A Strange Cosmic Flicker Reveals Merging Black Holes

The two black holes of the PG 1302-102 binary system are currently orbiting each other at a distance not much larger than the diameter of our own solar system, which in cosmic terms is just down the hall. Within a 100 million years, the two bodies will eventually collide, releasing the energy equivalent of 100 million supernovae. It will be a hell of a show.

The existence of PG 1302-102 isn't a complete slam dunk. The first hints of its existence came only earlier this year via imagery collected by the ground-based telescopes of the Catalina Real-Time Transient Survey, which was then described in a paper from astronomers at Caltech. The latest evidence, offered in a study published this week in Nature, adds some crucial support in the form of ultraviolet data from the Hubble and GALEX space telescopes.

PG 1302-102, it seems, is real.

Evidence for the binary system revolves around (heh) a mysterious light signal—a cyclical flickering within the heart of a galaxy some 3.5 billion years from Earth. Every five years, the object appears to dim dramatically before returning to normal, a pattern which we can observe thanks to the relatively long-term (20 year) archival data sets provided by Hubble and GALEX.

"It's as if a 60-Watt light bulb suddenly appears to be 100 Watts," explains Daniel D'Orazio, lead author of the new study and an astronomer at Columbia University, in a NASA statement. "As the black hole light speeds away from us, it appears as a dimmer 20-Watt bulb."

The variation is the result of motion. One of the two black holes is moving at a completely astounding velocity of around 7 percent of the speed of light. This is fast enough to traverse the entire span of our solar system in five years, and, if the black hole in question were orbiting the Sun along the same path as Earth, it would experience a full year about twice a day.

This velocity translates into a really big blue shift. As an object moves toward Earth, the light it emits (black holes emit a lot of light) winds up having to travel a shorter distance and so it gets "bunched up," in a sense. The result is that shorter wavelengths of light are observed here on Earth, at least relative to the same body not moving rapidly toward Earth or moving away from Earth. This is just the Doppler effect and it explains the system's regular flickers.

D'Orazio and his group took the blue shift observed by the Caltech team and figured out how it would look at much shorter ultraviolet wavelengths. They reached the conclusion that the same effect should magnified by about 2.5 times, and this is what the Hubble and GALEX data shows.

Eventually, the two black holes will merge in a completely epic collision. But just before that happens, as the two bodies are spinning around each other at unimaginable velocities, they should kick out some serious gravitational waves—ripples in space-time itself.

Physicists are very interested in detecting gravitation waves, something that has so far proved to be elusive. We won't be bagging those waves from PG 1302-102, but being able to observe such a system as it decays could offer important insights into the problem. So far we know precious little about the mechanics of merging black holes, which are among the universe's grandest shows.