Clash of the Titans: two supermassive black holes about to collide

 Clash of the Titans: two supermassive black holes about to collide 

Tick … tick … boom? A new study report observing signs of a supermassive black hole that has a closely orbiting companion. Supermassive black holes—very dark and dense objects hundreds of millions of times larger than the Earth's Sun—are in the center of most, if not all, galaxies in the universe. While astronomers believe most current supermassive black holes resulted from at least one merger between two smaller supermassive black holes, scientists lacked the observations that could give insight, since only one pair of supermassive black holes on the way to a merger had been found. The new study of two giant black holes that appear to be circling each other like fighters in a galactic boxing ring may help to confirm that hypothesis.

According to a study recently published on February 2, the black hole sharing the name PKS 2131021 is in a dance destined for about 9 billion light-years from Earth. PKS 2131-021 is one of 1,800 blazars that a group of researchers at Caltech in Pasadena has been monitoring with the Owens Valley Radio Observatory in Northern California for 13 years as part of a general study of blazar behavior. However, this specific blazar exhibits a strange behavior: Its brightness shows regular ups and downs as predictably as the ticking of a clock. Researchers believe that this regular variation is the result of pulling the first black hole as it orbits the other black hole about every two years. Each of the two black holes in PKS 2131021 is estimated to have hundreds of millions of solar masses. Moreover, the diameter of the orbit of the binary is 10 to 100 times smaller than the diameter of the only other known super-large binary, and this pair merges in about 10,000 years. It may seem long, but it will take a total of about 100 million years for black holes of this size to begin to orbit each other and finally converge. Therefore, more than 99% of this pair is heading for a collision. 

Even though the collision itself happened eons ago, there are imperceptible gravitational waves generated before the collision that are hitting us right now. The waves of these black hole activities will increase but will not affect the Earth. Nevertheless, they can help improve our understanding of how our universe has evolved. In the future, a pulsar timing array consisting of a series of pulsating dead stars closely monitored by a radio telescope should be able to detect gravitational waves from a supermassive black hole of this size. Future Laser Interferometer Space Antenna (LISA) missions will detect the coalescence of black holes with a mass of 1,000 to 10 million times greater than the mass of our sun. So far, no gravitational waves have been registered from any of these heavier sources, however PKS 2131-021 provides the most promising target yet.

References:

https://www.jpl.nasa.gov/news/astronomers-find-two-giant-black-holes-spiraling-toward-a-collision

https://www.caltech.edu/about/news/colossal-black-holes-locked-in-dance-at-heart-of-galaxy

https://earthsky.org/space/supermassive-black-holes-closest-orbit-collision-pks2131/