The merger event of two black holes, known as GW190521 and detected in 2019, may have been misinterpreted. A new study suggests that rather than an impact between binary objects, black holes found each other more “by chance”.
The collision was discovered through detection of gravitational waves — the most intense and strangest ever observed. The data showed that the black holes were probably between 85 and 66 solar masses, forming a new black hole of 142 solar masses.
With the analysis of gravitational waves, scientists concluded, at the time, that the objects must inhabit a stellar cluster and shared the same orbit, possibly being a binary system since the formation of the progenitor stars.
This means that before black holes were “born”, they were two massive stars formed within a star cluster and orbited each other. After they collapsed into black holes, they remained as a binary system, spiraling into each other on an imminent collision course.
But some things didn’t make much sense. For example, a black hole of 85 solar masses could not have arisen directly from the collapse of a single star — they cannot become that massive. Furthermore, the gravitational wave signal was strangely brief: it lasted less than a tenth of a second.
This led to new hypotheses, such as a kind of “cosmic billiards” or, even, a collision between two stars of a very strange class. Now, a new study suggests a different story for these black holes. After trying to reproduce the 2019 signal by simulating the collision with trajectory, spin and mass tweaks, they concluded that they were not born as a binary system.
According to astronomer Matteo Breschi, from the University of Jena and co-author of the study, the objects were independent and traveled through space in a highly eccentric (elliptical) orbit. As they approached, they were gravitationally “trapped” until they formed a spiral orbit and, finally, collided.
This scenario is more likely to happen in a high-density star cluster, as in environments like this this type of encounter is more common to happen. The team also concludes that the black holes were actually 81 and 52 solar masses.
The new mass measurement for the larger black hole is still too high for theoretical models. However, if these objects were in a dense cluster, it is likely that it evolved from smaller masses through collisions with other objects, such as small stars.
The study was published in Nature Astronomy🇧🇷