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Black hole merger
This graphic shows the ripples in spacetime created by gravitational waves emanating from the merger of two black holes. (Max Planck Institute / NCSA Illustration)

Astronomers have detected their fourth gravitational wave from the merger of two black holes, but this one marks a new milestone.

It’s the first wave picked up by the Virgo gravitational-wave detector in Italy — and the first opportunity to triangulate on its location with the twin detectors of the Laser Interferometer Gravitational-wave Observatory, or LIGO, in Louisiana and Washington state.

The Aug. 14 event, known as GW170814, showed that the ripples in spacetime were emitted by the smash-up of two black holes about 31 times and 25 times as massive as the sun, located about 1.8 billion light-years away. The merger created a single black hole about 53 times the sun’s mass.

Three solar masses were converted directly into gravitational-wave energy, in accordance with Albert Einstein’s famous equation E=mc2.

All that follows the model set by LIGO with its three previous detections since September 2015. The new twist involves folding in the data from Virgo, which started its first full-fledged advanced run in league with LIGO on Aug. 1.

The detectors pick up the faint signal of a passing gravitational wave by bouncing laser beams around in miles-long tunnels. The interference patterns can track ripples in the fabric of spacetime as small as a thousandth of the width of a proton.

Having three detectors made it possible for astronomers to narrow down the location of the black hole merger by a factor of 20. In this first case, the wave was picked up initially by Louisiana’s LIGO detector, and then 8 milliseconds later by the LIGO detector at Hanford, Wash., and then 6 milliseconds later by the Virgo detector near Pisa in Italy.

The readings pointed to a region of the night sky centered on the southern constellation Eridanus. Details of the observation and the analysis are being published in a research paper accepted for publication in Physical Review Letters.

Being able to zero in on a gravitational-wave detection will be a key step in turning more traditional telescopes toward the source of the emission, which could give astronomers a better understanding of the origins and effects of black hole mergers.

“Our ambition is, with our capabilities, to open this new field of multi-messenger astronomy, and I think now we have taken the first step in that process,” Virgo scientific spokesperson Jo van den Brand said today during a news conference conducted in Turin in conjunction with a meeting of G7 science ministers.

Black hole merger locations
A 3-D global projection of the Milky Way in the night sky shows the probable locations of four confirmed black hole mergers, plus an event of lower significance known as LVT151012. The green blip at lower left shows the regions of 90 percent and 10 percent probability for the location of the most recent LIGO-Virgo detection. (LIGO / Virgo / Caltech / MIT Illustration / Leo Singer)

For what it’s worth, observing teams working with Virgo and LIGO weren’t able to detect any significant emissions in the electromagnetic spectrum that corresponded with August’s gravitational-wave event.

David Shoemaker, spokesperson for the LIGO Scientific Collaboration, said the case of GW170814 is “just the beginning of observations with the network enabled by Virgo and LIGO working together.”

The detectors are currently being fine-tuned for their next observing run, due to begin in the fall of 2018. Shoemaker said that run should increase the sensitivity of the network by a factor of two, which means that events should be detectable eight times farther away than was the case for the just-concluded run.

“We can expect such detections weekly or even more often,” Shoemaker said in a news release.

The gravitational-wave detectors just might pick up the signals of other types of cosmic clashes, such as collisions involving neutron stars. Still more detectors are due to join the network in the years to come, such as IndiGO in India and KAGRA in Japan.

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