Team including Northwestern scientists detect waves caused by merger of neutron star, mystery object
EVANSTON, Ill. (CBS) -- Northwestern University astrophysicists were among those involved in an international research collaboration that has detected a gravitational-wave signal caused by a merger of what is believed to be a neutron star, and a mystery object.
A neutron star is formed when a star runs out of fuel and collapses. The result is that all the protons and electrons in the star are crushed into neutrons – and the mass of the incredibly dense object remains comparable to the sun, but the size is comparable to a city, as explained by NASA.
Neutron stars develop from the collapse of stars measuring up to three times the mass of the sun. Stars with higher masses will collapse even further into black holes, which are so dense that not even light can escape their surfaces. Black holes gobble up galactic material and have such strong gravity that all the matter they acquire is squished down to a point of essentially infinite density called the singularity, NASA explained.
The mystery object discovered in the research collaboration involving Northwestern measures within what is called the "mass gap" – heavier than the heaviest known neutron star, but lighter than the lightest known black hole.
What is this mystery object? The gravitational-wave signal is not enough to find out, Northwestern noted. But it is known to be 2.5 to 4.5 times the mass of the sun, and located 650 million light-years from the earth.
Future detections of activity from the object, especially when accompanied by bursts of electromagnetic radiation, could be crucial to solving the mystery of what it might be, Northwestern said.
"While previous evidence for mass-gap objects has been reported both in gravitational and electromagnetic waves, this system is especially exciting because it's the first gravitational-wave detection of a mass-gap object paired with a neutron star," Northwestern's Sylvia Biscoveanu, who managed the scientific paper for the study and contributed to the analysis, said in a news release. "The observation of this system has important implications for both theories of binary evolution and electromagnetic counterparts to compact-object mergers."
Biscoveanu is a NASA Einstein Fellow at the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern. CIERA director and Northwestern physics and astronomy professor Vicky Kalogera also worked on the study, as did Michael Zevin, a CIERA visiting scholar and astrophysicist at the Adler Planetarium.
The gravitational signal was detected by what is known as the LIGO-Virgo-KAGRA Collaboration – which involves three different gravitational wave detection systems in different parts of the world. The Laser Interferometer Gravitational-Wave Observatory has two locations – in Hanford, Washington, and Livingston, Louisiana – the Virgo Gravitational Wave Interferometer is located in Pisa, Italy; and the Kamioka Gravitational Wave Detector (KAGRA) is located in Japan.
The detection systems discovered the signal GW230529 in May of last year, shortly after the start of their fourth run of observations, Northwestern said. Astrophysicists analyzed the signal and found it came from the merger of two objects – one with a mass of 1.2 to 2 times the mass of the sun, the other with a mass of 2.5 to 4.5 times the mass of the sun.
The researchers first said the less massive of the two objects was likely a neutron star, while the more massive of the two was likely a black hole. But scientists now believe the more massive object is a mystery object that falls into the mass gap.
Gravitational-wave observations provided nearly 200 measurements of compact-object masses like neutron stars and black holes, but only once before has a signal been found from a mass-gap object, Northwestern said. Signal GW190814 was observed from the merger of a black hole and something that had a greater mass than the heaviest known neutron star, the university said.
This latest detection was the first in which a mass-gap object apparently merged with a neutron star.
"Before we started observing the universe in gravitational waves, the properties of compact objects like black holes and neutron stars were indirectly inferred from electromagnetic observations of systems in our Milky Way," Zevin said in a news release. "The idea of a gap between neutron-star and black-hole masses, an idea that has been around for a quarter of a century, was driven by such electromagnetic observations. GW230529 is an exciting discovery because it hints at this 'mass gap' being less empty than astronomers previously thought, which has implications for the supernova explosions that form compact objects and for the potential light shows that ensue when a black hole rips apart a neutron star."
