Gravitational-wave researchers increasingly think that globular clusters, dazzling, celestial “snow globes” –among the oldest objects in the Universe– populated with hundreds of thousands of densely packed stars, harbor dark “hearts” loaded with dozens to even hundreds of black holes–by far the greatest concentration of these exotic objects found anywhere in the universe.
Their hunches appear to have paid off, reported MIT, with the discovery in 2018 of new mergers of black holes and neutron starsbased on results from the National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-based VIRGO gravitational-wave detector regarding their searches for coalescing cosmic objects, such as pairs of black holes and pairs of neutron stars.. The LIGO and Virgo collaborations have confidently detected gravitational waves from stellar-mass binary black hole mergers and one merger of neutron stars, which are the dense, spherical remains of stellar explosions.
From September 12, 2015, to January 19, 2016, during the first LIGO observing run since undergoing upgrades in a program called Advanced LIGO, gravitational waves from three binary black hole mergers were detected.
The second observing run, which lasted from November 30, 2016, to August 25, 2017, yielded one binary neutron star merger and seven additional binary black hole mergers, including the four new gravitational-wave events being reported.
2021 –First detection of a collision between a black hole and a neutron star
Fast forward to 2021: for the first time, LIGO researchers have confirmed the detection of a collision between a black hole and a neutron star. In fact, the scientists detected not one but two such events occurring just 10 days apart in January 2020. The extreme events made splashes in space that sent gravitational waves rippling across at least 900 million light-years to reach Earth. In each case, the neutron star was likely swallowed whole by its black hole partner.
Gravitational waves are disturbances in the curvature of space-time created by massive objects in motion. During the five years since the waves were first measured, a finding that led to the 2017 Nobel Prize in Physics, researchers have identified more than 50 gravitational-wave signals from the merging of pairs of black holes and of pairs of neutron stars. Both black holes and neutron stars are the corpses of massive stars, with black holes being even more massive than neutron stars.
Now, in the new 2021 study, the gravitational waves were detected by the National Science Foundation’s (NSF’s) Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and by the Virgo detector in Italy. The KAGRA detector in Japan, joined the LIGO-Virgo network in 2020, but was not online during these detections.
The first merger, detected on January 5, 2020, involved a black hole about 9 times the mass of our sun, or 9 solar masses, and a 1.9-solar-mass neutron star. The second merger was detected on January 15, and involved a 6-solar-mass black hole and a 1.5-solar-mass neutron star.
Found! –The missing type of binary
Astronomers have spent decades searching for neutron stars orbiting black holes in the Milky Way, our home galaxy, but have found none so far. “With this new discovery of neutron star- black hole mergers outside our galaxy, we have found the missing type of binary. We can finally begin to understand how many of these systems exist, how often they merge, and why we have not yet seen examples in the Milky Way,” says Astrid Lamberts, a researcher at Observatoire de la Côte d’Azur, in Nice, France.
The first of the two events, GW200105, was observed by the LIGO Livingston and Virgo detectors. It produced a strong signal in the LIGO detector but had a small signal-to-noise in the Virgo detector. The other LIGO detector, located in Hanford, Washington, was temporarily offline. Given the nature of the gravitational waves, the team inferred that the signal was caused by a black hole colliding with a 1.9-solar-mass compact object, later identified as a neutron star. This merger took place 900 million light-years away.
“Even though we see a strong signal in only one detector, we conclude that it is real and not just detector noise. It passes all our stringent quality checks and sticks out from all noise events we see in the third observing run,” says Harald Pfeiffer.
Location of the merger uncertain
Because the signal was strong in only one detector, the location of the merger on the sky remains uncertain, lying somewhere in an area that is 34,000 times the size of a full moon.
“While the gravitational waves alone don’t reveal the structure of the lighter object, we can infer its maximum mass. By combining this information with theoretical predictions of expected neutron star masses in such a binary system, we conclude that a neutron star is the most likely explanation,” says Bhooshan Gadre, a postdoctoral researcher at the AEI.
