A search for neutrinos from fast radio bursts
Astronomers and astrophysicists expect that fast radio bursts (FRBs) happen all the time. Some estimates predict there are up to 10,000 per day. But they are not easy to detect. Telescopes with a wide field of view often don’t have good resolution. And the ones that have good resolution usually come with a much smaller field of view and might be missing them all the time.
FRBs are so short that regular follow-up observations by other radio or optical telescopes are not realistic. This could be a depressing scenario for scientists, but guess what: neutrinos might come to the rescue.
Although little is known about the origin of FRBs, many models point to cataclysmic scenarios, such as the birth of black holes from supramassive neutron stars or enormous magnetic eruptions. Such brutal environments may well produce hadronic emission. Thus, they might be the perfect culture for creating high-energy cosmic rays and neutrinos. More exotic phenomena such as cosmic strings or primordial black holes, which would both have originated in the early universe before any star had formed, could also produce bursts of both radio waves and neutrinos.
And this is what a team of researchers at the Wisconsin IceCube Particle Astrophysics Center (WIPAC) has looked for: neutrino emission in conjunction with FRBs. During the first year of operation of the complete IceCube Neutrino Observatory, the Parkes and Green Bank telescopes detected four FRBs, two near the celestial equator and two farther south. It was the Parkes telescope that in 2007 announced the discovery of the first FRB in archival data taken in 2001. As the co-discoverer Duncan Lorimer explained, it was so bright that it couldn’t be missed, and yet, no one knew what to do with it.
The first search for neutrinos from FRB sources did not find any of the IceCube neutrinos to be consistent with the time and direction of any of the four FRBs. However, the quest has just started.
“Astrophysical neutrinos and fast radio bursts are two of the most exciting mysteries in physics today. There may be a link between them. We will continue to study the many new FRBs detected after these four,” explains Justin Vandenbroucke, an assistant professor at the University of Wisconsin–Madison.
WIPAC researchers have set the first upper limits on neutrino emission from FRBs. The results, posted to arXiv in November and published today in The Astrophysical Journal, rule out very bright neutrino emission.
The IceCube Neutrino Observatory, a cubic kilometer telescope in Antarctica, views the entire sky all the time. Although a well-known method of neutrino telescopes is to study the opposite hemisphere of the sky with respect to where they are located—they use the Earth to filter out a huge background of atmospheric particles—IceCube also has excellent sensitivity in the southern sky. This is thanks to various analysis techniques including vetoing downgoing cosmic rays, applying a moderate energy threshold, or using cascades rather than muon tracks, which are not confused with atmospheric backgrounds. The full-sky sensitivity makes IceCube especially well-suited to follow up short transients, including the many FRBs being discovered by radio telescopes in the Southern Hemisphere.
As we learn more about FRBs, including whether or not many of them might repeat and whether they are all extragalactic, new radio interferometer arrays will soon detect as many as a few dozen per day. In a few years, searches for neutrino emission from thousands of stacked FRBs will either prove or rule out that hadronic or exotic processes are involved, hopefully pointing to specific classes of FRBs and revealing the environments where they were created. “A single analysis could help solve two outstanding mysteries. This is a very exciting possibility,” says Vandenbroucke.
+ info “A search for neutrinos from fast radio bursts with IceCube,” Samuel Fahey, Ali Kheirandish, Justin Vandenbroucke, and Donglian Xu. The Astrophysical Journal 845 (2017) 1, 14; doi.org/10.3847/1538-4357/aa7e28, arxiv.org/abs/1611.03062