Aerial view of the Virgo site showing the Mode-Cleaner building, the Central building, the three kilometer-long west arm and the beginning of the north arm. The other buildings include offices, workshops, computer rooms and the control room of the interferometer. Credit: The Virgo collaboration/CCO 1.0

In August, detectors on two continents recorded gravitational wave signals from a pair of black holes colliding. This discovery, announced today, is the first observation of gravitational waves by three different detectors, marking a new era of greater insights and improved localization of cosmic events now available through globally networked gravitational-wave observatories.

The collision was observed Aug. 14 at 10:30:43 a.m. Coordinated Universal Time (UTC) using the two National Science Foundation (NSF)-funded Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors located in Livingston, Louisiana, and Hanford, Washington, and the Virgo detector, funded by CNRS and INFN and located near Pisa, Italy.

The detection by the LIGO Scientific Collaboration (LSC) and the Virgo collaboration is the first confirmed gravitational wave signal recorded by the Virgo detector. A paper about the event, a collision designated GW170814, has been accepted for publication in the journal Physical Review Letters.

"Little more than a year and a half ago, NSF announced that its Laser Interferometer Gravitational Wave Observatory had made the first-ever detection of gravitational waves, which resulted from the collision of two black holes in a galaxy a billion light-years away," said NSF Director France Córdova. "Today, we are delighted to announce the first discovery made in partnership between the Virgo gravitational-wave observatory and the LIGO Scientific Collaboration, the first time a gravitational wave detection was observed by these observatories, located thousands of miles apart. This is an exciting milestone in the growing international scientific effort to unlock the extraordinary mysteries of our universe."

LIGO operates two detector sites -- one near Hanford in eastern Washington, and another near Livingston, Louisiana. The Livingston detector site is pictured here. Credit: LIGO Collaboration

The detected gravitational waves—ripples in space and time—were emitted during the final moments of the merger of two black holes, one with a mass about 31 times that of our sun, the other about 25 times the mass of the sun. The event, located about 1.8 billion light-years away resulted in a spinning black hole with about 53 times the mass of our sun—that means about three solar masses were converted into gravitational-wave energy during the coalescence.

"This is just the beginning of observations with the network enabled by Virgo and LIGO working together," says LSC spokesperson David Shoemaker of the Massachusetts Institute of Technology (MIT). "With the next observing run planned for fall 2018, we can expect such detections weekly or even more often."

LIGO has transitioned into a second-generation gravitational-wave detector, known as Advanced LIGO, that consists of two identical interferometers. Beginning operations in September 2015, Advanced LIGO has conducted two observing runs. The second observing run, "O2," began Nov. 30, 2016, and ended Aug. 25, 2017.

The Virgo detector, also now a second-generation detector, joined the O2 run Aug. 1, 2017 at 10 a.m. UTC. The real-time detection Aug. 14 was triggered with data from all three LIGO and Virgo instruments.

View of the LIGO detector in Hanford, Washington. LIGO research is carried out by the LIGO Scientific Collaboration, a group of more than 1,000 scientists from universities around the US and 14 other countries. Credit: LIGO Laboratory

"It is wonderful to see a first gravitational-wave signal in our brand new Advanced Virgo detector only two weeks after it officially started taking data," says Jo van den Brand of Nikhef and Vrije Universiteit Amsterdam, spokesperson of the Virgo collaboration. "That's a great reward after all the work done in the Advanced Virgo project to upgrade the instrument over the past six years."

When an event is detected by a three-detector network, the area in the sky likely to contain the source shrinks significantly, improving distance accuracy. The sky region for GW170814 has a size of only 60 square degrees, more than 10 times smaller than the size using data available from the two LIGO interferometers alone.

"Being able to identify a smaller search region is important, because many compact object mergers—for example those involving neutron stars—are expected to produce broadband electromagnetic emissions in addition to gravitational waves," says Georgia Tech's Laura Cadonati, deputy spokesperson for the LIGO Scientific Collaboration. "This precision pointing information enabled 25 partner facilities to perform follow-up observations based on the LIGO-Virgo detection, but no counterpart was identified—as expected for black holes."

"With this first joint detection by the Advanced LIGO and Virgo detectors, we have taken one step further into the gravitational-wave cosmos," says Caltech's David H. Reitze, executive director of the LIGO Laboratory. "Virgo brings a powerful new capability to detect and better locate gravitational-wave sources, one that will undoubtedly lead to exciting and unanticipated results in the future."

