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The international gravitational-wave observatory network, which includes support from UK research teams, has detected what appears to be gravitational ripples from a collision of two neutron stars. A new study confirms that this event on 25 April 2019, which was witnessed by only one detector in the network, LIGO Livingston, was indeed likely the result of a merger of two neutron stars. This would be only the second time this type of detection has ever been made.

The unique aspect of this event is that the combined mass of the two merging neutron stars appears to be three and a half times that of our Sun.  This is the largest coalescing system that has ever been observed in our galaxy and challenges researchers' expectations.

The global gravitational-wave observatory network includes the National Science Foundation's LIGO in the USA and the European Virgo detectors, and has substantial support from UK research teams supported by STFC from the Institute of Gravitational Wave Astronomy, University of Birmingham; Cardiff University’s Gravitational Physics Group: the University of Strathclyde and the University of Glasgow’s Institute for Gravitational Research.

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.”

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.

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.

Professor Alberto Vecchio, Director of the Institute of Gravitational Wave Astronomy, University of Birmingham, said: “The binary system that we observed on the 25th of April 2019 is a very interesting one. Its members are likely two neutron stars, and the sum of their masses is the highest ever observed. This begs the question of whether this is the tip of the iceberg of a new class of neutron star binary systems, different from those we have known so far. On the other hand, we cannot rule out that at least one of the members of the binary is a black hole. If this is the case then we would need to invoke some fairly unconventional formation processes.”

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.

Dr Vivien Raymond, from Cardiff University’s Gravitational Physics Group, said: "With this new binary neutron star detection, we are hinting at the presence of an unexpectedly heavy class of neutron stars in our universe. Gravitational-Wave astronomy is really enabling us to explore new environments."

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," says 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.

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.

An animation of the merger event is available at www.ligo.org/detections/GW190425.php

And https://www.youtube.com/watch?v=853sZWxVto4&feature=emb_logo

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 January 5 2020, at the 235th meeting of the American Astronomical Society in Honolulu, Hawaii.

NOTES:

The first detection of gravitational waves, announced on February 11, 2016, was a milestone in physics and astronomy; it confirmed a major prediction of Albert Einstein’s 1915 theory of general relativity, and marked the beginning of the new field of gravitational-wave astronomy. 

Then, on October 16, 2017, scientists announced that they had directly detected gravitational waves in addition to light from the spectacular collision of two neutron stars, marking the first time that a cosmic event has been viewed in both gravitational waves and light. That event was widely reported as helping usher in an era of multi-messenger astronomy.

Additional information about the gravitational-wave observatories:

LIGO is funded by the National Science Foundation (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. Approximately 1,300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php.

The Virgo Collaboration is currently composed of approximately 520 members from 99 institutes in 11 different countries including Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration groups can be found at ​http://public.virgo-gw.eu/the-virgo-collaboration/​. More information is available on the Virgo website at​ ​http://www.virgo-gw.eu​.

More information on Gravitational Waves

STFC – Gravitational waves: everything you need to know

Since the detectors first started operation in September 2015, the LIGO and Virgo Collaborations, which include the Universities of Birmingham, Cardiff and Glasgow and also the University of Strathclyde, have completed two observation runs. During these runs they have detected gravitational waves from a total of ten stellar-mass binary black hole mergers – compact objects likely formed by the gravitational collapse of massive stars. They have also detected one binary neutron star coalescence – generated by two neutron stars spiraling into each other.

UK teams have 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).

More about the UK’s involvement in gravitational wave research

• STFC – working together to ride the gravitational waves
• University of Glasgow Institute for Gravitational Research
• The Institute of Gravitational Wave Astronomy at Birmingham
• Gravitational Physics Group at Cardiff University
• University of Strathclyde Physics Dept