Salvatore Vitale describes how gravitational-wave signals suggest black holes completely devoured their companion neutron stars.

Recently, an international team of scientists, including researchers at MIT, announced the detection of a new kind of astrophysical system: a collision between a black hole and a neutron star — two of the densest, most exotic objects in the universe.

Scientists have detected signals of colliding black holes, and colliding neutron stars, but had not confirmed a merging of a black hole with a neutron star until now. In a study appearing today in The Astrophysical Journal Letters, the scientists report observing not just one, but two such rare events, each of which gave off gravitational waves that reverberated across a large swath of the universe before reaching Earth in January 2020, just 10 days apart.

The size of a tennis ball. The mass of the Earth.

But that could change soon.

Current gravitational wave observatories are sensitive to the mergers of stellar-mass black holes. We’ve observed a few mergers involving neutron stars, but most have been between black holes on the order of tens of solar masses.

We can’t yet observe the gravitational waves of supermassive black holes in other galaxies, nor can we observe those of planet-sized worlds. Proposed detectors such as eLISA will allow us to observe the former, but it will take a novel new idea to detect the latter.

Another missing piece has just been added to our knowledge of cosmic phenomena. The LIGO, Virgo and KAGRA collaborations have announced the first detection of gravitational waves^[1] resulting from the ‘mixed’ merger between a black hole and a neutron star.^[2] The discovery, published on June 29, 2021 in Astrophysical Journal Letters, involves CNRS researchers working within the Virgo scientific collaboration.

Although it has only been only a few years since the very first observation of gravitational waves, the technique has yielded an extensive repertoire of phenomena involving massive cosmic objects. The LIGO and Virgo detectors have already observed mergers of pairs (or binaries) of black holes and, less frequently, of neutron stars. However, gravitational waves detected in January 2020 provide evidence of the existence of a new type of system. The signals, named GW200105 and GW200115 from their dates of detection, were produced by a process that had been predicted but never observed until now: the coalescence of ‘mixed pairs’ called NSBH pairs, each made up of a neutron star and a black hole.^[3]

Gravitational waves contain valuable information about their source, such as the mass of the components making up the binary. Analysis of the signals revealed that GW200105 resulted from the merger, some 900 million years ago, of a black hole and a neutron star, respectively 8.9 times and 1.9 times more massive than the Sun, while GW200115 originated from an NSBH pair which coalesced around 1 billion years ago, with masses 5.7 and 1.5 times greater than the Sun. The difference in mass between the components of the system indicates that they are indeed mixed binaries: the mass of the heavier object corresponds to that of a black hole while the mass of the lighter object is consistent with that of a neutron star. The difference between the two masses could also explain why no light signals were detected by telescopes. When a neutron star approaches a black hole it can theoretically be torn apart by tidal forces, causing flares of electromagnetic radiation. However, in the two cases observed, the black hole, being much more massive, could have gobbled up the neutron star in a single mouthful, leaving no trace.

Black holes seem like the perfect spot for harvesting energy.

Researchers create a device to test a 50-year-old physics theory from the famed Roger Penrose.