An article published on the arXiv server reports the observation of gravitational waves emitted by the merger of two black holes with very different masses. The scientists from the LIGO and Virgo collaborations used the data collected by their detectors to examine this merger, the first in which the mass of the two objects is so asymmetric given that the estimates made indicate that they were 8 and 30 times the Sun’s. The event, cataloged as GW190412, produced waves with shapes different from those found so far in mergers between objects of similar masses and contain information that allowed to obtain more precise measurements of the pair’s physical properties. It also made it possible to conduct new tests of the theory of general relativity, confirmed once again.
The third observing run (O3) of the LIGO detectors conducted together with the Virgo detector between April 2019 and March 2020 except for a work interruption and resumption in October 2019 led to 56 gravitational wave event candidates. In the first month of operations, on April 12, 2019, a candidate was detected, then confirmed, cataloged as GW190412. The merger between two black holes is by far the most common type of event detected in this early stage of gravitational wave astronomy, but typically the two objects have similar masses. The event GW190412 showed different characteristics and therefore particularly interesting.
the analysis of the event GW190412 indicates that the merger of the two black holes occurred between 1.9 and 2.9 billion light-years from Earth. The most interesting result came from the estimate of the two objects’ masses, which were about 8 and 30 times the Sun’s. The image (Courtesy N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project. All rights reserved) shows a screenshot from a simulation of an event such as GW190412.
Alessandra Buonanno, director of the Max Planck Institute for Gravitational Physics in Potsdam, Germany, explained that the greater the diversity in masses, the richer the spectrum of gravitational radiation. She made a musical comparison explaining that two black holes with similar mass and rotation are like a pair of violins while two black holes that are very asymmetrical in terms of mass and other characteristics are like two different instruments.
The theory of general relativity predicted the existence of gravitational waves, and the confirmation of the event cataloged as GW150914 already verified that prediction. The subsequent confirmed events were similar, but the theory also included gravitational waves of a higher harmonic, of the type generated by the merger of two asymmetric objects. The event GW190412 provided yet another verification of the theory of general relativity.
The suspension of the third observing run of the LIGO and Virgo detectors due to the Covid-19 pandemic shortened it by a few weeks, but is still a great success with many events under study. It remains to be seen how to proceed for the next updates which will further increase the detectors’ sensitivity to take another step forward in gravitational waves astronomy.
Mergers between black holes are proving common and are useful to try to understand the origin of those pairs. The possibilities are that they’re separately born objects that came close enough to bond gravitationally, or binary systems with two stars that exploded in supernovae that left two black holes. Each new study of a confirmed gravitational wave event helps to understand how common the various types of pairs are.