Physicists Identify Second Gravitational-Wave Event

An international team of scientists has identified a second gravitational wave event in the data from the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO).

This image depicts two black holes just moments before they collided and merged with each other, releasing energy in the form of gravitational waves. Image credit: S. Ossokine, A. Buonanno, T. Dietrich and R. Haas, Max Planck Institute for Gravitational Physics / Simulating eXtreme Spacetime Project.

This image depicts two black holes just moments before they collided and merged with each other, releasing energy in the form of gravitational waves. Image credit: S. Ossokine, A. Buonanno, T. Dietrich and R. Haas, Max Planck Institute for Gravitational Physics / Simulating eXtreme Spacetime Project.

Gravitational waves are ‘ripples’ in space-time produced by some of the most violent events in the cosmos.

They carry unique information about the origins of the Universe and studying them is expected to provide important insights into the evolution of black holes, stars, supernovae and gamma-ray bursts.

However, they interact very weakly with particles and require incredibly sensitive equipment to detect.

Physicists on the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration are using a technique called laser interferometry.

LIGO’s first detection of gravitational waves and a pair of colliding black holes occurred September 14, 2015.

On December 25, 2015, at 10:38 p.m. EST (7:38 p.m. PST), the scientists observed gravitational waves for the second time.

The gravitational wave event, named GW151226, was detected by both of the twin LIGO detectors, located in Livingston, Louisiana, and Hanford, Washington.

“We report the observation of a gravitational-wave signal produced by the coalescence of two stellar-mass black holes,” the scientists said.

“The signal, GW151226, was observed by the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) on December 26, 2015 at 03:38:53 GMT. The signal was initially identified within 70 s by an online matched-filter search targeting binary coalescences.”

The team has concluded that these gravitational waves were produced during the final moments of the merger of two black holes — 14 and 8 times the mass of the Sun — to produce a single, more massive spinning black hole with 21 solar masses.

In comparison, the black holes detected September 14 were 36 and 29 times the Sun’s mass, merging into a black hole of 62 solar masses.

“It is very significant that these black holes were much less massive than those in the first detection,” said Louisiana State University scientist Prof. Gabriela Gonzalez, spokesperson of the LIGO Scientific Collaboration.

“Because of their lighter mass, they spent more time — about one second — in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our Universe.”

During the merger, which occurred about 1.4 billion years ago, a quantity of energy roughly equivalent to the mass of the Sun was converted into gravitational waves.

The GW151226 signal comes from the last 27 orbits of the black holes before their merger.

“GW151226 perfectly matches our theoretical predictions for how two black holes move around each other for several tens of orbits and ultimately merge,” said Prof. Alessandra Buonanno, LSC principal investigator and a physicist at the University of Maryland.

“Remarkably, we could also infer that at least one of the two black holes in the binary was spinning.”

Based on the arrival time of the signals — with the Livingston detector measuring the waves 1.1 milliseconds before the Hanford detector — the position of the source in the sky can be roughly determined.

The discovery is reported in the journal Physical Review Letters.

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B.?P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration). 2016. GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence. Phys. Rev. Lett. 116 (24): 241103; doi: 10.1103/PhysRevLett.116.241103

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