Â鶹´å

Study Reveals How Often Black Hole–Neutron Star Collisions Spark Kilonovae

When a black hole collides with a neutron star, the cataclysmic crash sends ripples through spacetime, and sometimes ignites a blaze of light called a kilonova. Kilonovae are rare, but they are key to understanding the origin of the universe’s heaviest elements and the extreme physics taking place inside neutron stars. A new study from Â鶹´å estimates the odds of spotting a kilonova from black hole–neutron star collisions, aiding future efforts to observe these rare cosmic emissions. 

In May 2023, the LIGO-Virgo-KAGRA detectors spotted GW230529, a gravitational wave event that was likely generated by the merger of a black hole and a neutron star. The black hole’s low mass meant that the collision was more likely to produce a kilonova. But because GW230529 was only spotted by one detector, a corresponding kilonova was hard to find. 

Keerthi Kunnumkai, a graduate student in Carnegie Mellon’s Department of Physics, tackles some of the challenges kilonovae present for observers on Earth. Kilonovae are rare, faint, and fast-fading, reaching peak brightness within one to two days after a merger and vanishing from view in less than a week. To catch them, astronomers need precise predictions, which gravitational-wave signals alone can’t provide. 

“This work essentially transforms gravitational-wave alerts into actionable observing strategies,” Kunnumkai explained. “It’s about making sure we don’t miss these rare cosmic signals.” 

Her latest research . 

Kunnumkai set out to estimate the likelihood that a kilonova was formed from GW230529. Using gravitational wave data, she modeled thousands of possible configurations for the two objects that are thought to have created GW230529, including their mass, spin and inclination, and then calculated how much mass would be ejected from the collision. Her results estimated the event had a 2–28% chance of producing a kilonova bright enough for ground-based telescopes to see. 

Next, she simulated a large population of neutron-star black hole systems in which the black hole lies in the lower mass gap, an intriguing mass range that may bridge the transition between the heaviest neutron stars and lightest black holes, according to Kunnumkai. 

In the simulation, she explored a wide range of masses, spins and equations-of-states that could produce gravitational waves capable of being detected in the upcoming LIGO/Virgo/KAGRA’s fifth observing run. The team found that such mass-gap neutron star–black hole mergers could generate kilonovae about 3% of the time — potentially yielding one or two detectable events per year. The result is a roadmap for telescope teams, telling them which events to prioritize, which filters to use, and when to look. 

“Every detected kilonova helps scientists probe the extreme physics inside neutron stars and the origin of the Universe’s heaviest elements: gold, platinum and uranium,” said Kunnumkai. But they’re elusive: only one kilonova has been confirmed through a gravitational wave detection, in 2017. 

Antonella Palmese, assistant professor of physics, said that the new findings have the potential to accelerate the research into neutron star-black hole mergers and the kilonovae they produce. 

“Neutron star-black hole mergers were not thought to be promising multimessenger sources,” Palmese said. “This is because we expected the black hole to engulf the neutron star entirely in most cases, leaving no matter to emit electromagnetic radiation. We show that neutron-star black-hole mergers in the lower mass gap, which was once considered uncommon, can realistically produce detectable kilonovae and are actually very promising multimessenger sources.” 

The LIGO/Virgo/KAGRA’s fifth observing run, slated to begin in late 2027, promises a richer harvest of gravitational-wave events. In tandem with gravitational wave detection, studies like Kunnumkai’s can help astronomers know when and where to look for the elusive kilonova, ensuring we don’t miss the next golden opportunity. 

In addition to Carnegie Mellon’s Kunnumkai and Palmese, an international team of astronomers were involved with the work, including: Mattia Bulla from the Department of Physics and Earth Science, University of Ferrara, Italy; Tim Dietrich from the Institut für Physik und Astronomie, Universität Potsdam, Germany; Amanda M. Farah from the Department of Physics, University of Chicago; and Peter Tsun Ho Pang from the Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University, The Netherlands.