Mystery Object Blurs Line between Neutron Stars and Black Holes

 From Scientific American.com (June 30, 2020):

For more than a decade astrophysicists have wondered why nature appears to show an odd restraint in the way it slays stars. In life, they range from pip-squeaks to behemoths. Small ones simply burn out and fade away, but something more curious happens to the jumbo-size variety. When such a star dies, its great bulk causes its innards to implode as a core-collapse supernova. The process sparks a cataclysmic explosion and compresses some of the remains into astrophysical exotica—often a neutron star or, for the very heaviest suns, a black hole. Yet a pronounced rift appears to divide the weight classes of these two types of massive stellar corpses. Although astronomers have spotted neutron stars weighing up to around two solar masses and black holes as light as five, middleweight cadavers have gone entirely missing—until now.

In June 2020 the Laser Interferometer Gravitational-wave Observatory (LIGO) Scientific Collaboration announced the first conclusive detection of a stellar remnant falling into the so-called mass gap between neutron stars and black holes. After months of calculations, researchers at LIGO and the Virgo gravitational-wave detector in Italy concluded that such waves rippling through Earth in August 2019—an event dubbed GW190814 that was initially classified as a black hole consuming a neutron star—actually came from a 23-solar-mass black hole swallowing a mysterious 2.6-solar-mass object. Whether the smaller body is the heaviest known neutron star or the lightest known black hole—or a truly exotic beast, such as a star made of particles distinct from those of normal stars—its existence suggests that the theories describing the most extreme stellar fates need updating.

“I would rank this as definitely the most exciting announcement we’ve seen from LIGO since the original binary black hole discovery and then the first detection of a neutron star collision,” says Duncan Brown, a gravitational-wave astronomer at Syracuse University, who was not involved in the research. “We’re probing a new piece of astrophysical understanding of the universe.”

The new finding hints that the cosmos may enjoy a wider freedom in how it disposes of stars than researchers had supposed. Whether that leeway means atomic building blocks have enough brawn to support more monstrous neutron stars or that supernovae can forge tinier black holes, LIGO’s detection shrinks the gulf between those two most plausible scenarios.

“The idea of a mass gap as a true gap with nothing in it, I think, is getting progressively destroyed,” says Philippe Landry, a LIGO member at California State University, Fullerton. “This is going to be one nail in the coffin.”

From a fundamental physics perspective, the line separating neutron stars from black holes is razor-thin. If you toss an apple onto a neutron star at the limit of what its constituent neutrons can bear, it will abruptly collapse into a black hole. The heftiest known neutron star weighs 2.14 times the mass of our sun. And nuclear theorists suspect the objects can grow somewhat heavier, with the most optimistic models putting the complete breakdown of matter at 2.5 solar masses. Based on such theories, the LIGO collaboration calculated that the chance of the lighter partner in GW190814 being a neutron star is less than 3 percent. A neutron star that heavy, Landry says, would be a “complete game changer.” [read more]