It's hard to follow a star performer that made a really big bang, but the second detected collision of two neutron stars has proven to be a real heavy hitter, even if it will never have the impact of the first.
The first detection of two neutron stars colliding was a contender for the scientific event of the decade, transforming our knowledge of many fields of physics and involving up to a quarter of the planet's astronomers. By contrast, the second initially seemed like a letdown. However, the first paper on this event has revealed a star system so unlike anything we have seen before, astronomers were unsure if it was even possible.
Neutron stars are known to have masses of up to 2.4 times that of the Sun, so 4.8 solar masses looks like the upper limit for orbiting pairs. However, there are 10 known Milky Way neutron star pairs with decaying orbits that will cause them to eventually collide, and none of them remotely approach this ceiling. The same goes for the first detected collision.
According to a paper in The Astrophysical Journal Letters, the second case, dubbed GW190425, involved one neutron star with twice the mass of the Sun, and another with 1.4 times. The combination is still far short of the theoretical maximum, but Professor Susan Scott of the Australian National University told IFLScience a 3.4-solar-mass combination is a complete outlier, five standard deviations outside the previously observed range.
It's possible this is just a coincidence – 10 pairs in our galaxy is not a huge sample; maybe nearby galaxies host much more massive examples.
Alternatively, Scott thinks our existing, non-gravitational, detection methods may be missing something. “One theory is that the more massive pairs may be in very tight orbits, and they are orbiting so quickly it would create a Doppler smear, making these pairs difficult for us to see with conventional telescopes,” Scott said.
An even more interesting possibility is that these stars formed in conditions somehow different from our local environment. GW190425 occurred 520 million light-years away – far more distant than the first detected collision, let alone the pre-collision pairs we have found. “This leads to the intriguing possibilities that the old binary system we’ve discovered formed diﬀerently to those observed in the Milky Way,” Scott said in a statement.
Sadly GW190425 can give us these questions, but few answers to those posed previously. Most of the detail from the first neutron star collision came from optical, X-ray, and radio telescopes, which compared emissions across the electromagnetic spectrum to learn about the energy released, and the materials produced, in the explosion. The primary contribution of gravitational wave detectors was to alert astronomers to the event, and pinpoint its location so conventional telescopes knew where to look.
Sadly when GW190425's waves reached Earth, the gravitational wave detection system was down one detector, preventing precise pinpointing of the collision’s location. Attempts to scan the general area led to nothing, so beyond the mass surprise, our opportunities for learning were limited.