Copyright © 2016 Hail Science

Hail Science

LIGO and Virgo detect rare mergers of black holes with neutron stars for the first time

Astronomy and Space

LIGO and Virgo detect rare mergers of black holes with neutron stars for the first time

Q: Tell us about these extreme, elusive systems. In general, what was known about collisions involving black holes and neutron stars prior to these detections?
A: Both neutron stars and black holes are left behind by massive stars once they run out of nuclear fuel. Since a large fraction of the stars in the universe are in binary systems, one would expect the existence of all possible pairwise combinations: two neutron stars, two black holes, or a neutron star and a black hole.
Neutron star binaries have been known for decades, discovered using electromagnetic radiation. Black hole binaries were observed for the first time in 2015, with the gravitational-wave detection GW150914. After that, gravitational-wave detectors such as LIGO and Virgo have discovered tens of binary black holes and two binary neutron stars. However, binaries with one neutron star and one black hole (NSBH) had never been found using electromagnetic radiation, nor with gravitational waves, at least until now.
Q: What can you tell from the signal about the possible scenarios that could have brought these objects together in the first place?
Sadly, not very much, at this stage! The most likely scenario is that the two objects in each binary have been together their whole life, as giant stars. As they ran out of fuel, they went through powerful explosions known as supernovae, leaving behind a neutron star and a black hole. The two objects in the binary then got closer and closer, since they lose energy through gravitational-wave emission, until they collide. LIGO and Virgo saw the last few seconds leading to the collision.
Theoretically these mergers could produce light, which is extremely exciting! However, for that to happen, one needs some matter to be left around the system after the collision. Unfortunately, if the black hole is too massive, or if it doesn’t rotate fast enough around its axis, it will entirely swallow the neutron star before this has a chance to get torn apart. When this happens, no matter is left behind, and hence no light. This is what might have happened with both of these gravitational-wave detections.
However, it is also possible that light was, in fact, emitted but was not detected by the telescopes that followed-up these systems. This is because their position in the sky — based on the gravitational-wave data — was rather uncertain, which implies telescopes might not have had a chance to find the electromagnetic counterpart before it faded away.
Q: What is the overall significance of this new detection? And what avenues does this open up in our understanding of the universe?
A: These two systems are important since they are the first clear discovery of neutron star black hole binaries, a type of source that had never been observed, with either electromagnetic or gravitational waves. It tells us that these systems do exist but are more rare than binary neutron stars. With only two sources, the numbers are still very uncertain, but roughly: for every 10 neutron star binaries, there is one NSBH merger.
The merger rate that we have calculated using these two signals, and the properties of the compact objects, will be a tremendous help to astronomers and modelers trying to understand formation and the evolution of NSBHs.
In fact, since none had ever been observed before, there wasn’t a good way to refine theoretical and numerical models. Those models are complicated and depend on many of the physical parameters of the binary system, as well as its history. For example: How violent is the supernova explosion that leaves behind neutron stars and black holes? Is it so powerful that it can destroy the binary system altogether?
Finally having access to NSBH mergers will help refine these models, and hence our understanding of the formation and evolution of compact objects.

Continue Reading

More in Astronomy and Space

To Top