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Bangs Without Flashes: Gravitational Waves and Elusive Neutrinos

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A photograph of a building and other infrastructure atop an ice sheet at night. Several stars are visible overhead.
The IceCube detector located at the Amundsen-Scott South Pole Station in Antarctica. [IceCube Collaboration]

The universe is crackling with gravitational waves and sparkling with neutrino emissions, but detecting both signals from the same source is an enormous challenge. A new publication summarizes the state of this effort and reports on non-detections of high-energy neutrinos associated with the merger events caught by LIGO/Virgo.

Multiple Messengers

Historically, astronomers studied our universe by analyzing the electromagnetic radiation (light) that lands on Earth after a journey through the depths of the cosmos. In the last decade, however, researchers have expanded upon this paradigm and now routinely scrutinize other “messengers” that collide with our home planet and leave their imprints on our scientific instruments. Two of these messengers include high-energy neutrinos, first observed in 2013, and gravitational waves, first spotted in 2015.

Certain astrophysical processes, including the mergers of compact objects like black holes and neutron stars, are thought to produce both of these messengers. Therefore, to wring as much information as possible from these distant events, astronomers are incentivized to detect coincident signals of different messengers from the same source. Unfortunately, pulling this off requires overcoming technical and instrumental hurdles across two different subfields of physics, and to date the positional uncertainties of gravitational wave detections especially have prevented any confident claims.

Vigilance and Patience

A histogram displaying the distribution of delays between initial detection of a gravitational wave and the release of IceCube's analysis. The median is marked with an orange vertical line marking 0.94 hours.

The distribution of delay times between a gravitational wave event and release of preliminary IceCube analysis. Most often the team turned around results in less than one hour. [Abbasi et al. 2023]

Between April 2019 and March 2020, a pair of gravitational wave detectors named LIGO and Virgo listened intently for the “bangs” of compact objects merging somewhere in the distant universe. Each time the detectors heard something, their teams quickly released a rough estimate of its source location on the sky. This in turn kickstarted a global scramble to try and detect the same merger with a different messenger, whether optical light, gamma rays, or high-energy neutrinos.

The IceCube team, a collaboration that searches for flashes of light created when high-energy neutrinos interact with Antarctic ice, watched carefully for these real-time estimates. Whenever one was released, they immediately checked their logs for high-energy neutrinos that came around the same time from around the same place as the gravitational wave, then released their own reports of any encouraging signals. Thanks to their preparation leading up to the run, this entire process happened usually within the hour following the initial LIGO/Virgo observations. Once the runs were complete and the gravitational wave data were collated into a more confident catalog, the team also ran an after-the-fact analysis to check if anything was missed in real time.

Close Calls

A heatmap with right ascension on the horizontal axis and declination on the vertical. Darker colors, which represent regions with a higher probability of hosting the gravitational wave event, form a vertical stripe down the middle. Overplotted on the heatmap is a circular contour marking the high-likelihood region for the neutrino's source region. The circle largely overlaps with the darkest region.

A sky map showing the overlap between high-likelihood regions of a gravitational wave event and of the most likely direction of a high-energy neutrino that arrived 360 seconds earlier. Though there is much overlap, it was not classified as statistically significant coincident emission. [Abbasi et al. 2023]

While the IceCube team did not confidently detect any high-energy neutrinos associated with a gravitational wave event, there were four close calls that prompted further scrutiny. Just as importantly, their non-detections enabled the calculation of upper bounds on neutrino emission for each gravitational wave event. As LIGO and Virgo gear up for another observing run and IceCube considers a next generation detector, hopefully the tools developed here will reveal the first confirmed neutrino emission from a merger event soon.

Citation

“IceCube Search for Neutrinos Coincident with Gravitational Wave Events from LIGO/Virgo Run O3,” R. Abbasi et al 2023 ApJ 944 80. doi:10.3847/1538-4357/aca5fc