When two black holes eat each other, they release a lot of energy.
A lot. A significant fraction of the mass of black holes is converted into energy, which is radiated as gravitational waves, waves in the structure of space-time. These can have as much energy as, and I’m not kidding here, tens of thousands of times as much as the sun will radiate throughout its useful life… and they fly away in some seconds. In other words, during that time black holes emit hundreds of millions of times more energy than all the stars in the galaxy combined.
That kind of power is terrifying. Crushing mind. However, despite everything, the event itself is completely invisible and does not emit any light. It is possible under Some specific circumstances Some Light will be emitted, but in general, the two black holes merge, scream their gravitational waves and become a single somewhat larger black hole … and all without a emitted photon.
So no light is produced directly. But indirectlyIt turns out that things can be different.
On May 21, 2019, the LIGO and Virgo gravitational wave observatories detected a pair of black holes merging somewhere in the Universe; the event received the designation S190521g. Triangulating the signal, it was discovered that it came from a point in the sky in the direction of the Coma Berenices constellation. In moments, an automatic alert was sent to telescopes across the planet to search for some kind of optical signal.
No flash of light was seen. At least, not immediately.
At Mount Palomar in California there is a 1.2 meter telescope with a 600 megapixel camera that can see a whopping 47 square degrees of sky at once, a great field of view. Called the Zwicky Transient Facility, it scans the sky for things that flash (or simply change the brightness) at night.
34 days after detecting LIGO-Virgo gravitational waves, he was looking towards Coma Berenices and saw a flare, a glow, coming from the distant galaxy AGN J124942.3 +344929 (let’s call it J1249 for short). This is a active galaxy, one with a supermassive black hole in its heart (100 million times the mass of the Sun, so decently huge) that it is actively eating material. As it falls, this material gets very hot and emits quite a bit of light, making this galaxy visible even from its prohibitive distance of 4.6 billion light years.
The detected flare lasted approximately 50 days. During that time, the amount of energy it emitted was enormous, about 1044 Joules: more energy than the Sun will emit during its entire useful life!
Active galaxies are known to glow, but this particular one had been silent, or at least had a constant brightness, for about a year around the flare. By looking at the history of the galaxy (and the stories of other galaxies) and applying some statistics, astronomers discovered that the probability that this eruption is due to some intrinsic variability in the galaxy is less than 0.01%, one chance in ten thousand . So very low.
They also looked at other potential sources, such as supernovae, a star that was ripped apart by a black hole (!), And even microlenses, but nothing matched the profile.
Than does However, the fit is a “small” black hole that slips through space at high speed and crashes into the disk of material around the supermassive black hole in the center of J1249. An event like this would create huge shock waves on the disk, causing it to heat up substantially and eject radiation for weeks. The gas that falls into the smallest black hole would also build up around it, heat up, and also emit a lot of light. These are extremely powerful events, releasing as much energy as the Sun will do throughout its life.
Hmmmmm
OK, you may be thinking, so what? What does this have to do with S190521g, the black hole merger that happened a month earlier?
Ah, here is the really fun part. If two uneven-mass black holes merge, the explosion of gravitational wave energy is emitted asymmetrically – more is sent from one side than the other. This produces great force on the remaining combined black hole, giving it an extremely powerful kick: remember that the energies we are talking about here are vast – and can be accelerated at high speeds through space.
And if that black hole fusion occurred near the center of a galaxy, say one with a supermassive black hole and a huge disk of matter spinning around it, then it could shoot through that disk at high speed, creating shock waves that They could heat up the material and create a gigantic two-month-long flash of light, one bright enough to travel more than 4 billion light-years and still be seen by a telescope in California.
Unbelievably, math works. If the gravitational wave event S190521g was two black holes in the active galaxy J1249 merging to form a single larger black hole of approximately 100 times the mass of the Sun, the kick from the powerful waves could accelerate it to a staggering 200 kilometers. per second – More than 700,000 kilometers per hour! Then a month later, this black hole sank through the disk around the supermassive black hole, and the above events occurred. The amount of energy emitted by such a catastrophe would be roughly what was seen, and would last as long as the galaxy’s flare.
My God.
This gets better. You need a pair of binary black holes to merge, and black holes form when massive stars explode. But it is difficult to make black holes with about 50 times the mass of the Sun each (or let’s say 80 and 20). You would need a bigger star than any we have ever seen. So maybe the two separately formed black holes, from two independent stars, grew over time by eating things around them, and then somehow joined together to form a binary system later. Good idea, but that’s almost impossible to do in an empty space. It is incredibly unlikely, and even then it is difficult to get them to orbit each other.
However, there is a place in the Universe where something like that is much easier: close to a supermassive black hole! There may be many massive stars in orbit that become black holes, and many to feed them. Making them orbit each other is easier when there are many other things that can safely bring them together. In fact, massive black holes are likely the result of previous black hole mergers. Even if they got one of those off-center kicks, the gravity of the much larger supermassive black hole nearby would prevent them from shooting out of the galaxy.
So this story stays together from start to finish. It could actually be correct.
Now notice, this is all circumstantial. Math works and physics works too, but that doesn’t mean this is what happened. I was very skeptical when I saw the press release. But reading the newspaper, I have to think that this series of Rube Goldberg events is actually the most likely scenario.
I would say “incredible” but it is literally credible. Amazing, certainly.
So the merger did not directly produce a flash of light, but it may have started a series of events that resulted in a huge flash of light a month later. Amazing. And if it is true, it means that this could be happening many times in the Universe, which means that we cannot simply look for immediate flashes of mergers, but also those that happen weeks or months later.
On the way. Given all the physics involved, astronomers think that the final black hole from the merger is still in orbit around the supermassive black hole, which means it can dive through the disk. again in the future. And not so much, either: just 1.6 years after the first event, late 2020 / early 2021. You can bet that astronomers will keep their telescopic eyes on galaxy J1249. If the fireworks start again, we’ll have a good idea of their cause.
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