Ghost (Particle) Hunting in Gamma-ray Bursts

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Search for 10–1,000 GeV Neutrinos from Gamma-ray Bursts with IceCube
Authors: IceCube Collaboration
Status: Published in ApJ

Particle Accelerators, but in Space!

Gamma-ray bursts are some of the most powerful explosions in the universe, releasing a “fireball” of particle-filled plasma in a powerful jet that accelerates these particles through the universe. Gamma-ray bursts are understood to originate either from compact object (neutron star or stellar-mass black hole) mergers (these are called short gamma-ray bursts and last less than two seconds!) or from core-collapse supernovae (these are called long gamma-ray bursts but are just defined as anything longer than two seconds). Gamma-ray bursts are the most powerful particle accelerators in the universe and are really useful for looking for new particles and new particle interactions!

Today’s authors look at gamma-ray bursts as a possible source of ghost particles, i.e., neutrinos. Neutrinos rarely interact with other matter, which makes them really hard to detect — like a ghost! The IceCube Neutrino Observatory sees neutrinos all over the sky but can’t pinpoint where they’re coming from. Since gamma-ray bursts could produce neutrinos in their outbursts, the authors search through all of IceCube’s data to see if there are any bursts of high-energy neutrinos that came in at the same time as a gamma-ray bursts.

How Many Gamma-ray Bursts Does It Take to Find a Neutrino?

Today’s authors search for coincident neutrinos in the time periods surrounding the 2,298 bursts that happened during the lifetime of IceCube-DeepCore (IceCube’s highest-energy neutrino detector). They do this by looking at each time window individually and by combining many time windows to add faint signals that might not be seen in individual windows, but together might show an association between neutrinos and gamma-ray bursts.

In the first search, the authors define search windows before and after each burst to look for neutrinos (see Figure 1). Since neutrinos don’t interact with matter very often, they can easily stream out of dusty environments from which photons struggle to escape, meaning that the neutrinos could actually be expected to arrive at Earth before gamma-ray (and other photon) emission. The authors search the entire sky for neutrinos in these windows and assess the probability that there is an excess of neutrinos coming from the source location compared to the neutrino background that we see all over the sky.

The second search looks at groups of gamma-ray bursts that are associated in location and time with neutrino events. The authors look at the combined probability of burst/neutrino association of all the events in this group. This makes it possible to correlate gamma-ray bursts with neutrinos even if the events don’t individually stand out. Using this method, the authors didn’t find any groups of neutrinos that are any more statistically significant than individual neutrinos that fall within gamma-ray burst time windows.

Trials Factors and Tribulations

The winning burst of the first search (i.e., the most significant neutrino–gamma-ray burst correlation) is GRB bn 140807500. (Since there are a lot of gamma-ray bursts recorded by burst-hunting instruments like the Fermi Gamma-ray Burst Monitor (Fermi-GBM) and the Swift Burst Alert Telescope (Swift-BAT), it’s too much of a hassle to give the bursts individual names. Instead, the bursts get “telephone numbers” corresponding to the date they were detected.) The corresponding neutrino falls within 100 seconds of GRB bn 140807500 and has a p-value of 4.6 x 10^-5, which is the probability that the correlation between the burst and the neutrino is just a lucky coincidence and not from actual correlation (i.e., small p-values mean a more likely detection of neutrinos from gamma-ray bursts!).

This probability seems really small, and at first glance it seems like the neutrino and the gamma-ray burst are most likely connected here! Unfortunately, this doesn’t take into account trials factors (also called the look-elsewhere effect), which quantify the statistical statement that if you look at enough gamma-ray bursts and neutrinos, there will be some events that line up with each other in space and time, just by chance. To account for this, the authors must correct for 2,268 trials, one for each burst. After correcting for trials, this leaves us with a much larger p-value of 0.097, meaning there’s a one in ten chance that the gamma-ray burst and the neutrino aren’t really connected. Generally particle (and astroparticle) physicists require a p-value of 3×10^-7 (about 1 in 3.5 million, corresponding to the [in]famous 5-sigma threshold!) to feel confident in saying that these events are actually correlated.

Don’t Forget About the BOAT

At the same time as this article was being prepared, the brightest of all time (BOAT) gamma-ray burst was detected. The authors didn’t include the BOAT directly in their dataset, but they made some predictions as to how it would measure up to the other gamma-ray bursts that were considered. The BOAT was so bright that the authors calculated the expected neutrino signal to be 6–8 times the combined expected signal of all 2,264 gamma-ray bursts used in this part of the study (see Figure 2)! This is because the BOAT is so much more energetic in gamma rays than other gamma-ray bursts, implying a large flux of high-energy neutrinos, which are localized to more precise regions in the sky than the lower-energy neutrinos expected to accompany other bursts. This means that we can more confidently associate any observed neutrinos with the location of the burst.

There’s still work to be done to see if there are any neutrino events that seem to come from the BOAT gamma-ray burst, but this leaves the idea open that neutrinos could come from gamma-ray bursts (or at least, that we can more confidently say that they don’t)! The universe sometimes throws surprises like the BOAT at us, allowing astronomers to study the high-energy universe a lot more easily. Luckily, we’ve entered into the era of time-domain astronomy where instruments like Fermi-GBM and Swift-BAT allow us to catch more bursts and explosions than ever before, giving us an increasingly large sample of gamma-ray bursts to study!

Original astrobite edited by Cole Meldorf.

About the author, Samantha Wong:

I’m a graduate student at McGill University, where I study high-energy astrophysics. This includes studying all sorts of extreme environments in the universe like active galactic nuclei, pulsars, and supernova remnants with the VERITAS gamma-ray telescope.