Black hole collisions they are so powerful that they distort the very structure of space-time, sending gravitational waves through the cosmos. Waves lap the Earth, and tuned detectors allow us to “hear” these collisions with impressive precision. However, we cannot “see” them. Black holes swallow light and radiation with their immense gravitational pull, making these mergers invisible to us.
Through S190521g, a candidate gravitational wave event detected on May 21, 2019.
When the wave crashed into Earth, it activated the gravitational wave detectors at the twin LIGO facilities in the US and the Virgo observatory in Italy. The signal told investigators they had just heard two giant black holes colliding in a distant region of space. Fortunately, almost at the same time, the Zwicky Transitional Facility at the Palomar Observatory in California had its telescopic eyes focused on the same region of space.
And by carefully studying the data, the researchers found an explosive explosion that occurred at almost the same time.
In a study published in the astrophysical journal Physical Review Letters on Thursday, astronomers detail the flare detected by ZTF and why they believe it is connected to the merger of S190521g black holes. If his theory is confirmed, it would be the first time anyone detects an electromagnetic counterpart, light, associated with a black hole collision.
“It would be amazing if the GW and EM signals were related,” says Rory Smith, an astrophysicist at Monash University in Australia, who was not affiliated with the study.
The new research is based on the theory that black hole mergers regularly occur in accretion disks surrounding supermassive black holes. The accretion disk is a rotating region filled with gas, dust, stars, and black holes, and in this extreme environment the cosmic beasts constantly come into contact with each other: they meet, dance, and potentially collide. Previous research predicted what an explosive explosion from a black hole fusion would look like if it took place on an accretion disk.
According to the team’s predictions, Matthew Graham, the ZTF chief scientist and first author of the study, and his team looked for this explosive flare in the ZTF data and eventually found their candidate near a distant supermassive black hole called J1249 + 3449. The team believes that a pair of black holes merged into the giant gas disk and the fusion caused a “recoil”, disturbing and heating the gas and debris. The disturbance is the flare that ZTF collected.
“The new fused black hole gets this kick and there is material being dragged along with it.” [It] it crashes into this gaseous environment around it and you get a head-on collision, that’s the initial cause of the eruption, “Graham explained.
This type of flash is not unknown. In April astronomers saw an eruption associated with a black hole passing through the gas disk of galaxy OJ 287. The phenomenon in OJ 287 is slightly different, but Graham explains that the team “was really happy” that other scientists might be familiar with the type of event they were proposing.
However, there are a number of reasons why you may see these kinds of flashes on an accretion disk, and the team was still not convinced.
“I was pretty skeptical at first,” says Saavik Ford, an astrophysicist at the American Museum of Natural History and co-author of the article. “This flash seemed interesting, but the gas disks around the black holes glow all the time, and I wasn’t sure how excited I was.”
Ford explains that eruptions can also occur when a star explodes or during tidal disruption events, when a black planet swallows a planet. “The flare really doesn’t look like any of those things,” Ford said. Accretion discs are also prone to burning, but again, Ford says the disc flares generally don’t look like the ZTF flash saw. Also, the disk at J1249 + 3449 has not fired in the past decade and a half.
“The only remaining option is that it is a new and very unusual type of flare from this gas disk, a discovery that would be very interesting on its own!” Ford says.
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Balance is restored.
A quick recap of the team’s theory: Two black holes merge into J1249 + 3449’s accretion disk. The event causes an outbreak, detected by ZTF. The event also creates a gravitational wave, detected by LIGO and Virgo. The researchers believe they have “seen” the explosion of light from a fusion of black holes. Now the newly formed and merged black hole has settled, the flare has disappeared, and balance appears to have been restored.
But what else can we learn?
The eruption can also inform astronomers about the physical characteristics of the two colliding black holes. Ford says that if the predictions are correct, this will be “the most massive merger” observed so far by LIGO and Virgo. Merged black holes are believed to be just under 100 solar masses – that is, they became 100 times more massive than our sun.
To get to that size, black holes would have to merge with each other over time, gradually swallowing and becoming more and more massive, like some kind of cosmic Katamari. And an accretion album could be the perfect place for this, because it is within them that they have the opportunity to court, dance, and eventually merge with each other. If this happens regularly on accretion disks, we should be able to detect these outbreaks more frequently.
Graham admits that the team’s predictions may ultimately be wrong. He even hopes that other scientists may have alternative explanations for the flare. But, the team’s predictions are testable, and when the fused black holes interact with the disk again in early 2022, the team will be watching.
“We hope to see a blazing event in a year and a half, if our model is correct,” he said. “That is good science.”
Additionally, LIGO-Virgo researchers are studying the gravitational wave event, S190521g, with attention. Detecting gravitational waves should allow astronomers to work backward and estimate the masses of the merging black holes.
“Assuming that the LIGO observation is a genuine astrophysical signal, the LIGO measurement of the black hole mass can be compared to the EM measurement once it is made public,” said Rory Smith, the Monash astronomer.
If they align with what Graham, Ford, and their colleagues predict, this could be a bona fide black hole bonanza and open up a new way to study these extreme cosmic events.
“This type of work complements discoveries like GW190814,” said Smith. “Joint observations of gravitational waves and EMs make the universe take a much sharper focus.”
It has been a good week for gravitational wave astronomy. On Tuesday, researchers from the LIGO and Virgo collaboration, including Smith, announced an event called GW190814. Collision between a black hole and a ‘mysterious object’ That could be the lightest black hole ever detected or the heaviest neutron star, raises new questions for gravitational-wave astronomers about some of the most extreme phenomena in the universe.