The Smoking Gun –No electromagnetic counterparts were detected for either system
“The inference that GW200105 was a NS+BH merger comes from the light mass of the secondary compact object: at ~ 2 solar masses this would be lighter than any known black hole while this mass is consistent with the heavier neutron stars known to exist based on pulsar observations,” wrote Caltech’s Albert Lazzarini, deputy director of the LIGO Laboratory, in an email to The Daily Galaxy. “In the absence of tidal effects near the time of merger or of mass outflow from tidal disruption of the neutron star after merger, the waveforms for both types of mergers would be the same.
“So the smoking gun for a neutron star+black hole merger, Lazzarinni wrote, “would have been (1) evidence of tidal effects in the waveforms at the end of the merger and/or (2) the observation of tidal disruption of the lighter object via electromagnetic-wave (EM) observations. Unfortunately evidence of tidal effects were not seen in the waveform; given the signal duration and signal strength, however, this is not surprising. Further, the event localization with only two detectors was not sufficient to allow EM followup. With regard to not yet having detected examples in the our own galaxy, it’s a question of rates: based on the few detections we have to date for both binary neutron star mergers and binary neutron star-black hole mergers, the latter are estimated to occur much less frequently, the rate being ~25% or less than the former.”
The Second Event 1 Billion Light-years from Earth
The second event, GW200115, was detected by both LIGO detectors and the Virgo detector. GW200115 comes from the merger of a black hole with a 1.5-solar mass neutron star that took place roughly 1 billion light-years from Earth. Using information from all three instruments, scientists were better able to narrow down the part of the sky where this event occurred. Nevertheless, the localized area is almost 3,000 times the size of a full moon.
Astronomers were alerted to both events soon after they were detected in gravitational waves and subsequently searched the skies for associated flashes of light. None were found. This is not surprising due to the very large distance to these mergers, which means that any light coming from them, no matter what the wavelength, would be very dim and hard to detect with even the most powerful telescopes. Additionally, the mergers likely did not give off a light show in any case because their black holes were big enough that they swallowed the neutron stars whole.
“The gravitational waveform for GW200105 differs from waveforms for binary black hole mergers primarily because the component-masses are different,” Harald Pfeiffer wrote in his email to The Daily Galaxy: “Both objects are less massive than the average masses of the binary black holes observed by LIGO/Virgo so far. Because of the lower mass, the waveforms have a longer duration during which they are detectable by the instruments (roughly a minute, rather than a few seconds for binary black holes). Interestingly, though, *if* the components of GW200105 were two black holes (rather than a black hole and one neutron star), then the waveforms would look indistinguishable to those observed. ”
“Not Like the Cookie Monster”
“These were not events where the black holes munched on the neutron stars like the cookie monster and flung bits and pieces about. That ‘flinging about’ is what would produce light, and we don’t think that happened in these cases,” says Patrick Brady, a professor at University of Wisconsin-Milwaukee and Spokesperson of the LIGO Scientific Collaboration.
Previously, the LIGO-Virgo network found two other candidate neutron star-black hole mergers. One event called GW190814, detected August 14, 2019, involved a collision of a 23-solar-mass black hole with an object of about 2.6 solar masses, which could be either the heaviest known neutron star or the lightest known black hole. Another candidate event, called GW190426, and detected on April 26, 2019, was thought to possibly be a neutron star-black hole merger, but could also simply be the result of detector noise.
Having confidently observed two examples of gravitational waves from black holes merging with neutron stars, researchers now estimate that, within one billion light-years of Earth, roughly one such merger happens per month.
“The detector groups at LIGO, Virgo, and KAGRA are improving their detectors in preparation for the next observing run scheduled to begin in summer 2022,” says Brady. “With the improved sensitivity, we hope to detect merger waves up to once per day and to better measure the properties of black holes and super-dense matter that makes up neutron stars.”
Image credit: Black Hole Dynamics in Globular Clusters YouTube