LIVINGSTON – Barely one month into its latest eavesdropping on the universe, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Livingston Parish has detected two new celestial events.
LIGO, its sister facility in Hanford, Wash., and the Virgo detector in Pisa, Italy, registered gravitational waves on April 25 from what appears to be a crash between two neutron stars — the dense remnants of massive stars that previously exploded, according to a LIGO spokesman.
The next day, April 26, LIGO in Livingston and Virgo detected another candidate that may have resulted from the collision of a neutron star and black hole, a first to be detected.
"The universe is keeping us on our toes," said Patrick Brady, spokesman for the LIGO Scientific Collaboration and a professor of physics at the University of Wisconsin-Milwaukee.
Brady and a group of researchers from MIT and Cal State — which run the two LIGO facilities for the National Science Foundation the LIGO Scientific Collaboration and Virgo Collaboration — briefed the media on Thursday, May 2, about the developments.
"We're especially curious about the April 26 candidate. Unfortunately, the signal is rather weak,” said Brady at the media briefing carried on social media.
“It's like listening to somebody whisper a word in a busy café; 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 to reach a conclusion about this candidate."
LIGO and Virgo resumed operations April 1, after undergoing a series of upgrades to increase their sensitivities to gravitational waves, he said.
The April 25 neutron star smash-up, dubbed S190425z, is estimated to have occurred 500 million light-years from Earth. The local LIGO facility and Virgo detected the signal. The LIGO facility in Hanford was offline.
Because only two of the three detectors registered the signal, where it occurred was not precise, leaving astronomers to survey nearly one-quarter of the sky for the source.
The possible April 26 neutron star-black hole collision, called S190426c, is estimated to have taken place roughly 1.2 billion light-years away. It was recorded by all three facilities, which helped better narrow its location to about 3 percent of the total sky.
"The latest LIGO-Virgo observing run is proving to be the most exciting one so far," said David Reitze of Caltech, executive director of LIGO.
"We're already seeing hints of the first observation of a black hole swallowing a neutron star.
“If it holds up, this would be a trifecta for LIGO and Virgo—in three years, we'll have observed every type of black hole and neutron star collision,” Reitze said.
“But we've learned that claims of detections require a tremendous amount of painstaking work — checking and rechecking — so we'll have to see where the data takes us."
In addition to the two new detections involving neutron stars, the LIGO-Virgo network has observed three possible black-hole mergers.
Since making history with the first-ever direct detection of gravitational waves in 2015, the network has spotted evidence for two neutron star mergers, 13 black-hole mergers and one possible black hole-neutron star merger.
The first detection of gravitational waves, announced Feb. 11, 2016, was a milestone in physics and astronomy -- it confirmed a prediction of Albert
Einstein’s 1915 theory of general relativity, and marked the beginning of the new field of gravitational-wave astronomy.
LIGO was awarded the Nobel Prize in physics in 2017 for the achievement.
Then, on Oct. 16, 2017, scientists announced they had directly detected gravitational waves in addition to light from the collision of two neutron stars.
This marked the first time that a cosmic event has been viewed in both gravitational waves and light.
When two black holes collide, they warp the fabric of space and time, producing gravitational waves, according to LIGO. When two neutron stars collide, they not only send out gravitational waves but also light.
That means telescopes sensitive to light waves across the electromagnetic spectrum can witness these impacts together with LIGO and Virgo.
One event occurred in August 2017: LIGO and Virgo initially spotted a neutron star merger in gravitational waves and then, in the days and months that followed, about 70 telescopes on the ground and in space witnessed the explosive aftermath in light waves, including everything from gamma rays to optical light to radio waves.
"The search for explosive counterparts of the gravitational-wave signal is challenging due to the amount of sky that must be covered and the rapid changes in brightness that are expected," Brady said.
"The rate of neutron star merger candidates being found with LIGO and Virgo will give more opportunities to search for the explosions over the next year."