Collision between black hole and ‘mysterious object’ puzzles astronomers

Illustration of the space object GW190814

GW190814: A colliding black hole and neutron star, or something even more unusual?

Carl Knox Center of Excellence / OzGrav ARC

On August 14, 2019, a gravitational wave, a massive wave through the fabric of space-time, spread across Earth. The wave was detected by sophisticated and tuned lasers in the United States and Italy. And it was amazing. While lasers had previously collected black hole collisions and neutron star collisionsThey were now suggesting something unprecedented: a black hole crashing into a neutron star.

The signal was one of the strongest ever seen by gravitational wave scientists at the Laser Interferometer Gravitational Wave Observatory and the Virgo Observatory in Italy. After an alert was sent moments after detection, teams of astronomers around the world rotated their telescopes to the point in space from which the wave emanated.

But his searches came out empty. Without light, without X-rays, without infrared, without gamma rays.

The event was puzzling. And it became more puzzling as scientists began to study the data carefully. On Tuesday, researchers from the LIGO and Virgo collaborations detail their comprehensive analysis of gravitational wave detection, dubbed GW190814, in The Astrophysical Journal Letters. It is the first detailed study of the epic cosmic collision, and it only deepens the mystery.

“I think GW190814 is the first time we have seen gravitational waves where the source of the waves is really puzzling,” said Rory Smith, an astrophysicist at Monash University in Australia. “I’ve been in LIGO for just over 10 years, and this is without a doubt one of the most exciting events we’ve seen.”

The key to the investigation are the two LIGO facilities and the Virgo facility that can detect gravitational waves. Extreme astronomical objects like black holes and neutron stars send waves through the cosmos when they collide. The facilities essentially listen to the sounds of massive cosmic beasts colliding with each other, and then work backwards to understand their physical characteristics.

Smith and his colleagues have been working on simulating these types of collisions using supercomputers, which help perform that inverse calculation and can infer what the objects are, their probable masses, and their whereabouts.

“We use sophisticated parallel algorithms that can run our analyzes on a supercomputer cluster that contains many hundreds or thousands of individual computers,” he said. “Running the same analysis on your laptop would have taken about 50 to 100 years.”

Observations show that the pair GW190814 collided in a deep corner of space, 800 million light-years away. Half of the pair is definitely a black hole, about 23 times more massive than our sun. But his dance partner is mysterious: The other object is only 2.6 times more massive than our sun, putting him in a strange position. That could Being a neutron star, that possibility is still on the table, but it could also be a black hole. And that’s a little problem.

“It is difficult to explain how a black hole or a neutron star could have around 2.6 solar masses,” said Smith.

Scientists have never detected a black hole that is so light. Neutron stars are not expected to be this heavy – they collapse into black holes when they get too big. So the mysterious object appears to be some kind of Goldilocks star that doesn’t fit our current understanding. Whatever it is, it will rewrite our knowledge on one of the two extreme objects.

Interestingly, if it is an ultra-heavy neutron star, Smith says that “perhaps even new physics would be required to explain it.” If it is a clear black hole, our understanding of how and where light-starved cosmic beasts will form will be rewritten. It is a win-win scenario for science.

GW190814 is only the second time that a gravitational wave detection has found a significant discrepancy in the mass of objects. A collision between two black holes detected on April 12, 2019 and named GW190412, showed a difference in mass of more than 20 solar masses. These large differences are incredibly useful: They allow researchers to test Einstein’s theory of general relativity. Both GW190814 and GW190412 fit Einstein’s predictions, so we haven’t broken physics (yet).

GW190814 is exceptionally rare. We have only seen one of these events in three years of observation and it will be a while before we find more. The LIGO and Virgo detectors have been turned off since March, ending their last observation prematurely due to the coronavirus pandemic and won’t be back online until the end of next year.

“Our detectors are being updated, so they will be more responsive when they turn on,” Smith said. “At that time, we hope to see not only more systems like GW190814, but also other unexpected sources of gravitational waves.”

That leaves plenty of room to try to explain the mysterious object. Is it a black hole? Is it a neutron star?

“Theorists have a lot of fun ahead trying to explain GW190814!” Smith said.

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