Last Dance of Neutron Star Pair
The LIGO observatory near Richland has detected the apparent gravitational ripples from a black hole swallowing a neutron star.
It could be a scientific first.
Just after 2 p.m. Aug. 14, the Hanford Laser Interferometer Gravitational-wave Observatory detected the signal of the possible collision of a black hole and neutron star 870 million light years from Earth.
Two other gravitational-wave observatories also measured the waves.
Hanford LIGO and its twin LIGO observatory in Louisiana detected gravitational waves through space and time for the first time in 2015.
The first waves detected were created when two black holes spiraled together and collided in a distant galaxy more than 1 billion light-years away.
Since then, they have checked the detection of another event off their list — the gravitational waves from a fiery collision of two neutron stars, spewing material that radioactively decayed to create gold and platinum.
That left the highly anticipated detection of a merger of a black hole and a neutron star yet to be reported as the two U.S. LIGO observatories, joined by the Virgo gravitational wave observatory in Italy, started their most recent collaborative observing run April 1.
Just weeks into the observing run on April 26 researchers spotted the signal of what might be gravitational waves from a black hole swallowing a neutron star about 1.2 billion light years from Earth.
“Unfortunately, the signal is rather weak,” Patrick Brady, spokesman of the LIGO Scientific Collaboration, said earlier this spring. “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.”
Gravitational wave detection
Scientists have yet to confirm or rule out the April detection.
They are more confident in the August measurements of possible gravitational waves from the black hole and neutron star merger.
The possibility remains that the signal could be a false alarm caused by noise, but such a signal being caused by noise would be “very, very rare,” according to Keita Kawabe, the lead detector at the Hanford LIGO. “I personally feel that this is real.”
LIGO is a new type of observatory that looks for gravitational waves that provide evidence of past events far from Earth, rather than observing the heavens with light.
It has opened up a new way for scientists to learn about the universe, particularly when observances of both light and gravitational waves can be paired, creating the new field of “multi-messenger” astronomy.
Unlike when black holes merge, neutron stars send out not only gravitational waves, but also light.
When the first neutron star event was detected in August 2017, telescopes that detect different type of light waves were able to search the sky.
The merger of the two neutron stars where seen using gamma rays, optical light and radio waves.
There have been no confirmed reports so far of light being observed from the possible merger of a black hole and neutron star Aug. 14, although searching and checks of data continue, Kawabe said.
Was it a neutron star?
It’s not entirely certain that even if the Aug. 14 detection is confirmed to be gravitational waves that they were created by a black hole and a neutron star merger.
Researchers initially say a black hole is anything heavier than five times the Earth’s sun, and neutron stars are anything lighter than three solar masses, based on the masses of black holes and neutron stars found to date.
If the Aug. 14 event turns out to be real, the black hole may have swallowed a neutron star of the “tiniest black hole ever seen,” Kawabe said.
“Either way it’s going to be cool,” he said.
The National Science Foundation has been working on detecting gravitational waves for 40 years before the first detection was made in September 2015.
LIGO observatories at Hanford and in Louisiana operated from 2002 to 2010 without a detection.
It’s taken significant upgrades to reach the current sensitivity of the Hanford LIGO.
Before the planned 12-month operating run began in April the Hanford and Louisina LIGOs had observed 10 black hole mergers and one collision of two neutron stars.
But enhancements to the sensitivity of the LIGO and the Virgo observatories has led to more frequent possible detections in their current operating run.
“It’s been a really great run so far,” Kawabe said.
The collaboration has announced 23 possible detections, not counting several possible candidates that had been announced and then retracted as likely false alarms.
It appears that 20 of the candidate detections are mergers of black holes. A possible merger of two neutron stars also has been reported, but researchers’ confidence in that detection is low.
The remaining two are the possible April and August black hole and neutron star mergers.
The LIGO Hanford and Louisiana observatories are scheduled to take a break for the month of October to allow engineers to install new components and make upgrades and repairs.
The operating run will be extended through April 2020 to bring the observing time to a full 12 months.
$7.7 million education center
In other LIGO Hanford news, the observatory has received the $7.7 million in Washington state money approved in the last legislative session to build a “STEM Exploration Center” at the observatory.
The center, expected to open in fall 2021, will include a classroom and a maker-space for hands-on science education.
Since LIGO announced its first detection of gravitational waves, the number of schools wanting to take field trips to the observatory has nearly doubled.
The observatory also is open for monthly public tours.
The next free monthly tour of LIGO Hanford will be Sept. 14. 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.