We’d like to introduce you to GW200105 and GW200115. These two gravitational wave detections, just ten days apart, originated from the mergers of pairs of black holes and neutron stars. Detected by the LIGO and Virgo detectors, these discoveries allow astronomers to essentially lay down a royal flush of detections.

In September 2015, LIGO and Virgo made their first observations of a pair of black holes merging and sending gravitational waves rippling through the universe, where they would be detected as oscillations in the space between detectors as space literally expanded and contracted in reaction to this event. Just like different bells produce different kinds of sounds, and you can measure the size of a bottle by the kind of sound it makes when blown into, the size of merging objects can be measured by the gravitational waves they make in the space-time continuum.

Yes, folks, you, like the detectors, expand and contract by ever so small an amount as these waves pass through you.

These waves are tiny and comparable to the size of a particle, not to the size of a person, and we’ll never notice them with our senses. It is frankly amazing we can detect them at all. I have to admit, I didn’t think we’d be able to ever make these detections from Earth, but determined scientists and engineers have a habit of finding a way.

From that first black hole-black hole merger, LIGO and Virgo have gone on to make numerous discoveries, including the August 2017 first discovery of two neutron stars merging. When it comes to generating detectable gravitational waves, there aren’t a lot of options out there. Two neutron stars can do it. Two black holes can do it, and the combo of a neutron star and a black hole can do it.

A quick literature review was unable to find a paper on the detection of any existing black hole-neutron star binary systems, although one 2005 paper, led by Eric Pfahl, postulated they should be very rare, with one being born only once per ten million years or less. They also should only last for about 100 million years before they merge. This means they are rare and short-lived and number among the hardest things to observe, although a paper from 2020 paper led by Debatri Chattopadhyay indicates that the Square Kilometre Array may be able to start detecting them if MeerKAT doesn’t get to it first.

It turns out LIGO and Virgo got to it first. That final detectable pair, the black hole-neutron star combination, has been seen not once, but twice at the moment of their merger into a bigger black hole.

Royal flush: that makes a black hole-black hole merger in 2015, a neutron star-neutron star merger in 2017, and now a black hole-neutron star merger in 2021.

That two of these were seen in ten days may seem a bit hard to fathom, but this is in part due to the ever-improving abilities of the detectors and the fact that random events sometimes will occur one after the other. Now that LIGO and Virgo can detect these, we look forward to seeing more and more combos of more and more different sizes and orbits to show themselves in their shaking of time and space.

More Information

CNRS press release

Northwestern press release

University of Wisconsin-Milwaukee press release

“Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences,” R. Abbott et al., 2021 June 29, The Astrophysical Journal Letters