Gravitational ripples from the likely collision of two neutron stars have been observed for only the second time ever.

The ripples were picked up on April 25 2019 by a gravitational-wave observatory network that includes the National Science Foundation's LIGO – to which Lancaster University is affiliated - and the European Virgo detectors.

The first such detection, 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 cosmic collision resulted in a merged object with an unusually high mass.

Dr Matthew Pitkin, a physicist at Lancaster University and member of the LIGO Scientific Collaboration, said: “Gravitational-waves from colliding neutron stars leave a characteristic ‘chirp-like’ signal in gravitational-wave detectors, with a both rising pitch and amplitude over the course of about 100 seconds. The way the pitch changes allow us to find out the masses of the two merging stars and to learn about the extremely dense state of matter that they are made from.”

"From conventional observations with light, we already know of 17 binary neutron star systems in our own galaxy and we have estimated the masses of these stars," said Ben Farr, a LIGO team member based at the University of Oregon. "What's surprising is that the combined mass of this binary is much higher than what was expected."

"We have detected a second event consistent with a binary neutron star system and this is an important confirmation of the August 2017 event that marked an exciting new beginning for multi-messenger astronomy two years ago," said Jo van den Brand, Virgo Spokesperson and professor at Maastricht University, and

Nikhef and VU University Amsterdam in the Netherlands.

Multi-messenger astronomy occurs when different types of signals are witnessed simultaneously, such as those based on gravitational waves and light.

The study, submitted to the Astrophysical Journal Letters, is authored by an international team that comprises 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 the 235th meeting of the American Astronomical Society in Honolulu, Hawaii.

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.

LIGO, which stands for the Laser Interferometer Gravitational-Wave Observatory, 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 project's first detection of a neutron star merger took place in August of 2017: both LIGO detectors, one in Livingston, Louisiana, and one in Hanford, Washington, detected the event, 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 to ultimately pinpoint the event’s location in the sky.

In the case of the April 2019 event, only a single detector in the LIGO-Virgo network picked up the gravitational-wave signal—LIGO Livingston. The LIGO Hanford detector was temporarily offline, and, at a distance of more than 500 million light-years, the event was too faint to be detected with Virgo's current sensitivity, in addition to being located in a region of sky where Virgo is less sensitive. Using the Livingston data, combined with information derived from Virgo’s observations, 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.

"This is our first published event for a single-observatory detection," saids Caltech's Anamaria Effler, a scientist who works at LIGO Livingston Observatory. "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 that of the mass of our sun. Typically, in our galaxy, neutron star collisions are known to produce final masses of 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, and that their merger resulted in a newly formed black hole of about 3.4 solar masses.

"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," said Surabhi Sachdev, a LIGO team member based at Penn State. "And this could have interesting implications for how the pair originally formed."

Neutron star pairs are thought to form either early in life, when companion massive stars successively die one by one—or they are thought to come together later in life within dense, busy environments. 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.