A newfound cosmic lighthouse is shedding light on a yearslong mystery from the first detected collision of neutron stars, revealing that one member of this doomed stellar pair was far larger than the other, a new study finds.
Detecting more of these unequal mergers could one day help solve a cosmic mystery surrounding how quickly the universe is expanding, as well as the ultimate fate of the cosmos, researchers said.
In 2017, astronomers witnessed a never-before-seen event — two neutron stars merging. Neutron stars are corpses of large stars that perished in catastrophic explosions known as supernovas. Although neutron stars are usually small, with diameters of about 12 miles (19 kilometers) or so, they are extraordinarily dense. A neutron star's mass may be about the same as that of the sun, and a teaspoon of neutron-star material has a mass of about a billion tons, all in all making neutron stars the universe's densest objects besides black holes. (Their name derives from how the gravitational pull of stellar remnants is strong enough to crush together protons and electrons to form neutrons.)
When stars collide
The 2017 discovery was made when scientists detected ripples in the fabric of space-time known as gravitational waves, which radiated outward from a crash between a pair of neutron stars located about 130 million light-years away from Earth, a merger dubbed GW170817. Astronomers quickly followed up this find with observations from conventional telescopes around the world, marking the first time both gravitational waves and electromagnetic waves were seen from an astronomical event.
A better understanding of neutron star mergers could shed light on the origins of the universe's heaviest elements. Recent findings have suggested that much of the gold and other elements heavier than iron on the periodic table were born in the aftermath of colliding neutron stars.
As scientists followed up on this discovery, the huge amount of matter ejected from the collision and the brightness of this debris proved an unexpected mystery. One possible explanation was that the merger involved neutron stars of different sizes. However, until now, the nine known binary systems composed of neutron stars orbiting each other closely enough to merge within the age of the universe all involved pairs of roughly equal mass.
Now scientists have found a binary composed of neutron stars of different sizes, which supports the possibility that GW170817 was a merger between such neutron stars.
Pulsar solves mystery
Using the Arecibo Observatory, a giant radio telescope in Puerto Rico, and the volunteer distributed computing project Einstein@Home, the researchers analyzed the pulsar PSR J1913+1102, which is located about 23,290 light-years from Earth and was first discovered in 2012. Pulsars are rotating neutron stars that emit twin beams of radio waves from their magnetic poles. These beams appear to pulse because astronomers see them only when a pulsar pole is pointed at Earth — hence hence the name "pulsar," which is short for "pulsating star."
PSR J1913+1102 is part of a binary system with another neutron star. The scientists estimated these neutron stars, which are separated by a distance less than the width of our sun, will likely collide in about 470 million years.
The researchers discovered the pulsar is significantly larger than its companion, harboring about1.62 times the mass of the sun compared to 1.27 solar masses. This is the most unequal pairing seen yet between neutron star binaries that will likely one day merge.
"This is the first system we've found of its kind — a double neutron star binary system in which the relative masses of the two neutron stars in the system are so significantly different," study lead author Robert Ferdman, an astrophysicist at the University of East Anglia in England, told Space.com.
Since the pulsar is significantly larger than its companion, the pulsar's gravitational pull will distort the shape of its neighbor, stripping off large amounts of matter before they actually merge, and potentially ripping it completely apart. Such a disruption would eject more hot material than expected between mergers between neutron stars of equal mass, potentially helping to explain the mystery of GW170817.
All in all, asymmetric binaries could make up about 10% of all merging neutron star binaries, Ferdman said.
Lopsided pulsar dance
Since asymmetric neutron star mergers may often lead larger neutron stars to warp smaller companions, investigating such unequal partnerships may help researchers "delve deeper into understanding what makes up a neutron star," Ferdman said. "What sort of matter makes up the interior of a neutron star is quite a mystery, so observing the distortions a neutron star undergoes might help us understand them better."
In addition, the researchers noted that gravitational merging neutron stars might help shed light on a cosmic mystery concerning how quickly the universe is expanding. The cosmos has continued expanding since it was born about 13.8 billion years ago. By measuring the present rate of the universe's expansion, known as the Hubble constant, scientists can deduce the age of the cosmos, as well as details on how it evolved over time. They can even use the number to try to deduce the fate of the universe, such as whether it will expand forever, collapse upon itself or rip apart completely.
Currently, the two primary methods scientists use to measure the Hubble constant are yielding conflicting results. However, recent findings suggest researchers could measure the Hubble constant using gravitational waves from merging neutron stars. Asymmetric neutron star mergers that produce detectable gravitational and electromagnetic signatures could lead to very precise measurements of the Hubble constant and help solve the so-called Hubble constant conflict, Ferdman noted.
In the future, the researchers want to find more of these asymmetric neutron star binaries "to get an even better idea of how common they are," Ferdman said.
The scientists detailed their findings online July 8 in the journal Nature.
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