Before the Merge: Spiraling Black Holes. This artist's conception shows two merging black holes similar to those detected by LIGO. The black holes—which will ultimately spiral together into one larger black hole—are illustrated to be orbiting one another in a plane. However, the black holes are spinning in a non-aligned fashion, which means they have different orientations relative to the overall orbital motion of the pair. There is a hint of this phenomenon found by LIGO in at least one black hole of the GW170104 system.

Image Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet).

For the third time, a telltale signal of two colliding black holes has been caught by the dual detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO). Not only does the new detection reinforce LIGO’s capabilities and previous detections , it also provides clues about how these black hole systems form and just how common they are. In addition, each new detection is a chance to test the predictions of general relativity—predictions that can’t be tested in a lab.Referred to as GW170104 for its detection date, the newest gravitational wave signal originated when two heavy black holes violently collided, one about 20 times the mass of our sun and the other about 30 times the mass of the sun. On colliding, they merged into a single black hole approximately 50 times the mass of our sun. The intense event created ripples in the fabric of space-time that reached the LIGO detectors on January 4, 2017, after traveling a distance of three billion light years.Today’s announcement was made by scientists from the LIGO Scientific Collaboration and the Virgo Collaboration, an international team working to bring a third gravitational wave detector, Virgo , online this summer in Italy. Their discovery and analysis is outlined in a paper in the American Physical Society’s journaland adds to the information the scientists gained from the first two detections.On September 14, 2015, LIGO detected gravitational waves for the first time ever , originating from the merger of two black holes each more than 25 times the mass of the sun. This was a huge accomplishment and a bit of a surprise, as prior to that detection scientists didn’t know that collapsing stars could form black holes that massive. On December 25, 2015, LIGO detected gravitational waves for the second time , in this case originating from the merger of two smaller black holes. Both of these detections occurred during the first stretch of time LIGO was operational after a $200 million upgrade called Advanced LIGO.The first Advanced LIGO observing run lasted about four months, ending in January of 2016. Over the next several months, the team increased the sensitivity of the detectors by reducing internal noise, replacing parts, and otherwise improving the quality of the data. The second run started in November of 2016 and is expected to go until the end of this summer. It was during this run that GW170104 was spotted.The new data not only supports the existence of heavy stellar black holes—black holes more than 20 times the mass of the sun—they also provide clues about how binary black hole systems form. Prior to a black hole merger like the ones detected by LIGO, two black holes orbit around one another. Over time they get closer and closer together, losing energy in the form of gravitational waves , until they eventually collide and merge.Scientists don’t know yet how these black hole binary systems form, but there are two general theories. The first is that the black holes form together: If two stars already in a binary system eventually collapse into black holes, it makes sense that the black holes would remain in orbit around one another. The second theory suggests that the black holes form independently within dense clusters of stars. Over time, heavy black holes sink to the center of star clusters and, if conditions are right, can pair off into binary systems.This new detection slightly favors the second theory. The clue comes in the form of spin information captured in the gravitational wave signal. In a binary system, each of the two black holes spin individually as they spiral around one another. If the black holes formed together, each one should spin in the same direction as the overall orbital motion (the spiral). If they formed independently, the black holes can spin in any direction relative to the overall orbital motion. At least one of the black holes in the binary system whose merger created GW170104 doesn’t seem to align with the overall orbital motion, according to the researchers. However, they caution, more data is needed to rule out either theory.Moving forward, LIGO should be able to detect more and more of these black hole mergers. Although they are relatively rare events, when LIGO reaches full sensitivity (around 2021), it should be able to detect about 1-7 mergers per week, the researchers estimate. They are also eagerly looking forward to the detection of gravitational waves from two merging neutron stars. Such an event would produce gamma rays that could be detected by space-based gamma ray detectors, enabling a new kind of cooperative astronomy. Perhaps that will be the next LIGO announcement!For more on gravitational waves, check out these stories from Physics Central: