Today, a huge scientific team announced that humanity has added a third gravitational wave detector to its arsenal. And, only two weeks after Europe's VIRGO detector joined forces with the two LIGO detectors, the three combined to pick up a new black hole merger. While the three have worked together for less than a month so far, there are plans for a substantial observation run next autumn.

Waves in space

Gravitational waves are produced as two massive objects spiral inward toward a collision. Once they get close enough, their rapid circling distorts space itself, sending gravitational waves rippling out. Immediately after their collision, the object formed by their merger vibrates like a bell, briefly producing a different pattern of waves. These ripples in space will alternately expand and contract the distance between two objects by an infinitesimal amount—but one sufficient for our most sensitive instruments to pick up.

The new event, GW170814, is similar to the ones detected earlier. It involves stellar-mass black holes, 31 and 25 times the mass of the Sun. Those are heavier than theoretical work indicates should be possible to form through the collapse of stars. This suggests that either these were formed through earlier mergers or some alternate route of formation exists. The resulting black hole is 53 times our Sun's mass. The missing material—three solar masses' worth of black hole—was converted to energy in the form of gravitational waves. The event occurred about 1.8 billion light years away, though, so don't be surprised that you didn't notice anything.

The software that runs the detectors is set up to do a quick-and-dirty analysis to recognize any potential signals in the data. It then sends out a "trigger" to alert traditional telescopes that there's a region of sky they might want to look at closely. Some of these signals will be false alarms that go away upon detailed analysis, but the chance of understanding these events better makes those a worthwhile risk. In this case, there was no indication of any light (visible or otherwise) produced by the black hole merger.

Earlier this year, rumors circulated that LIGO had also spotted a neutron star merger that was associated with a burst of energetic photons called gamma rays. Neutron stars are less massive than a black hole, so the corresponding release of gravitational energy should be smaller. But these events are almost certain to liberate photons, either visible or at other wavelengths, something that black holes are notoriously stingy about. Several telescopes, including the Hubble, Chandra, and Fermi orbiting observatories all pursued observations of the galaxy NGC 4993 about this time, as did several ground-based observatories. The Chandra's observation log indicated it had done so in response to a trigger from LIGO/VIRGO.

If this has happened, the team is keeping quiet about it, although there's a press conference happening, and I expect the matter to be raised. We'll update the story if they drop any hints.

And then there were three

The announcement was significant in ways that go beyond the discovery, however, as it's the first discovery by the combined LIGO-VIRGO team. VIRGO, built in Italy, uses the same approach to spotting gravitational waves as LIGO: bouncing light off mirrors spaced kilometers apart and looking for subtle changes in the interference pattern that results when the light returns. Passing gravitational waves will alter the distance between the mirrors, altering the interference pattern.

At three kilometers long (compared to LIGO's four), VIRGO is less sensitive than the existing detectors, but its presence can make a big difference, as the two teams have agreed to combine and jointly analyze their data. Seeing a weak signal in three different detectors increases the probability it's real, and having a third detector allows us to make far better estimates of where the source of the gravitational waves resided. Detecting an event with all three detectors allows us to pinpoint the event to a smaller volume of the Universe—the volume is 20 times smaller than we could localize things with only two detectors.

An upgraded VIRGO detector joined the two LIGOs on August first. This was followed by a 24-day period where all three detectors were searching simultaneously; GW170814 was picked up on August 14. The joint observations ended when the LIGOs ended their observational run and shut down for a period of maintenance and upgrades. The VIRGO detector teased that this was a productive period, with its website declaring: "Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis."

The next joint observation run is planned for the Autumn of 2018. MIT's David Shoemaker, a spokesman for the collaboration, indicated the enhanced sensitivity of adding a third detector will make a big difference: “With the next observing run planned for Fall 2018 we can expect such detections weekly or even more often.”

Which is another way of saying that we should be prepared for black hole mergers to become as boring as finding a new exoplanet. Any "merger fatigue," however, would mark the transition from simply learning about the events to providing a greater sense of the overall statistics of mergers and the black holes that fuel them, which would tell us significant things about the Universe as a whole... which seems a fair tradeoff.

We're listening in on the press conference and will update this story if anyone says anything they shouldn't. A paper describing the merger has been accepted for publication in Physical Review Letters, and the LIGO team has placed a copy online.