Image credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
On April 25, 2019, the LIGO Livingston Observatory picked up what appeared to be gravitational ripples from a collision of two neutron stars. LIGO Livingston is part of a gravitational-wave network that includes LIGO (the Laser Interferometer Gravitational-wave Observatory), funded by the National Science Foundation (NSF), and the European Virgo detector.
This would be only the second time this type of event has ever been observed in gravitational waves.The first such observation, which took place in August of 2017, made history for being the first time that both gravitational waves and light were detected from the same cosmic event. The April 25 merger, by contrast, did not result in any light being detected. However,through an analysis of the gravitational-wave data alone, researchers have learned that the collision produced an object with an unusually high mass.
The results were presented at a press briefing on January 6, at the meeting of the American Astronomical Society in Honolulu, Hawaii.
Neutron stars are the remnants of dying stars that undergo catastrophic explosions as they collapse at the end of their lives. When two neutron stars spiral together, they undergo a violent merger that sends gravitational shudders through the fabric of space and time. LIGO became the first observatory to directly detect gravitational waves in 2015; in that instance, the waves were generated by the fierce collision of two black holes. Since then, LIGO and Virgo have registered dozens of additional candidate black hole mergers. The August 2017 neutron star merger was witnessed by both LIGO detectors, one in Livingston, Louisiana,and one in Hanford, Washington, together with a host of light-based telescopes around the world (neutron star collisions produce light, while black hole collisions are generally thought not to do so).This merger was not clearly visible in the Virgo data, but that fact provided key information that ultimately pinpointed the event’s location in the sky.The April 2019 event was first identified in data from the LIGO Livingston detector alone. The LIGO Hanford detector was temporarily offline at the time, and, at a distance of more than 500 million light-years, the event was too faint to be visible in Virgo’s data.
Using the Livingston LIGO data, combined with information derived from Virgo’s data, the team narrowed the location of the event to a patch of sky more than 8,200 square degrees in size, or about 20 percent of the sky. For comparison, the August 2017 event was narrowed to a region of just 16 square degrees, or 0.04percent of the sky. The LIGO data reveal that the combined mass of the merged bodies is about 3.4 times the mass of our sun. In our galaxy, known binary neutron star systems have combined masses up to only 2.9 times that of sun. One possibility for the unusually high mass is that the collision took place not between two neutron stars, but a neutron star and a black hole, since black holes are heavier than neutron stars. But if this were the case, the black hole would have to be exceptionally small for its class. Instead, the scientists believe it is much more likely that LIGO witnessed a shattering of two neutron stars. They might form from binary systems of massive stars that each end their lives as neutron stars, or they might arise when two separately formed neutron stars come together within a dense stellar environment. The LIGO data for the April 25 event do not indicate which of these scenarios is more likely, but they do suggest that more data and new models are needed to explain the merger’s unexpectedly high mass.
Additional information about the gravitational-wave observatories:
LIGO is funded by the NSF and operated by Caltech and MIT, which conceived of LIGO and lead the project. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project.
The University of Florida has been a member of the LIGO Science Collaboration since 1996. UF has designed and built components of the LIGO interferometers and has devised some of the data analysis methods used by the collaboration to search for signals in the LIGO data.