After several as yet unclear candidates, astronomers have now captured the clear gravitational wave signal from the collision of a neutron star with a black hole twice. In these events, almost a billion light years away, a stellar black hole devoured a smaller, lighter neutron star, releasing energy that set space-time vibrating. The LIGO and Virgo observatories captured these vibrations in the form of gravitational waves in January 2020. The data now provide valuable insights into the circumstances of such unequal mergers and their history.
Since the commissioning of the two LIGO gravitational wave detectors in the USA and the European detector Virgo in Italy, astronomers have captured the characteristic gravitational wave signatures of a cosmic collision more than 50 times. Mostly it was the merging of two stellar black holes. In 2017, the detectors also registered the collision of two neutron stars, the extremely dense relics of massive stars, for the first time. What was missing so far, however, was the observation of a mixed collision: “Gravitational waves made it possible for us to detect the collision of pairs of black holes and pairs of neutron stars, but the collision of a black hole with a neutron star was the missing piece in the family picture of the mergers compact objects, ”explains co-author Chase Kimball of Northwestern University in Evanston. Although there were some gravitational wave events that were considered candidates for such a “mixed duo”, the data situation was not clear enough.
Two mixed collisions in a row
That has changed since January 2020. Because at an interval of just ten days, the gravitational wave observatories detected two events that astronomers attribute to the collision of a neutron star with a black hole. The GW200105 event was captured on January 5, 2020 by the LIGO detector in Livingston and the Virgo detector in Italy, but in the latter case the signal was only weak and therefore difficult to filter out of the background noise. “Even though we only see a strong signal from one detector, it is real and has passed all of our quality tests,” emphasizes Harald Pfeiffer from the Max Planck Institute for Gravitational Physics in Potsdam. Analyzing the data, it appears that the signal comes from around 900 million light years away and was caused by the merging of a black hole with nine solar masses and a lighter object with 1.9 solar masses.
“The gravitational waves alone do not tell us the structure of the lighter object, but we can determine its maximum mass,” explains Pfeiffer’s colleague Bhooshan Gadre. “By comparing this information with the theoretical predictions for neutron star masses in such binary systems, we can conclude that a neutron star is the most likely explanation for the second object.” This is corroborated by the second event GW200115, which occurred on January 15, 2002 by both LIGO Detectors and the Virgo detector. This gravitational wave signal also comes from the unequal collision of a black hole and a neutron star, as the astronomers report. A black hole with a weight of six solar masses and a neutron star with 1.5 solar masses were involved in the merger, which is around a billion light-years away. Using triangulation, the scientists succeeded in narrowing the position of this event to an area of the sky that corresponds to the size of around 3,000 full moons.
Neutron star was swallowed whole
Immediately after the detection of the gravitational wave signals, additional teams of astronomers were alerted to search for possible electromagnetic signs of the collision in the corresponding area of the sky – for example in the form of bursts of radiation. However, they did not find anything. The researchers attribute this to the great distance and the fact that the neutron star was probably swallowed as a whole by the black hole. “These were not events in which the black holes nibble at the neutron stars and throw debris around them like the cookie monster,” explains LIGO spokesman Patrick Brady of the University of Wisconsin-Milwaukee. “This ejection would generate radiation, but we don’t think that happened in these cases.” Co-author Susan Scott of the Australian National University adds: “It’s more like PacMan: the black hole has its neutron star partner completely swallowed. “
As the astronomers explain, the evidence of these mixed collisions now offers new opportunities to explore the formation, evolution, and end of such mixed pairs in more detail. “There is so much that we don’t know about neutron stars and black holes – how small or big they can get, how fast they rotate, how they pair up,” explains Maya Fishbach of Northwestern University. “With more gravitational wave data we will get the statistics we need to answer these questions and learn more about the most extreme objects in our universe.” To achieve an even higher sensitivity in 2022. As a third member of the group, the Japanese KAGRA detector has also been in operation since 2020.
Source: LIGO and Virgo collaboration; The Astrophysical Journal Letters, doi: 10.3847 / 2041-8213 / ac082e