Astronomers have definitively detected a black hole devouring a neutron star for the first – and second – time. These cataclysmic events created ripples in space-time called gravitational waves that travelled more than 900 million light-years to reach detectors on Earth.
The first of the two collisions was detected on 5 January 2020 by the Virgo observatory in Italy and one of the two detectors that make up the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US – the second LIGO detector was temporarily offline. It consisted of a black hole about 8.9 times more massive than the sun consuming a neutron star about 1.9 times the sun’s mass.
The second, spotted on 15 January 2020 with all three detectors, was a black hole about 5.7 times the sun’s mass swallowing a neutron star about 1.5 times the sun’s mass. While LIGO has detected other events that could have been collisions between black holes and neutron stars, these two detections are significantly more clear and definitive.
Because the two events were so far away, astronomers were not able to spot any light in the sky from the collisions. Even if they had been closer, though, it’s possible there was no visible light produced at all because the black holes were so much more massive than the neutron stars. “Simulations suggest that the neutron star would be swallowed whole, not shredded,” says LIGO team member Astrid Lamberts at the Côte d’Azur Observatory (OCA) in France. “It might just disappear into the black hole.”
Observations like these could help us figure out how such strange, unmatched partners form – a black hole and a neutron star could be born as a pair, from stars that already orbited one another, or they could meet later in their lifetimes. There are tentative indications that the latter may be true for the second detection, but nothing concrete enough to say for sure.
LIGO’s next observing run is set to begin in mid to late 2022, so we should be able to detect more of these odd couples then, as well as other types of objects. “We’ve seen binary black holes, we’ve seen binary neutron stars, and now we’ve definitely seen a binary with both,” says Nelson Christensen, also a LIGO researcher at OCA. “Now we need a supernova or a spinning pulsar, that’ll be the next big deal.”
Journal reference: The Astrophysical Journal Letters, DOI: 10.3847/2041-8213/ac082e
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