Five years ago today, the LIGO and VIRGO gravitational wave detectors made what is still their most fruitful detection: two neutron stars during their last minute or so of orbiting about one another and then merging together, exploding spectacularly, and forming … well, we still aren’t sure whether they formed a black hole (probably) or a more massive neutron star. We call the event GW170817.
This is what the waves from the last minute of the orbit and the merger looked like and sounded like, as inferred from the GW data. Listen carefully at the very end! The other signals in this video are black hole mergers, which last much much less time.
GW170817 was fruitful scientifically because the explosion — something you don’t get when two invisible black holes merge into a bigger invisible black hole — was visible from Earth and was observed by astronomers using not only optical telescopes, but also radio telescopes and satellites detecting X-rays and γ-rays. We listened to it with GWs and watched it with light. This combination of different ways of observing has led to thousands of research publications (Google Scholar today lists over 13,000, though this may include much second-hand reporting) on the physics and astrophysics of neutron stars.
GW170817 also established gravitational wave detection as a full partner to the other branches of astronomy. And it effectively created a new form of astronomy called multimessenger astronomy, the combining of information brought to us by different messengers (GWs, electromagnetic waves, cosmic rays, neutrinos).
I wrote about this event in a previous blog post on the first anniversary of the detection in 2018, so I am not going to repeat all that. What is remarkable is that, just since 2019 there have been over 1200 research papers which mentioned GW170817 in their abstracts. The event clearly continues to stimulate research across many areas of astrophysics!
And yet the event itself has remained unique. We have detected a few further mergers of neutron stars with other neutron stars and even with black holes, but these events have been so far away that it has not proved possible for astronomers to identify the counterpart. That’s because an event that is further away is weaker, so we can’t pinpoint its location on the sky very well, so astronomers have to search over too large an area. What is more, no gamma rays have been detected from these, presumably because the narrow jet emitted by the merger, which radiates the gamma rays, has been directed away from our line of sight. We have come to appreciate how lucky an event GW170817 really was!
As we look forward to the re-starting of observations by LIGO, Virgo, and now the Japanese KAGRA (with LIGO-India under construction), by the end of this year or early next, we have great hopes that we will finally get to do joint observations again with other astronomers on a handful of merger events involving neutron stars. The detectors will have improved their reach by a factor of 1.5 or more, which opens up an observing volume more than (1.5)3 = 3.4 times larger than before. Meanwhile, optical astronomers have also been improving their ability to survey the skies for short-lived explosions.
The improved sensitivity during the upcoming one-year-long observing run O4 will produce an abundance of events. Black-hole binary mergers may be detected once every one or two days, and the science from them may well answer many questions left by previous observations. How are these black hole binaries formed? Do they constitute a large part of the so-far invisible dark matter of the Universe? Can we use them to measure the expansion rate of the Universe (the Hubble-Lemaître constant) accurately enough to resolve the tension among current determinations? And, of course, we will be waiting with fingers crossed to see more GW170817’s!