Did LIGO Just Discover Two Fundamentally Different Types Of Neutron Star Mergers?
“The first neutron star-neutron star merger ever directly observed was seen in both gravitational waves and in various forms of light, giving us a window into the nature of short gamma ray bursts, kilonovae, and the origin of the heaviest elements of all. The second one, however, had no robustly confirmed electromagnetic counterpart at all. The only major physical differences were the combined mass (2.74 vs. 3.4 solar masses), the initial object formed (neutron star vs. black hole), and the distance to the event (130 vs. 518 million light-years).
It’s possible that there really was an electromagnetic counterpart, and we simply weren’t able to see it. However, it’s also possible that binary neutron star mergers that directly lead to a black hole don’t produce electromagnetic signatures or enriched, heavy elements at all. It’s possible that this binary neutron star system, the most massive one ever discovered to date, represents a fundamentally different class of objects than have ever been seen before. This incredible idea should get put to the test over the next few years, as gravitational wave detectors continue to find more and more of these mergers. If there are two different classes of neutron star mergers, LIGO and Virgo will lead us to that conclusion, but we have to wait for the scientific data to know for sure.”
Neutron stars, when they merge, can produce gamma ray bursts, ejecta, the heaviest elements of all, and electromagnetic afterglows that cover nearly the full spectrum of where light can exist. We saw this with a gravitational wave + gamma ray signal that occurred on August 17, 2017, leading to a revolutionary understanding of kilonovae. But our second merging binary neutron star system, which was seen in gravitational waves on April 25, 2019, displayed no such signal at all.