The hunt for neutrinos from gamma-ray bursts

In 2012, shortly after the IceCube Neutrino Observatory was completed, the IceCube Collaboration announced in Nature an important and unexpected result in neutrino astrophysics: gamma-ray bursts (GRBs), which were one of the two leading candidates for sources of high-energy neutrinos and cosmic rays, did not report any neutrino excesses. 

Since then, IceCube has continued to improve the analyses, which now include real-time searches, and to establish ever stronger limits on the contribution of GRBs to the observed diffuse neutrino flux.

The IceCube Collaboration, in collaboration with Fermi Gamma-ray Burst Monitor (GBM), presents a new study that now includes neutrino emission up to a two-week window before and after a GRB detection. The results of these analyses also show no excess of neutrinos, but provide us with new limits on neutrino emissions from extended timescales. These results, recently submitted to The Astrophysical Journal, once again constrain the contributions of GRBs to the astrophysical neutrino flux seen by IceCube and provide a means for testing current GRB models, some of which have been excluded.

The black and grey lines show the integrated and differential limits of the precursor analysis, respectively. They are compared to the astrophysical neutrino flux seen by IceCube and to different model predictions. This analysis can exclude the “red” model (progenitor with an outer hydrogen layer) but not the “green” model (progenitor with an outer helium layer).

GRBs are considered to be caused by the collapse of massive stars or due to compact binary mergers and are among the most energetic sources in the universe, which makes them great candidates for sources of ultra-high-energy cosmic rays (UHECRs) and neutrinos.  

Until now, IceCube had performed several searches for neutrinos correlated with the gamma-ray emissions from the main phase, also known as prompt gamma rays.  

Prompt gamma-ray emission is hypothesized to be the output of a relativistic fireball, emerging from a plasma of electrons, photons, and hadrons that are accelerated to the highest energies. The same environment would produce neutrinos, which could potentially be seen by IceCube and would have temporal and spatial correlation with gamma-ray emissions. Detecting neutrino emission correlated with GRBs would have established these gamma-ray sources not only as sources of neutrinos but of cosmic rays as well.  

However, GRBs have been known to produce gamma-ray emissions before and after the main burst known as precursor and afterglow emissions, and there is a possibility of neutrino emissions from these phases as well. 

These extended emissions have motivated the analyses presented in this paper, which look for statistical correlations between neutrinos and GRBs up to 14 days before and after the start of the prompt phase for GRBs in the period between May 2011 and October 2018.

IceCube members, in collaboration with two Fermi GBM team members, have performed four different analyses, each one based on slightly different assumptions. In total, 2209 GRBs were examined. 

Extended TW. Here researchers examined 10 time windows around each GRB, ranging from five seconds before and after the gamma-ray emission to a window of 15 days—from a full day before to 14 days after the emission. 

Precursor/Afterglow. In this case, each GRB was fit with a precursor time window, ranging from the beginning of the prompt phase up to 14 days prior, and an afterglow time window, ranging from the beginning of the prompt phase up to 14 days after.

GRB Precursor. The time windows were taken to be the time intervals during which gamma-ray precursor activity was observed from GRBs.

Stacked Precursor. The time window of each GRB was taken to be 250 seconds, covering the time period in which gamma-ray precursor activity is seen.

None of the four analyses found evidence of a correlation between neutrino events and GRBs. 

90% CL upper-limit on the contribution to the quasi-diffuse neutrino flux from the total population of all long GRBs for different durations of precursor or afterglow neutrino emission. Any model predicting neutrino emission above the red line for the respective emission window (precursor or afterglow) is rejected.

Artist’s impression of a relativistic jet of a gamma-ray burst (GRB), breaking out of a collapsing star and emitting very high energy photons. Credit: DESY, Science Communication Lab

Measurements were used to set new limits on the contribution of cosmic GRBs to the IceCube neutrino flux and test different emission hypotheses. Prompt neutrino emission from GRBs is limited to <1% of the observed diffuse neutrino flux, and precursor/afterglow emission from long GRBs on timescales up to 10,000 seconds is constrained to 24% of the total diffuse flux.

This is not the end of GRB searches in IceCube though, because nondetections do not demonstrate that GRBs are not high-energy neutrino sources. In fact, these searches are expected to continue with the proposed IceCube-Gen2 detector, which will have increased sensitivity to observe populations of GRB sources. 

Each new limit helps in testing old and new hadronic models. GRBs examined to date allow for more segmented searches that look for neutrino emission from specific subclasses of GRBs, such as choked GRBs. These are interesting events that result from a core-collapse supernova and that are not electromagnetically bright. And yet, those sources should also produce high-energy neutrinos.

GRBs are rapid, transient sources that can emit neutrinos in a window that lasts from milliseconds to thousands of seconds. Bursts shorter than two seconds are known as short GRBs and tend to have a harder energy spectrum than long bursts. 

Current efforts to look for real-time emission that can motivate multimessenger observations are now in the process of implementing an “Extended TW” analysis technique and will look at GRBs from 2018 to now. On top of those, offline comprehensive studies should be performed every few years to study the significance of the collection of GRB results. 

+ Info: “Searches for Neutrinos from Gamma-Ray Bursts using the IceCube Neutrino Observatory” IceCube Collaboration: M.G.Aartsen et al, Fermi GBM: A. Goldstein and J. Wood. Submitted to The Astrophysical Journal.