atmosphere.
Both the number of doublets and of triplets represent a small excess with respect to the expected values of 312.7 doublets and 0.341 triplets estimated from background. But the significance of this excess does not provide substantial support for declaring the neutrinos as originating in astrophysical sources.
IceCubers have used the low number of triplets to set limits on the contributions of two classes of short transients to the total neutrino flux observed by IceCube. For transients similar to GRBs, their contribution would be between 5 and 30% depending on the energy spectrum of the astrophysical neutrino flux. Minute-long transients can only account for the complete astrophysical flux if there are a large number of faint sources. For sources similar to core-collapse supernovae with choked jets the population has to be 100 to 100,000 times more frequent than GRBs to produce the observed astrophysical neutrino flux without triggering the detection of a source with the follow-up program.