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June 1, 2017 | Rachel Gaal
The Laser Interferometer Gravitational-wave Observatory (LIGO) continues to make waves within the physics community. On June 1, they published their third gravitational wave detection from an observation in early January 2017. This new detection may help vet previously established theories of how these massive mergers behave, and has already provided a new round of observational data to push forward this new kind of astronomy.
The confirmed results, published in Physical Review Letters, tell a story of two black holes that coalesced 3 billion light-years from Earth. So far, this is the farthest source from which gravitational waves have been detected (the first and second detections were located 1.3 and 1.4 billion light-years away, respectively). The black hole formed by the merger has a mass about 49 times that of our sun. This fills in a gap between the masses of the two merged black holes detected previously by LIGO, with solar masses of 62 and 21 times that of our sun.
The newest observation also provides clues about the directions in which the black holes are spinning. The data suggests that at least one of the black holes involved in the merger may have been rotating about its axis in the opposite direction of the system's overall orbital motion. More observations with LIGO are needed to say anything definitive about the spins of binary black holes, but these early data offer clues about how these pairs may form.
In one well-known theory of how binary pairs of black holes can be formed, the black holes come together later in life within crowded stellar clusters, pairing up after they sink to the center of a star cluster. In this scenario, the black holes can spin in any direction relative to their orbital motion. Because LIGO sees some evidence that the black holes are non-aligned, the data slightly favor this dense stellar cluster theory.
"We're starting to gather real statistics on binary black hole systems," said Keita Kawabe of Caltech, an author of the paper, who is based at the LIGO Hanford Observatory, in the PRL press release. "That's interesting because some models of black hole binary formation are somewhat favored over the others even now and, in the future, we can further narrow this down."
The new detection occurred during LIGO's current observing run, which began November 30, 2016, and will continue through the summer. Its observations are carried out by twin detectors — one in Hanford, Washington, and the other in Livingston, Louisiana — operated by Caltech and MIT with funding from the National Science Foundation (NSF). A body of more than 1,000 international scientists who perform LIGO research together with the European-based Virgo Collaboration make up the LIGO Scientific Collaboration (LSC), which represents one of the largest international science collaborations in recent history.
"We have further confirmation of the existence of stellar-mass black holes that are larger than 20 solar masses — these are objects we didn't know existed before LIGO detected them," announced MIT's David Shoemaker, the newly elected spokesperson for the LSC in a press release. "It is remarkable that humans can put together a story, and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us. The entire LIGO and Virgo scientific collaborations worked to put all these pieces together."
To learn more about the recent LIGO detection, read the latest Physics Synopsis.