On May 29, 2023, one of the two U.S. LIGO detectors recorded a gravitational wave with unexpected properties. The source is believed to be a collision of a neutron star and an unusual black hole.
The signal was measured by the LIGO detector at Hanford shortly after the May 2023 start of the fourth measurement period of the LIGO-Virgo-Kagra collaboration. With only one active detector, the origin of the signal could not be pinpointed on the celestial sphere. But the space-time vibration itself indicates an unexpected origin.
Analyses of the vibration pattern of GW230529 reads on a collision of an object of 1.2 to 2 solar masses, presumably a neutron star, with an object that was slightly more than twice as heavy.
Mass gap
That second object in particular intrigues experts because, in theory, black holes between 2 and 5 solar masses do not actually seem to be able to exist. This is called the mass gap and seems to have to do with the formation history of black holes that remain when a star burns up and implodes.
The signal found does not by itself provide sufficient support for the conclusion that the mass gap does not exist, says official LIGO spokesman Jess McIver about the observation. However, it does make it clear that LIGO-Virgo-Kagra will also be able to pick up this type of signal.
Interferometers
LIGO and Virgo are so-called interferometers that can measure minute distance variations in a system of laser beams. To this end, LIGO detectors have two laser arms 4 kilometers long at right angles to each other. In Virgo near Pisa, the optical arms are three kilometers each. A passing gravitational wave distorts the arms, causing the laser patterns to change momentarily. Such signals were first measured in 2015.
The consortium’s current fourth O4 measurement period began in 2023 with mainly the two LIGO detectors and briefly Kagra in Japan. At that time, the European Virgo detector decided not to participate because its sensitivity had not yet proved optimal.
Virgo joining
Next week the second half of measurement round O4, which will last until spring 2025, begins, and now Virgo is also participating with sufficient sensitivity. Nikhef is one of the main partners in Virgo.
The results of event GW230529 was presented Friday at a conference in the US and pre-published on the arxiv website. The analyses also involved experts from Nikhef.