Last Dance of Neutron Star Pair
Along with two other collaborating gravitational wave observatories, it possibly detected the gravitational waves created as a black hole swallowed a neutron star about 1.2 billion light years from Earth.
A neutron star is the smallest, densest type of star known to exist. A teaspoon-sized chunk of a neutron star weighs a billion tons.
Gravitational waves, or ripples through space and time, from the collisions of black holes have been detected several times since 2015.
And one detection has been made of gravitational waves from a fiery collision of two neutron stars, spewing material that radioactively decayed to create gold and platinum.
But if confirmed, the gravitational waves detected April 26 would be the first time a collision of a black hole and neutron star has been observed.
The Hanford LIGO, its twin in Louisiana and the Virgo gravitational wave observatory in Italy have made five possible detections of gravitational waves from violent collisions in space since they started their latest collaborative observing run April 1.
“The universe is keeping us on our toes,” said Patrick Brady, spokesman for the LIGO Scientific Collaboration.
Black holes and neutron stars
One of the latest detections was of gravitational waves from a collision of two neutron stars 500 million light years from Earth, which would make it the second neutron star collision observed.
The other three possible detections were of gravitational waves from black hole collisions. If confirmed, they would bring the total detected to 13.
Scientists are least sure about the possible gravitational waves from the neutron star and black hole collision.
“Unfortunately, the signal is rather weak,” Brady said. “It’s like listening to somebody whisper a word in a busy cafe. It can be difficult to make out the word or even to be sure that the person whispered at all.”
It will take some time for researchers to reach a conclusion, he said.
Unlike black hole mergers, when neutron stars collide they send out not only gravitational waves, but light.
Since the first neutron star collision was detected in August 2017, the new field of “multi-messenger” astronomy was launched with researchers collecting data from observations of both gravitational waves and light.
In the days and months after the August 2017 event, telescopes that can detect different types of light waves across the electromagnetic spectrum were able to witness the fiery collision of two neutron stars. It was seen using gamma rays, optical light and radio waves.
Telescopes still searching
But so far, the light from either of the possible events involving neutron stars from the latest observing run has proven elusive despite searches by hundreds of astronomers.
A detection at all three observatories allows scientists to triangulate data to better determine the source of the gravitational waves in the sky. But without LIGO Hanford on line on April 25, scientists have been able to narrow its location only to about a quarter of the sky.
The weak signal from the possible neutron star and black hole collision has been narrowed to 3 percent of the total sky.
Gravitational waves were detected for the first time in human history at the Hanford and Louisiana observatories in September 2015, 100 years after Albert Einstein’s general theory of relativity predicted their existence.
The National Science Foundation had been working toward a detection for 40 years, and LIGO detectors operated from 2002 to 2010 without a detection.
Operations were stopped for five years to make them more sensitive, leading to the first detection of gravitational waves.
Since the initial Advanced LIGO run in 2015, the observatories have been shut down two more times for incremental improvements.
During the last shutdown, the LIGO detectors’ and Virgo’s sensitivity was significantly enhanced for the current operating run. They now can detect gravitational waves from a larger area of the universe than before.
LIGO observing run continues
The improvements appear to have paid off, leading to detections more than weekly in the first month.
“We see the LIGO-Virgo collaborations realizing their potential of regularly producing discoveries that were once impossible,” said France Cordova, National Science Foundation director. “The data from these discoveries, and others sure to follow, will help the scientific community revolutionize our understanding of the invisible universe.”
The operating run of Hanford LIGO and the two other observatories will continue for 11 more months.
For the first time in this operating run, information is being released immediately about possible detections before they are confirmed, a process that can take months.
The information allows astronomers around the world to immediately start searching for any light in the sky associated with the events.
The public also can sign up for an app for Apple systems that provides notifications of detections.
The observatory in the Tri-Cities backyard is making scientific history, but it still is accessible to the public.
Mid-Columbia students make field trips to LIGO Hanford each spring, and public tours also are offered.
The next free monthly tour of LIGO Hanford will be May 11. Walking tours that last about an hour start at 1:30 and 3:30 p.m. A LIGO staff member will give a talk at about 3 p.m.
To reach LIGO, search for “LIGO Hanford Observatory” on Google Maps. Or drive northwest from Richland on Highway 240 and turn right on Hanford Route 10 and drive about five miles.