LIGO and Virgo Find an NSBH

Conceptual animation of the gravity waves generated as a neutron star orbits and collapses into a stellar mass black hole.

Remember how excited everyone was when scientists detected the first gravity waves? It got the key players the 2017 Nobel Prize in Physics. Well, there are four different gravity wave detectors operating now, with a fifth scheduled to come on line in 2025. And gravity waves have been detected and tracked back to their source 48 times now, as of April 2021.

Probably we all need a quick refresher. Gravity waves, at least the ones we can presently detect, are made when extremely dense, extremely heavy objects move suddenly. Current gravity wave detection technology uses a laser reflected off a kilometers-distant object that is a very precisely known distance away. The reflection is timed very precisely. As a gravity wave moves through, it distorts the very time space, the fabric of our universe, making the object a tiny bit further away and a tiny bit closer. With three detectors, triangulation can determine where in the sky the event occurred. Sometimes astronomers can find visual proof with a hint where to look. LIGO and Virgo are the names of the first two gravity wave detectors.

Up until now, LIGO and Virgo had found two types of gravity wave generating events: those made by the collision/merger of two black holes, and those made by the collision/merger of two neutron stars. But until now, they had not detected a neutron star-black hole collision/merger, a so-called “mixed” or NSBH collision. Now the detectors have found two.

The first was on April 26, 2019 (it takes a while to process the data), and involved a black hole of about nine solar masses and a neutron star of about 1.9 solar masses about 900 million light years away. The second was on January 15 of this year, and involved a black hole of six solar masses merging with a neutron star of about 1.5 solar masses about 97 million lightyears away.

A neutron star is what’s left after a large star (one of 10 to 25 solar masses) explodes as a supernova. Most of the star’s mass is ejected in the explosion – think of the Crab Nebula – and what’s left undergoes extreme compression as the star’s core collapses. What’s left is neutronium, the densest form of matter known. A neutron star might have a radius of just 10 kilometers, but have a mass greater than Sol, our sun. There’s believed to be a lot of these buggers; maybe as many a a billion in the Milky Way galaxy alone. A few of them have inflating normal matter. A that matter is accelerated by the unbelievably intense gravity of the neutron star, they generate x-rays.

A stellar mass black hole forms when an even larger giant star, one of 25 solar masses or more, explodes and the remnant of the giant star is more than 3 to 4 solar masses, that remaining core will collapse under its own gravity to the point where even neutronium collapses, and becomes a black hole.¹ The collision of stellar mass black hole and a neutron star would leave only the black hole as a survivor, with the neutronium collapsed to whatever state of matter exists inside a black hole.

It’s a titanic enough collision that it can be detected and measured from 900 million lightyears away.

¹ A stellar mass black hole is a black hole generated by a supernova. It may get larger as it accretes more matter. They get unimaginably larger; the supermassive black hole at the center of the Milky Way, Sagittarius A, masses something like 4.3 million solar masses, but has a radius of just 0.002 lightyears.