Scientists Observe Collisions of Neutron Stars with Black Holes

For the first time, researchers have confirmed the detection of a collision between a black hole and a neutron star.

In a new study published June 29 in The Astrophysical Journal Letters, scientists have announced the detection of gravitational waves from two rare events, each involving the collision of a black hole and a neutron star.

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. An additional detector in Japan called KAGRA joined the LIGO-Virgo network in 2020, but was not online during these detections.

Montclair State University faculty members Marc Favata, Shaon Ghosh and Rodica Martin are part of the international LIGO team that made the discovery. Approximately 1,400 scientists from around the world participate in the effort to operate the detector and analyze the data through the LIGO Scientific Collaboration.

“From its inception, LIGO and its partners expected to detect three classes of compact stellar binaries: pairs of two black holes, pairs of two neutron stars, and mixed pairs – a black hole and a neutron star orbiting each other,” says Favata, who is chairperson of Montclair State’s Department of Physics and Astronomy and PI of the University’s LIGO group. “Since 2015, we’ve found 48 black hole pairs and two neutron star pairs. Now, we’ve finally found this third type of compact binary and can estimate the rate at which these events happen – it took gravitational-wave detectors to make this possible.”

Gravitational waves are disturbances in the curvature of space-time created by massive objects in motion. The waves were first measured in 2015, a finding that led to the 2017 Nobel Prize in Physics. They are primarily produced by merging pairs of black holes or neutron stars. Both black holes and neutron stars are the corpses of massive stars, with black holes being even more massive than neutron stars.

The extreme events announced today, which occurred 10 days apart in January 2020, made splashes in space that sent gravitational waves rippling across at least 900 million light-years to reach Earth.

The first merger, detected on January 5, involved a black hole about nine times the mass of our sun and a 1.9-solar-mass neutron star. The second merger involved a 6-solar-mass black hole and a 1.5-solar-mass neutron star. In each case, the neutron star was likely swallowed whole by its black hole partner.

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 weak signal 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.

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.

“A binary system consisting of a black hole and a neutron star is of great interest to the astrophysics community,” says Ghosh. “Having both neutron stars and black holes, these systems bring the `best of both worlds.’ They are louder gravitational-wave sources than binary pairs with only neutron stars, and can be seen farther out in the universe. Unlike pairs with only black holes, they can possibly emit electromagnetic waves along with the gravitational waves they produce.”

Ghosh served as part of the team that drafted the paper reporting this discovery. An analysis code he helped develop led to a rapid determination that these systems contained a neutron star and black hole pair, and also that there would be only a small chance for these collisions to produce an electromagnetic signal. For the two events, the black holes were large enough to swallow the neutron stars whole, without giving off a light show. Additionally, these mergers were far enough away that any light coming from them would be dim and hard to detect with even the most powerful telescopes.

Due in part to the computational infrastructure developed by Ghosh and his collaborators, astronomers were alerted to both events soon after they were detected in gravitational waves and subsequently searched the skies for associated flashes of light. As predicted, no electromagnetic signals were found.

Having now confidently observed two examples of gravitational waves from black holes merging with neutron stars, researchers estimate that, within 1 billion light-years of Earth, roughly one such merger happens per month.

“With every step that increases the detectors’ sensitivity, we observe new signals and more confidently confirm the origins of gravitational waves, adding to our understanding of the universe,” says Martin, who was a member of the team that helped design and install the upgrade to LIGO that enabled the first-ever detections of gravitational waves in 2015. “The next generation of gravitational-wave detectors will have the sensitivity to detect black hole and neutron star collisions from much earlier in the universe’s history.” Martin works with a large team of Montclair State undergraduates, designing optical components that will be used in a future improvement to the LIGO detectors.

Montclair State University joined the LIGO Scientific Collaboration in 2013 and is one of 127 institutions in the 1,400+ member collaboration. Current Montclair State students involved in gravitational-wave research include Claudia Barone, Michael Camilo, Ariella Hernandez, Kevin Johansmeyer, Mariam Mchedlidze, John Notte, Jonathan Reyes, Jacob Santos, and Ricky Wilde.

“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 Patrick Brady, a professor at the University of Wisconsin-Milwaukee and spokesperson for the LIGO Scientific Collaboration. “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.”

More information about this discovery can be found at ligo.org.

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