Astronomers announced late last month that they had discovered something out in the dark: a stellar corpse too heavy to be a neutron star — the remnant of a supernova explosion — but not heavy enough to be a black hole. Whatever it once was, it is long gone. About 780 million years ago — and 780 million light-years away — it was eaten by a black hole 23 times more massive than the sun. That feast left behind an even heavier black hole — a vast, hungry nothing with the mass of 25 suns.
News of the event only recently reached Earth, in space-time ripples known as gravitational waves. These evanescent vibrations were felt on Aug. 14, 2019, by an array of antennas in Italy and the United States called the International LIGO-Virgo Collaboration; the results were published in Astrophysical Journal Letters.
According to a theory that has been the backbone of decades of astrophysical excitement, a star can wind up in one of three final states, depending on its mass: a perpetually cooling cinder known as a white dwarf; a dense star, with the mass of a couple of suns compressed into a ball only 12 or so miles wide, known as a neutron star; or a black hole, a beast reluctantly predicted by Albert Einstein to be so dense that nothing, not even light, can escape its gravity.
The victim in this collision weighed in at 2.6 solar masses, according to the LIGO-Virgo calculations. That is heavier than the accepted limit of 2.5 suns for a neutron star. But the lightest black hole ever measured was about five solar masses. So the mystery object lies squarely in what astrophysicists call the “mass gap.” Astronomers have long wondered what, if anything, could occupy this astronomical no-man’s land.
“We’ve been waiting decades to solve this mystery” Vicky Kalogera of Northwestern University, one of the main authors of the paper, said in an interview. “We don’t know if this object is the heaviest known neutron star or the lightest known black hole, but either way it breaks a record.” She added: “If it’s a neutron star, it’s an exciting neutron star. If it’s a black hole, it’s an exciting black hole.” In a statement issued by the Science and Technology Facilities Council in Britain, Charlie Hoy, a graduate student at Cardiff University and Kalogera’s co-author, said, “I did not believe the alert when I first saw it come through.”
The LIGO observatory made history in 2016 when it detected gravitational waves from a pair of colliding black holes, proving the existence both of gravitational waves, a century after Einstein predicted them, and of black holes. The instrument consists of twin L-shaped antennas in Hanford, Washington, and Livingston, Louisiana. Since then, LIGO has been joined in its exploration of the darkness by another antenna known as Virgo, in Cascina, Italy. The combined LIGO-Virgo Collaboration consists of about 2,000 scientists around the world. The alphabetical listing of their names and institutions takes up the first 5 1/2 pages of the new paper.
The puzzling collision recorded last August was one of 56 possible gravitational wave events — most of which appear to be black hole collisions — detected during the observatory’s third run, which went from April 2019 until March 2020, when the coronavirus pandemic shut down most scientific activities around the world. The collaboration is still reviewing the data in an effort to analyse and confirm them. Kalogera said that the event was exciting for several reasons. The ratio of the two colliding masses was the most extreme — 9-to-1 — of the gravitational wave collisions that have been observed so far. Astronomers have difficulty imagining how such unmatched stars could get together in a binary double-star system to begin with. “This is very hard for formation theories to explain,” she said.
The signal — a characteristic “chirp” caused by the colliding objects circling faster and faster as they approach their moment of ultimate doom — lasted about 10 seconds. “Due to the favourable circumstance of having observed such a loud signal with quite different component masses and for about 10 seconds, we achieved the most precise gravitational-wave measurement of a black hole spin to date,” Alessandra Buonanno, of the Albert Einstein Institute in Potsdam, Germany, said in a statement issued by the institute’s arm in Hannover, Germany.
A black hole’s spin carries important information about the birth and evolution of the black hole, Buonanno noted. In this case, it revealed that the black hole was spinning “rather slowly,” less than one-tenth the rate allowed by the strictures of Einstein’s theory. Nobody had any immediate explanation or candidate for what kind of entity could fill this mass gap — a “dearth,” Kalogera called it — except to affirm that the calculations were robust.
Gordon Baym, an expert on neutron stars at the University of Illinois, pointed out that the collision of a pair of neutron stars in 2017, which produced a cosmic fireworks spectacle, left behind a neutron star with about 2.7 solar masses, thus briefly occupying the mass gap. But that object is thought to have collapsed into a black hole almost immediately. Most well-measured neutron stars have masses of around 1.4 suns, Baym said, and only a handful have masses more than two. In theoretical calculations, he said, “it is very hard to make matter stiff enough, using reasonable physics,” to conjure a neutron star in the range of 2.6 solar masses.
Daniel Holz, an astronomy professor at the University of Chicago who is a member of the LIGO collaboration, but not one of the principal authors of this paper, mused that neutron stars and black holes were in some sense “polar opposites.” “A neutron star is composed of the densest matter in the universe, and is in some sense the ultimate star,” he said in an email. “A black hole is just warped space and time. It doesn’t even have a physical surface! And the interior of a black hole is in some sense not even part of our universe, since nothing can come out of it.”
He added: “What is astounding is that, despite their profound differences, in this particular case we can’t tell which is which!” All the clues disappeared into the resultant black hole. “So we’re not sure if this object is a neutron star or a black hole, and either way it’s exciting and we learn something new,” Holz said. “It’s a win-win! Lots of theorists are now sharpening their pencils to try to explain what we’ve seen.”
Dennis Overbye c.2020 The New York Times Company
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