Last year the gravitational wave detectors LIGO and Virgo faced a completely new type of collision: not two neutron stars, not two black holes, but a neutron star and a black hole.
Scientists were delighted: for the first time such a binary system was discovered.
Now, exploring the outer space in which the collision occurred, an international team of astronomers noticed the consequences, or rather, their absence.
Using some of the world's most powerful astronomical instruments, ElectromagNetic astronomers in collaboration with the VEry Large Telescope (ENGRAVE) failed to detect even a brief burst of light associated with the collision. Their research, pending peer review, has been published on the arXiv preprint server.
This does not mean that there was no event called S190814bv. That doesn't even mean there was absolutely no burst of electromagnetic radiation – what's called an 'electromagnetic counterpart' for detecting gravitational waves.
This means that astronomers have a little more information – the beginning of a database that will help us learn more about these elusive mergers in the future. And that could allow scientists to impose some tentative restrictions on the actions of a black hole devouring a neutron star – if that really was what really happened.
It is still not entirely clear what this event was – the analysis of the gravitational wave data is still being carried out. But the data suggests that the collision occurred between an object three times the mass of the Sun and another five times the mass of the Sun.
Both neutron stars and black holes are superdense remnants of dead stars, but we have never seen a black hole less than 5 solar masses or a neutron star greater than 2.5 solar masses.
So S190814bv could very well be that elusive double collision of a neutron star and a black hole.
Despite the fact that the signal from the gravitational wave S190814bv was strong, finding this hypothetical flash of light from afar – about 800 million light years – was not an easy task.
It is also possible that the neutron star was not torn apart until it was already inside the black hole's event horizon – preventing any light from escaping from the black hole.
And even if S190814bv was not a neutron star and black hole collision, there is a lot to learn. Astronomers also hunt for what is called a 'mass gap', when one or both of the colliding bodies are between the upper mass limit of neutron stars (2.5 solar masses) and the lower limit of black holes (5 solar masses).
We may not be able to know if what's in that gap is a tiny black hole or a chunky neutron star from data gathered from S190814bv. But the team has demonstrated that their collaboration works, and they are ready and waiting to collect the next round of observations, and the next, and the next after that.
The study was submitted to Astronomy & Astrophysics and is available on the arXiv website.
Sources: Photo: UCSC Transients
