The Boundary Between Black Holes & Neutron Stars

**SPEAKER_1** (0:00)
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**Matt O'Dowd** (1:00)
When we detected the very first gravitational wave, a new window was opened to the mysteries of the universe. We knew we'd see things previously thought impossible. And we just did. An object on the boundary between neutron stars and black holes, which promises to reveal secrets of both.
By now, we're becoming used to announcements that a new gravitational wave event has been detected. As though it's no big deal that we regularly read the infinitesimal ripples in the fabric of space time due to a cataclysmic collision of black holes billions of light years away.
As the LIGO and Virgo gravitational wave observatories spot event after event, the excitement is shifting from the holy crap we did it phase to giddy excitement about what we're actually learning. And the latest event is one of the most informative, and perhaps the most surprising so far. It seemed innocuous enough at first glance to compact bodies spiraling together. From the shape of the gravitational waveform, and based on calculations using Einstein's general theory of relativity, the masses of those bodies were calculated. One was a hefty 23 times the mass of our sun, making it definitely a black hole, and pretty similar to other LIGO mergers. Its companion was puny, by comparison, a mere 2.6 solar masses.
And there lies the surprise that that mass is pushing the limit for what was thought possible for a neutron star, and it's lighter than what was thought possible for a black hole. So what exactly is this thing? Today on Space Time Journal Club we're going to try to figure it out, and we'll do that by studying the paper that reported this detection, published by the LIGO Science Collaboration just a few weeks ago.
We've done gravitational wave astronomy before, but this event is so mysterious we had to cover it. But here's a quick refresh nonetheless. The LIGO observatories in Washington state and Louisiana, and the Virgo observatory in Italy, consist of kilometers long vacuum tubes set at right angles. A laser beam is split, sent down these tubes, then recombined. The passage of a gravitational wave causes extremely tiny changes in these arm lengths, which in turn causes the peaks and valleys of the laser's electromagnetic wave to line up differently. And so those changes can be measured. On August 14th, 2019, a gravitational wave hit the LIGO and VIRGO observatories, one after the other in close succession, consistent with a wave traveling through the entire earth at the speed of light. From the shape of the waveform, the masses were figured out as 23.2 and 2.59 solar masses. And we'll get back to why those are so weird. From the arrival times at the three observatories, the location of the event could be the same.

**SPEAKER_4** (4:00)
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**Matt O'Dowd** (4:30)
We narrowed down to some small arcs on the sky.
Unfortunately, there are countless galaxies in a region that size. So to start with, we have no idea in which galaxy the merger happened. Nonetheless, many telescopes quickly swiveled to scan that region, hoping to spot a faint flash of light, any indication that the merger of these objects may have been accompanied by an explosive event.