Bordering Binary Black-Holes Could Bang
An anomalous light signal from a quasar may be due to the smallest separation ever observed between two supermassive black holes that may merge one day.
Come on…you’ve thought about it I’m sure. What would happen if two black holes smashed into one another. Everyone loves collisions and explosions. Just look at the internet. After cats and porn, my guess is that crashing things are #3.
Sadly however, astronomers have never seen two black holes collide. Worse yet, supercomputer simulations of such a merger do not work, so we cannot yet even imagine accurately what would happen. This specific situation actually has a cool name, The Final Parsec Problem.
A parsec is an astronomical unit of distance equal to 3.26 light years or 31 trillion kilometers. Astronomers often use parsecs and mega-parsecs to describe vast distances in deep space. The Final Parsec problem describes the fact that our theoretical models do not predict what happens in the final stages of a black hole merger, nor even how long it could take. Another way to state this problem is that two black holes form a bound binary system at around 3 to 30 parsecs from each other. Gravitational waves, which would be instrumental in causing them to collide, are not significantly emitted however until their separation is much less than a parsec. What then would cause them to merge?
Observations have not been much help either since all potential mergers we’ve come across involve distances of thousands of parsecs. Until now that is.
The January 7 issue of Nature reports a finding by professor of astronomy and director of the Center for Data-Driven Discovery at Caltech, George Djorgovski and his colleagues. They describe a very unusual light signal from the heart of a quasar that might be the result of 2 black holes very close to each other and in the final stages of merging.
The galaxy, PG 1302-102, is about 3.5 billion light-years away in the constellation Virgo, and is a quasar. Quasars have feeding supermassive black holes at their centers that are so active that they are 100 times as luminous as the Milky. Many are visible from distances so vast that the universe was in its infancy when their light left. The brightest among these are estimated to consume the equivalent of 1000 suns per year or 600 earths per minute (nom earth nom).The light signal being received from this quasar however was not typical of the over 200,000 that have been observed to date. Usually these signals are bright and steady with some natural variations due to the uneven nature of food around the black hole. This signal though was periodic with a sine-wave-like waxing and waning that repeated every 5 years.
This was an attention-getter. A signal like that had never been observed before in a quasar (though 20 more were found later when it was specifically looked for). The fact that such a signal was rare was not the main reason it was interesting. The only thing known that could cause an accretion disk to behave like this is…another black hole.
The precise mechanism producing this signal is still up for grabs. Astronomers speculate that it could be caused by warps in the accretion disks around the holes or perhaps by the funneling of matter from the disks into the twin jets emitted by the holes which could also be precessing around each other like lighthouses.
Whichever mechanism they come up with though, they all can be explained by one thing. Two black holes very close to each other. How close? A fraction of a parsec, about 180 billion miles! That’s roughly the distance from the sun to the Oort cloud of comets at the edge of our solar system. This is well within the separation range between black holes that have been causing us problems and can therefore be most helpful in figuring out details about what happens when such titans collide.
One big problem though. It could still take a million years for them to close the distance. If you were expecting to see pictures of them smashing together in the near future, well, that’s not gonna happen.
To cheer you back up, let’s consider what might happen. When two supermassive black holes merge, it is thought that they could release the equivalent of 100 million supernovas. Damn that’s a lot. Most of the energy though would not be released as a huge ball of light. Most of it would be in the form of gravitational waves but that’s ok though because ordinary supernovas release most of their energy as ghostly and almost undetectable neutrinos. Despite that, they still put on a nice display in terms of a light-show. There may very well be a nice electromagnetic display for our robot descendants to see once these black holes collide.
Hopefully by that time we will have perfected gravity wave detector technology so the show can be enjoyed in its full glory.
Santiago Lombeyda, Center for Data-Driven Discovery, Caltech