As one of the most powerful classes of explosions in the universe, gamma-ray bursts (GRBs) have long been considered a possible astrophysical source of neutrinos—tiny “ghostlike” particles that travel through space and large amounts of matter unhindered. These high-energy neutrinos are of particular interest to the IceCube Neutrino Observatory, a gigaton-scale neutrino detector at the South Pole.
IceCube has previously performed searches for neutrino emission from GRBs—above the TeV energy level and, most recently, at MeV to PeV levels from the record-shattering GRB 221009A. However, thus far, a correlation has not been found between high-energy neutrinos and GRBs.
In a paper submitted to The Astrophysical Journal, the IceCube Collaboration presents the first GRB neutrino searches using low-energy neutrinos in the 10–1,000 GeV range, as theoretical models suggest the production of tens of GeV neutrinos. The searches looked at a GRB catalog and, specifically, at GRB 221009A. No evidence of neutrino emission was found among the GRBs in the catalog.
The study was conducted using more than a million low-energy neutrinos and correlating those with 2,268 GRBs detected over an eight-year period by satellites. The neutrino data was taken using IceCube’s DeepCore, a subdetector of IceCube sensitive to neutrinos above approximately 10 GeV, or about 100 times lower in energy than the main IceCube detector. For the correlation studies, three measurements were used: direction, time, and energy.
“We used a likelihood method to represent how likely the correlation is, given a specific GRB and a set of neutrinos,” says Chujie Chen, a former PhD student at the Georgia Institute of Technology and study co-lead. “For each GRB, neutrinos within six time windows around the GRB detection time were studied. Those time windows, as short as 10 seconds and as long as 500 seconds, cover the precursor, prompt, and early afterglow phases of GRBs.”
Since a single correlation between one GRB and some neutrinos might be weaker to detect, Chen and his collaborators also tested for a correlation between a group of GRBs and a set of neutrinos.
In addition to testing the GRB catalog, the same methods were used to analyze GRB 221009A with two exceptions: the analysis looked at 1) two different time windows and 2) neutrino emission consistent with the predictions of what is known as the subphotospheric emission model.
In the subphotospheric emission model, neutrinos in the 10-1,000 GeV energy range are thought to be produced in GRBs as a result of collisions between protons and neutrons in the GRB outflow. According to this model, the amount of neutrinos produced is directly related to the fraction of the GRB’s energy that is devoted to accelerating those protons and neutrons to high energies, a quantity known as the GRB’s baryon loading.
“For GRB 221009A, we found that our observed upper limits were significantly lower than the level of neutrino emission predicted by this model,” says Bennett Brinson, a physics PhD student at the Georgia Institute of Technology and study co-lead. “This means that, at least for this specific GRB, the baryon loading is lower than previously assumed, and consequently, less energy went into accelerating protons and neutrons in the GRB jet than was expected.”
The nondetection of neutrino emission from GRBs across a wide range of energies only adds to the growing body of evidence that GRBs are not the major source of neutrino emission. Although GRBs cannot be ruled out entirely, this study suggests that neutrino emission from GRBs in the 10-1,000 GeV energy range is not at a high enough level to be observed by current detectors. However, the IceCube Upgrade will install seven more closely spaced and more densely instrumented strings of sensors, which will improve IceCube’s sensitivity to low energies.
“As more of these results pile up, further research will be needed to refine or perhaps even entirely rethink our current models for neutrino emission from GRBs,” says Brinson.
+ info “Search for 10–1,000 GeV neutrinos from Gamma Ray Bursts with IceCube,” IceCube Collaboration: R. Abbasi et al., arxiv.org/abs/2312.11515