Last summer, the gravitational wave observatory known as LIGO caught its second-ever glimpse of two neutron stars merging. The collision of these incredibly dense objects — the hulking cores of long-ago supernova explosions — sent shudders through space-time powerful enough to be detected here on Earth. But unlike the first merger, which conformed to expectations, this latest event has forced astrophysicists to rethink some basic assumptions about what’s lurking out there in the universe. “We have a dilemma,” said Enrico Ramirez-Ruiz of the University of California, Santa Cruz.
The exceptionally high mass of the two-star system was the first indication that this collision was unprecedented. And while the heft of the stars alone wasn’t enough to cause alarm, it hinted at the surprises to come.
In a paper recently posted to the scientific preprint site arxiv.org, Ramirez-Ruiz and his colleagues argue that GW190425, as the two-star system is known, challenges everything we thought we knew about neutron star pairs. This latest observation appears to be fundamentally incompatible with scientists’ current understanding of how these stars form, and how often. As a result, researchers may need to rethink years of accepted knowledge.
Before 2017, when LIGO captured its first neutron star merger, everything we knew about neutron stars came from observations of relatively nearby specimens in our own Milky Way galaxy. (Of the 2,500 or so known neutron stars, 18 coexist in orbiting pairs known as binary neutron stars.) GW190425, by contrast, is nearly 5,000 Milky Ways away.
The first puzzling thing about it is its mass: The new system has a total mass of around 3.4 suns. All previously known examples of binary neutron stars weighed somewhere around 2.6 suns. LIGO’s first binary neutron star pair fell right into this lower range.
But the high combined mass is just the first of the merger’s mysteries. More bewildering still is the inferred abundance of big neutron stars: Based on the recent observation, LIGO scientists estimate that these heavy pairings should be almost as common as the lighter binary star systems that astronomers have been studying for decades. Big neutron star pairs should be all over the universe, including our own Milky Way. Why, then, have they never been spotted before?
One possibility is that these mergers are hard to detect because they happen so rapidly.
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