Lucky find. See what Richland astronomers discovered this time

Gravitational waves from another collision of black holes were detected at the LIGO observatory near Richland, thanks to a little luck.

It’s the third announcement of the merger of black holes detected in the latest operating run of the Laser Interferometer Gravitational-wave Observatory on Hanford land.

The detection was made on June 7, but scientists’ efforts have been focused on the previously announced first-of-a-kind finding from the run — the first direct observation of two neutron stars colliding.

The June 7 discovery appeared to be from a collision involving the lightest black holes yet detected by LIGO.

The LIGO Hanford almost missed it, but not because of their small mass.

A month before the detection, LIGO paused its second observation run since a major upgrade. It needed to open the vacuum systems that stretch in tubes set at right angles 2.5 miles across the Hanford shrub-steppe landscape for maintenance.

LIGO Hanford’s twin detector in Louisiana, LIGO Livingston, had finished some similar maintenance in late May. But LIGO Hanford had additional issues that delayed its return to observation mode.

On the afternoon of June 7, LIGO Hanford was finally able to stay online reliably, and staff were making final preparations to again “listen” for gravitational waves.

Routine adjustments still were being made about 7 p.m. when gravitational waves passed through Hanford LIGO and were detected in Louisiana about 7 milliseconds later.

Hanford LIGO’s automatic detection system was not engaged, but LIGO Livingston quickly reported the possible detection.

The Hanford LIGO adjustments only affected a narrow frequency range, so it had still collected the data that showed the waves after excluding those frequencies.

The data indicated that relatively light black holes — seven and 12 times the mass of the sun — spiraled toward each other and collided at a distance of about a billion light years from Earth.

The merger left behind a final black hole 18 times the mass of the sun, meaning that energy equivalent to about one solar mass was emitted as gravitational waves, or ripples through space and time, during the collision.

The collision was similar to one of the black hole mergers detected during the first operating run of LIGO after it had been upgraded and dubbed Advanced LIGO, said Michael Landry, head of the LIGO Hanford observatory.

The first run for Advanced LIGO was from September 2015 to Jan. 2016. The second, latest run was Nov. 30, 2016, to Aug. 25, 2017.

On Christmas day 2015, the merger of black holes that were about 14 and eight times the mass of the sun was detected. They were much smaller than those in the history-making first direct detection of a black hole three months earlier, with gravitational waves created by black holes with 36 and 29 times the mass of the sun combining.

Smaller black holes like those detected June 7 have been detected indirectly with observatories that search for electromagnetic radiation, such as X-rays. Black holes do not produce light.

The June 7 detection will help astronomers compare the properties of black holes detected from gravitational waves and those detected through X-ray studies, according to LIGO.

The latest discovery will make a significant contribution to the growing field of “multimessenger astronomy,” in which gravitational wave astronomers and electromagnetic astronomers work together to learn more about the objects in space, according to an announcement by LIGO.

The recently announced LIGO detection makes the sixth black hole collision detected at Hanford LIGO, in collaboration with a twin LIGO in Louisiana, since the first detection in September 2015 confirmed Albert Einstein’s theory of relativity. One also was detected jointly at the Virgo observatory in Italy, but the European gravitational wave observatory was not collecting data June 7.

The count of six includes an Oct. 12, 2015, finding not widely announced from the first Advanced LIGO because it initially could not be confirmed as a gravitational wave detection. However, as the science of gravitational wave observation advances, it now appears more likely to be a detection of gravitational waves.

The LIGO observatories are searching for unimaginably small movements caused by gravitational waves passing through Earth.

Gravitational waves stretch objects in one direction and compress them in the other. A circle would become an ellipse.

At the end of each miles-long vacuum tube at the Hanford LIGO hangs a mirror suspended on fine wires. A high power laser beam is split to go down each tube, bouncing off the mirrors at each end. If the beam is undisturbed, it will bounce back and recombine perfectly.

But if a powerful enough gravitational wave is pulsing through the Earth, the beam will be disturbed as the waves slightly stretch one vacuum tube and compress the other. The movement is so small that it would take 10 trillion such movements to equal the width of a human hair.

To confirm that the movement was caused by a gravitational wave — rather than other movement the observatory it may detect, such as vibrations from waves on the Pacific Coast — data at the Hanford and Louisiana LIGOs are compared to help verify a detection.

Both LIGO observatories are now offline for further upgrades to improve sensitivity. Another operating run is planned to begin next fall.

“With expected increases in detector sensitivity in the third advanced detector network observing run, … detection of black hole binaries will be a routine occurrence,” predicted the paper announcing the latest LIGO detection published in The Astrophysical Journal Letters.

This story was originally published November 20, 2017 at 7:44 PM with the headline "Lucky find. See what Richland astronomers discovered this time."