The third detection of gravitational waves (GW170104) suggests that binary black holes exist, those in orbit around each other, but there are still many unknowns about how they form. Now researchers think they have learned something about their birth – they were given a kick by the supernovae that produced them.
To discover this, researchers from the Rochester Institute of Technology (RIT) and Caltech took another look at GW151226, the second gravitational wave event, produced when two black holes merged. Their findings, reported in Physical Review Letters, look at how a “natal kick” from two massive stars exploding might have produced a misaligned pair of black holes. This is often seen in neutron stars, which are produced by smaller stars going supernova.
Black hole pairs like the ones we have observed producing gravitational waves have two rotations: They orbit each other and they spin on their axis. In both GW151226 and in the more recent GW170104, the black holes were misaligned, with their spin being perpendicular to the plane they were orbiting.
The researchers, who presented their study at the American Astronomical Society meeting in Austin, Texas today, reanalyzed the data from the Laser Interferometer Gravitational-Wave Observatory (LIGO) and think that the supernova ricochet is the best explanation for the observed tilt.
In GW151226's case, the larger black hole (14 solar masses) orbits the smaller one (eight solar masses) with a slight tilt. The third gravitational wave detection also involved misaligned black holes, suggesting it may have formed in in a similar way.
“My collaborators and I tried to constrain the strength of these natal kicks based on LIGO’s observation,” lead author Professor Richard O’Shaughnessy from RIT said in a statement. “If it [the binary black hole pair] formed from an isolated pair of stars, we conclude strong black hole natal kicks were required. That’s an exciting challenge for models of how massive stars explode and collapse.”
The findings show just how useful gravitational wave astronomy can be. At the moment, it's our best way of studying these black holes, located billions of light-years away. And it's telling us more about stars that exploded billions of years ago, too. It's possible these binary black holes may have formed in another way, but this seems to be a good line of thought at the moment.
“Using essentially freshman physics, we drew new insights about the most violent events in the universe,” O’Shaughnessy said. “These measurements not only allow us to measure a property of ancient stellar explosions, if stellar siblings are responsible for LIGO’s sources. These kinds of measurements will soon allow us to tell whether stellar siblings are indeed principally responsible for what LIGO’s seen.”