Two black holes collided in deep space. We may have seen it happen for the first time


Artist’s concept of a supermassive black hole and its surrounding disk of gas. Embedded within this disk are two smaller black holes orbiting one another.

Caltech/R. Hurt (IPAC)

Black hole collisions are so mighty they distort the very fabric of space-time, sending out gravitational waves across the cosmos. The waves wash over the Earth, and fine-tuned detectors allow us to “hear” these collisions with impressive accuracy. However, we can’t “see” them. Black holes gobble up light and radiation with their immense gravitational pull, so these mergers have remained invisible to us.

Until S190521g, a candidate gravitational wave event detected on May 21, 2019.

As the wave crashed into the Earth, it triggered the gravitational wave detectors at the twin LIGO facilities in the US and the Virgo observatory in Italy. The signal told researchers they’d just heard two mammoth black holes colliding in a distant region of space. Fortunately, around the same time, the Zwicky Transient Facility at the Palomar Observatory in California, had its telescopic eyes focused on the same region of space. 

And poring over the data, researchers found an explosive flare that occurred right around the same time.  

In a study, published in astrophysics journal Physical Review Letters on Thursday, astronomers detail the flare detected by ZTF and why they believe it’s connected to the merging black holes of S190521g. If their theory is confirmed, it would be the first time anyone has detected an electromagnetic counterpart — light — associated with a black hole collision. 

“It would be incredible if the GW and EM signal are related,” says Rory Smith, an astrophysicist at Monash University in Australia who was not affiliated with the study. 

Dancing mad

The new research draws on a theory that black hole mergers occur regularly in accretion disks surrounding supermassive black holes. The accretion disk is a swirling region full of gas, dust, stars and black holes, and in this extreme environment the cosmic beasts constantly come into contact with each other — meeting, dancing and potentially colliding. The previous research predicted what an explosive flare from a black hole merger might look like if it took place in an accretion disk.

Based on the team’s predictions, Matthew Graham, the principal scientist at ZTF and first author on the study, and his team went looking for this explosive flare in the ZTF data and eventually found their candidate near a distant supermassive black hole dubbed J1249+3449. The team believe a pair of black holes merged in the giant gas disk and the merger caused a “kick back,” disturbing and heating the gas and debris. The disturbance is the flare ZTF picked up. 

“The new new merged black hole gets this kick and there’s material dragged along with that. [It] slams into this gaseous environment around it and you get a shock front — that’s the initial cause of the flare,” explained Graham.

This type of flare isn’t unheard of. In April, astronomers saw a flare associated with a black hole crashing through the gaseous disk of the OJ 287 galaxy. The phenomenon in OJ 287 is slightly different, but Graham explains the team “were really happy” other scientists might be familiar with the type of event they were proposing. 

However, there are a number of reasons you might see these types of flare in an accretion disk — and the team wasn’t yet convinced.

“I was initially quite skeptical,” says Saavik Ford, an astrophysicist at the American Museum of Natural History and co-author on the paper. “This flare looked interesting, but gas disks around black holes flare all the time, and I wasn’t sure how excited to be.”  

Ford explains flares also might occur when a star explodes or during tidal disruption events — when a planets gets gobbled up by a nearby black hole. “The flare doesn’t really look like either one of those things,” said Ford. Accretion disks are also prone to flaring but, again, Ford says disk flares don’t generally look like the flare ZTF saw. Additionally, the disk at J1249+3449 hasn’t flared for the last decade and a half. 

“The only remaining option is that it’s a brand new and very unusual kind of flare from this gas disk — a discovery that would be very interesting all by itself!” says Ford.

Balance is restored

A quick recap on the team’s theory: Two black holes merge in the accretion disk of J1249+3449. The event causes a flare, detected by ZTF. The event also creates a gravitational wave, detected by LIGO and Virgo. Researchers believe they’ve “seen” the explosion of light from a black hole merger. Now the newly formed, merged black hole has settled down, the flare has disappeared and balance appears to have been restored.

But what else can we learn?

The flare can also tell astronomers about the physical characteristics of the two colliding black holes. Ford says if the predictions are correct, then this will be “the most massive merger” observed by LIGO and Virgo yet. The merged black holes are thought to be just shy of 100 solar masses — that is, they became 100 times more massive than our sun.

To get to that size, black holes would need to merge with each other over time, gradually swallowing each other up and getting more and more massive, like some sort of cosmic Katamari. And an accretion disk could be the perfect place for this, because it is within that they have the chance to court, dance with and eventually merge with each other. If this is happening in accretion disks regularly, we should be able to spot these flares more often.

Graham admits the team’s predictions may, ultimately, pan out to be wrong. He even expects other scientists may have alternate explanations for the flare. But, the team’s predictions are testable and when the merged black holes interact with the disk again in early 2022, the team will be watching. 

“We expect to see a flaring event in a year and a half, if our model is right,” he said. “That’s good science.”

In addition, researchers at LIGO-Virgo are studying the gravitational wave event, S190521g, intently. The gravitational wave detection should enable astronomers to work backwards and estimate the masses of the black holes that merged. 

“Assuming that the LIGO observation is a genuine astrophysical signal, then LIGO’s measurement of the black hole mass can be compared to the EM measurement once it’s made public,” said Rory Smith, the astronomer from Monash.   

If they line up with what Graham, Ford and their colleagues are predicting, this could be a bonafide black hole bonanza and open up a new way to study these extreme cosmic events.    

“This kind of work complements discoveries like GW190814,” said Smith. “Joint gravitational-wave and EM observations bring the universe into much sharper focus.”

It’s been a good week for gravitational wave astronomy. On Tuesday, researchers from the LIGO and Virgo collaboration, including Smith, announced an event dubbed GW190814. The collision, between a black hole and a ‘mysterious object’ that might be the lightest black hole ever detected or the heaviest neutron star, poses new questions for gravitational wave astronomers about some of the most extreme phenomena in the universe.   

Artist's concept of a supermassive black hole and its surrounding disk of gas. Embedded within this disk are two smaller black holes orbiting one another.

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