On August 17, 2017, the LIGO Scientific Collaboration and the Virgo Collaboration made the first-ever detection of minute ripples of space-time known as gravitational waves from merging neutron stars. Following the signal, other telescopes and observatories around the world detected the same source in visible light, gamma rays, ultraviolet light, X-rays, and radio waves.
It was the first gravitational wave event observed with an electromagnetic counterpart, crowning a new era of multimessenger astronomy—the coordination of observations from different “messengers.” Multimessenger observations provide unique insight into astrophysical sources: Different messengers are emitted during different times and from different subprocesses, allowing us to observe different epochs of the interaction and understand the whole process more completely.
Other than gravitational waves and electromagnetic radiation, subatomic particles called neutrinos also make excellent messengers. Chargeless and nearly massless, neutrinos are capable of traveling for billions of light-years undisturbed. It is the goal of the IceCube Neutrino Observatory at the South Pole to detect them.
While we have seen neutrinos and electromagnetic radiation with a common origin, researchers have yet to detect neutrinos and gravitational waves coming from the same place. So the IceCube Collaboration recently performed an analysis to look for neutrino emission that correlates with gravitational waves detected by the LIGO and Virgo Collaborations during their first two observing runs, O1 and O2. Their results are described in a paper published today in The Astrophysical Journal Letters. No coincidence was found, but the researchers are already at work on further analyses.
To date, all signals detected by the LIGO and Virgo interferometers have come from compact binary mergers: orbiting pairs of massive and dense—”compact”—objects, such as neutron stars or black holes. For this IceCube analysis, collaborators from the University of Wisconsin–Madison, the University of Florida, and Columbia University in New York City searched for neutrinos from the 11 compact binary mergers from LIGO-Virgo’s first two observing runs: 10 binary black hole mergers and one binary neutron star merger.
These mergers release an enormous amount of energy, some of which could go into accelerating cosmic rays to very high energies—possibly leading to neutrino production.
“Observing neutrino emission from these mergers would allow us to probe the underlying physics better and understand the properties of the objects involved,” says Doğa Veske of Columbia University, one of the leads on this analysis. “We could also associate a source with high-energy neutrinos.”
The researchers were searching for neutrino emission in IceCube data consistent with the time and location of a source responsible for the gravitational wave emission detected by LIGO-Virgo. It is estimated that there can be up to 500 seconds separating high-energy neutrino emission and gravitational wave emission from the same source, so researchers looked for neutrinos that arrived at IceCube in a 1000-second time window centered on the reported gravitational wave event time.
One challenge was that LIGO-Virgo events are very poorly localized. “These gravitational wave observations are like hearing a sound rather than seeing something: It is hard to tell where, precisely, they come from in the sky,” says Veske. So the researchers had to rely on statistical methods to pick out a significant signal within the large localization region reported by LIGO-Virgo and and determine which—if any—neutrinos were coming from the same source as the gravitational wave.
The searches did not identify significant neutrino emission from any of the 11 events reported by LIGO-Virgo. Nevertheless, the results allowed the researchers to set upper limits on the neutrino emission from each source as well as on the total energy emitted in high-energy neutrinos from each gravitational-wave event.
Raamis Hussain, a graduate student at UW–Madison and another lead on the analysis, says that the null results aren’t too much of a surprise because most of the events are binary black hole mergers, which aren’t necessarily expected to produce neutrinos. In the case of the one binary neutron star merger, “We could have just gotten unlucky,” he says. “We’ll need more events from LIGO-Virgo to either confirm or rule out compact binary mergers as a neutrino source.”
LIGO-Virgo just finished their third observing run, O3, in late March. Throughout O3, IceCube researchers tuned into LIGO-Virgo Open Public Alerts—high-significance gravitational-wave candidate events that were broadcast in near-real time—with two neutrino follow-up pipelines. This allowed the researchers to apply their searches to any new signals promptly.
“This O1 and O2 analysis laid the groundwork for the ongoing real-time search for neutrinos from the gravitational wave sources reported nearly every week during O3,” says Justin Vandenbroucke, a UW–Madison physics professor and another lead on the paper. “A detection would revolutionize multimessenger astrophysics, and we can report coincident neutrino directions within about an hour of gravitational wave detection in order for telescopes to search a much smaller region of the sky for electromagnetic counterparts.”
There are also other analyses in the works, one of which will perform the same search but on longer timescales. This will test some models that have predicted neutrino emission up to two weeks after the initial merger event.
In another forthcoming analysis, the IceCube researchers plan on expanding the search to include a second type of neutrino event—”cascades”—in addition to the “tracks” that they currently examine, which will increase the number of neutrinos they can test.
On the gravitational-wave side, a large set of event candidates that dig deeper into the noise floor of the LIGO and Virgo detectors was recently identified. The Columbia-Florida IceCube team will use this set to expand the horizon of the current search.
“As one of the founders and an early pioneer of this field, I have been dreaming for a decade and a half about gravitational-wave–high-energy neutrino joint discovery,” says Zsuzsa Márka, a Columbia University scientist and a lead of the project. She and her collaborators hope to find evidence of such a source soon, which would be a major discovery for both high-energy neutrino sources and compact objects.
“The science we will derive will be astonishing,” says Márka.
info “IceCube Search for Neutrinos Coincident with Compact Binary Mergers from LIGO-Virgo’s First Gravitational-Wave Transient Catalog,” The IceCube Collaboration: M. G. Aartsen et al. 2020 The Astrophysical Journal Letters 898 L10 iopscience.iop.org arxiv.org/abs/2004.02910