On April 25, 2019, the observatory known as LIGO Livingston picked up what appeared to be gravitational ripples from a collision of two neutron stars – only the second time this type of event has ever been observed by gravitational wave astronomy.

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.

Physicists from the University of Glasgow’s Institute for Gravitational Research played important roles in the development and construction of both LIGO and the data analysis which allows the collaboration to pick out the gravitational wave signals. The UK’s contribution to the collaborations is funded by the Science and Technology Facilities Council (STFC).

The first such observation of neutron star collisions, 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 event resulted in a merged object with an unusually high mass.

The study, submitted to The Astrophysical Journal Letters, is authored by an international team comprised of the LIGO Scientific Collaboration and the Virgo Collaboration, the latter of which is associated with the Virgo gravitational-wave detector in Italy. The results were presented at a press briefing today (Monday 6 January), at the 235th meeting of the American Astronomical Society in Honolulu, Hawaii. 

Professor Sheila Rowan, director of the University of Glasgow’s Institute for Gravitational Research, said: “We’re thrilled to have observed a second neutron star collision, less than two years after the first detection of this type of event, and we’ve been pleasantly surprised by the unexpectedly high mass of the binary.

“It’s a reminder that there’s still so much that gravitational wave astronomy has yet to reveal about how our universe works. We’ve made many discoveries since the first-ever detection in September 2015 but gravitational wave astronomy is still very much in its infancy, and we’re excited to see what our next detection can teach us.”

Neutron stars are the remnants of dead stars that exploded. When two neutron stars spiral together, they undergo a violent merger that sends gravitational shudders through the fabric of space and time.

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 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 0.04 percent of the sky.

Anamaria Effler, a Caltech scientist who works at LIGO Livingston, said: “This is our first published event for a single-observatory detection, but Virgo made a valuable contribution. We used information about its non-detection to tell us roughly where the signal must have originated from.”

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 more likely that LIGO witnessed a shattering of two neutron stars.

Surabhi Sachdev, a LIGO team member based at Penn State, said: “What we know from the data are the masses, and the individual masses most likely correspond to neutron stars. However, as a binary neutron star system, the total mass is much higher than any of the other known galactic neutron star binaries, and this could have interesting implications for how the pair originally formed.”

Neutron star pairs are thought to form in two possible ways. 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 unexpectedly high mass.