Gravitational wave detectors have detected a cosmic collision in which a giant black hole swallowed a mysterious object apparently too heavy to be a neutron star, but too light to be a black hole. Weighing 2.6 times the mass of the Sun, the object falls into a hypothetical "mass gap," a desert between the heaviest neutron star and the lightest black hole predicted by some theories, suggesting that the gap was not exists and that those theories need to be amended.
"People who thought there was a mass gap will have to rethink it, for sure," says Cole Miller, an astrophysicist at the University of Maryland, College Park, who was not involved in the observation. However, he adds: "People will not join cults because they cannot survive this change in their worldview."
The data comes from physicists working with the Laser Interferometer Gravitational Wave Observatory (LIGO), a pair of detectors in Louisiana and Washington state, and Virgo, a similar detector in Italy. All three consist of huge and exquisitely sensitive optical instruments that can detect the fleeting stretch of space itself when two massive objects, like black holes, swirl together. Since LIGO first detected such gravitational waves in 2015, physicists have detected dozens of mergers. And on August 14, 2019, the LIGO and Virgo detectors detected a fusion of objects with masses 23 and 2.6 times that of the Sun, the joint collaboration of LIGO-Virgo announced yesterday.
It's the 2.6 mass solar object that raises its eyebrows because it falls directly into the mass gap, says Vicky Kalogera, an astrophysicist and member of the LIGO team at Northwestern University. "Now, for the first time, we have seen such an object," she says. Feeling only the gravitational waves from the collision, LIGO and Virgo can't say for sure what the object is, he says. But nuclear physics suggests that a neutron star heavier than about 2.2 solar masses cannot support its own weight, making the object "almost certainly" a black hole, Miller says.
However, it is not easy to form a black hole with this light, explains Feryal Özel, an astrophysicist at the University of Arizona. Before the advent of LIGO and Virgo, the only observational evidence for black holes came from the study of about 30 in our own galaxy, each of which orbits a companion star that feeds hot matter. Neither of these black holes weighs less than five solar masses, Ozel and his colleagues observed in 2010. Therefore, they postulated a mass gap between approximately 2.5 and five solar masses in which neither neutron stars nor black holes should exist. But that notion is based on observation, Özel emphasizes. "Is there a fundamental physical reason why black holes cannot form below the five solar masses? We certainly don't believe it, ”she says. "But, there must be something about the wave that massive stars evolve that makes it very difficult."
Later, theorists explained why this can be so. A neutron star or black hole can form when a massive star runs out of hydrogen fuel and its core begins to collapse. If the star is light enough, the nucleus will collapse into a neutron star in a supernova explosion that expels the rest of the star. However, if the star is too massive, its core will shrink to an infinitesimal point, leaving only its super-intense gravitational field - a black hole. Theories suggest that for these heavy stars, all but the outermost layers of the star fall, increasing the mass of the black hole to five solar masses or more.
The new observation may take its toll on that theory, which Miller says has already met with some skepticism. "The supernova theorists doing the actual modeling basically say, 'Look, show us for sure that there is a mass gap and we'll work on it,'" he says. And Özel, one of the original proponents of the mass gap, does not seem disturbed by the finding. "It is very exciting," she says. Furthermore, LIGO and Virgo have shown that it is possible to form a low-mass black hole in a different way. In August 2017, they saw the merger of two neutron stars, which presumably produced a 2.7-solar-mass black hole.
The real puzzle may be the extreme mismatch in black hole masses in the new observation, says Brian Metzger, a theoretical astrophysicist at Columbia University who was not involved in the work. Just a few weeks ago, LIGO and Virgo announced an event in which one black hole outnumbered the other by a ratio of four to one. In the new event, the ratio is nine to one. "The interesting thing is the extreme mass ratio, which is difficult to produce through most [models] people have focused on, "says Metzger.
No one knows how closely in orbit pairs of black holes form in the first place. Most theorists have focused on two general scenarios, Metzger says: either in orbit of pairs of stars that collapse to form black holes or individual black holes that churn in tiny ancient galaxies called globular clusters that somehow manage to pair up. . Both scenarios tend to form pairs in which black holes have similar masses. Therefore, theorists may have to dream of new incubation rooms for twisted pairs of black holes, Metzger says, perhaps starting at the dense centers of large galaxies.