Explore further New gravitational wave data analysis now underway</s>Black holes are objects in space whose gravitational pull is so strong that even light is unable to escape, but despite that, there is some amount of radiation that actually goes outward from black holes. Highly energetic jets traveling at near the speed of light, called active galactic nuclei (AGN) jets, take energy from that radiation…</s>"We have known about the existence of these clumps of material streaming along black hole jets, and that they move close to the speed of light, but not much is known about their internal structure or how they are launched," says Harish Vedantham, a Caltech Millikan Postdoctoral Scholar. "With lensing systems like this one, we can see the clumps closer to the central engine of the black hole and in much more detail than before." Vedantham is lead author of two new studies describing the results in the Aug. 15 issue of The Astrophysical Journal. The international project is led by Anthony Readhead, the Robinson Professor of Astronomy, Emeritus, and director of the OVRO.

Many supermassive black holes at the centers of galaxies blast out jets of gas traveling near the speed of light. The gravity of black holes pulls material toward them, but some of that material ends up ejected away from the black hole in jets. The jets are active for one to 10 million years—every few years, they spit out additional clumps of hot material. With the new gravitational lensing system, these clumps can be seen at scales about 100 times smaller than before.

"The clumps we're seeing are very close to the central black hole and are tiny—only a few light-days across. We think these tiny components moving at close to the speed of light are being magnified by a gravitational lens in the foreground spiral galaxy," says Readhead. "This provides exquisite resolution of a millionth of a second of arc, which is equivalent to viewing a grain of salt on the moon from Earth."

A critical element of this lensing system is the lens itself. The scientists think that this could be the first lens of intermediate mass—which means that it is bigger than previously observed "micro" lenses consisting of single stars and smaller than the well-studied massive lenses as big as galaxies. The lens described in the new paper, dubbed a "milli-lens," is thought to be about 10,000 solar masses, and most likely consists of a cluster of stars. An advantage of a milli-sized lens is that it is small enough not to block the entire source, which allows the jet clumps to be magnified and viewed as they travel, one by one, behind the lens. What's more, the researchers say the lens itself is of scientific interest because not much is known about objects of this intermediate-mass range.

"This system could provide a superb cosmic laboratory for both the study of gravitational milli-lensing and the inner workings of the nuclear jet in an active galaxy," says Readhead.

The new findings are part of an OVRO program to obtain twice-weekly observations of 1,800 active supermassive black holes and their host galaxies, using OVRO's 40-meter telescope, which detects radio emissions from celestial objects. The program has been running since 2008 in support of NASA's Fermi mission, which observes the same galaxies in higher-energy gamma rays.

In 2010, the OVRO researchers noticed something unusual happening with the galaxy in the study, an active galaxy called PKS 1413+ 135. Its radio emission had brightened, faded, and then brightened again in a very symmetrical fashion over the course of a year. The same type of event happened again in 2015. After a careful analysis that ruled out other scenarios, the researchers concluded that the overall brightening of the galaxy is most likely due to two successive high-speed clumps ejected by the galaxy's black hole a few years apart. The clumps traveled along the jet and became magnified when they passed behind the milli-lens.

"It has taken observations of a huge number of galaxies to find this one object with the symmetrical dips in brightness that point to the presence of a gravitational lens," says coauthor Timothy Pearson, a senior research scientist at Caltech who helped discover in 1981 that the jet clumps travel at close to the speed of light. "We are now looking hard at all our other data to try to find similar objects that can give a magnified view of galactic nuclei."

The next step to confirm the PKS 1413+ 135 results is to observe the galaxy with a technique called very-long-baseline interferometry (VLBI), in which radio telescopes across the globe work together to image cosmic objects in detail. The researchers plan to use this technique beginning this fall to look at the galaxy and its supermassive black hole, which is expected to shoot out another clump of jet material in the next few years. With the VLBI technique, they should be able to see the clump smeared out into an arc across the sky via the light-bending effects of the milli-lens. Identifying an arc would confirm that indeed a milli-lens is magnifying the ultra-fast jet clumps spewing from a supermassive black hole.

"We couldn't do studies like these without a university observatory like the Owens Valley Radio Observatory, where we have the time to dedicate a large telescope exclusively to a single program," said Readhead.

The two studies, titled, "Symmetric Achromatic Variability in Active Galaxies: A Powerful New Gravitational Lensing Probe?" and "The Peculiar Light Curve of J1415+1320: A Case Study in Extreme Scattering Events," are published in The Astrophysical Journal.

Explore further: Discovery of a rare quadruple gravitational lens candidate with Pan-STARRS
LIGO and Virgo observatories detect the gravitational wave signal from two merging black holes.