The LIGO Scientific Collaboration and the Virgo collaboration have identified a second gravitational wave event in data from the Advanced LIGO detectors. The event was detected early on the morning of 26 December 2015, following on from the first direct detection of a gravitational wave in September.
The December gravitational waves were produced during the final moments of the merger of two black holes around 1.4 billion years ago. The black holes, 14 and 8 times the mass of the sun, merged to produce a single, more massive spinning black hole that is 21 times the mass of the sun.
“It is very significant that these black holes were much less massive than those observed in the first detection,” said Gabriela Gonzalez, LIGO Scientific Collaboration (LSC) spokesperson and professor of physics and astronomy at Louisiana State University. “Because of their lighter masses compared to the first detection, 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, a quantity of energy roughly equivalent to the mass of the sun was converted into gravitational waves. The detected signal comes from the last 27 orbits of the black holes before their merger. 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.
“In the near future, Virgo, the European interferometer, will join a growing network of gravitational wave detectors, which work together with ground-based telescopes that follow-up on the signals,” said Fulvio Ricci, the Virgo Collaboration spokesperson. “The three interferometers together will permit a far better localization in the sky of the signals.”
The detection of another gravitational wave events demonstrates that the era of gravitational wave astronomy has begun. Advanced LIGO’s next data-taking run will begin in the autumn, with further improvements in detector sensitivity. The Virgo detector is expected to start its observations during the second half of the new observing run.