A highly unusual gravitational wave signal, detected by the LIGO and Virgo observatories in the US and Italy, was generated by a new class of binary systems (two astronomical objects orbiting around each other), an international team of astrophysicists has confirmed.

Scientists from the LIGO and Virgo Collaboration, which includes researchers from the Institute for Gravitational Wave Astronomy at the University of Birmingham, detected the signal, named GW190814, in August 2019.

In a new paper, published in the Astrophysical Journal Letters, the team has announced that the signal was generated by a compact object (a neutron star or a black hole) 2.6 times the mass of our sun (2.6 solar masses), merging with a black hole of 23 solar masses.

The new observation is important because it challenges astrophysicists' understanding both of how stars die and how they pair up into binary systems. Although the precise nature of the lighter member of the binary that generated GW190814 is unknown, scientists have confirmed that it is a record breaker: it is more massive than any neutron star and lighter than any black hole yet observed.

"This merger event is one of the most unusual ones observed in gravitational waves to date", says Dr Patricia Schmidt, Lecturer at the Institute for Gravitational Wave Astronomy and member of the LIGO team. "It pushes our understanding of the nature of the lighter companion and how it is formed to the limits. This will keep astrophysicists occupied for a while."

Scientists have so far believed that dying stars do not leave remnants, whatever their nature, with a mass between 2.5 and 5 times the mass of the Sun. Now this desert has been populated by one of the objects that produced GW190814.

"From the very outset it was clear that this was a special event," says Dr Geraint Pratten, a researcher at the Institute for Gravitational Wave Astronomy, who was involved in producing the initial sky-maps for optical telescopes' follow-ups. "It highlights the need for ever better theoretical models of the emitted gravitational-wave signal, such as those produced here in Birmingham, to mine as much information as possible from the data and understand how such high mass-ratio binaries are formed."

An additional aspect requires further investigation. The disparity in masses between the two objects, with the black hole nine times more massive than its companion object, challenges existing theories about how binary systems of black holes and neutron stars are formed. What is certain, according to the research team, is that GW190814 was produced by a binary system that is quite different from other systems detected so far by LIGO and Virgo.

"We have been itching with excitement since this candidate showed up on our screens," says co-author Professor Alberto Vecchio, director of the Institute for Gravitational Wave Astronomy.

"We thought the Universe would be kind of lazy in producing binaries of objects with such different masses, if it did so at all. And guess what, we were wrong! We now know there are cosmic factories hiding somewhere that are actually rather efficient at generating these systems. The journey to figure out what they are and how they work is going to keep us busy for quite some time, but more and better data from LIGO and Virgo are just about a year away, and we are bound to have new surprises".

A further mystery surrounding GW190814 has been its elusiveness for astronomers looking for light from the event. When new gravitational waves are detected, an alert is sent to astronomers world-wide, triggering dozens of ground- and space-based telescopes to start searching for a fireball ignited by the collision. For GW190814, no such glow has yet been detected.

Dr Matt Nicholl is a Lecturer at the Institute for Gravitational Wave Astronomy, and followed up the event as part of the European ENGRAVE team using the ESO Very Large Telescope, and the US-led team using the Magellan telescopes.

He says: "Observatories around the world carried out an intensive search for any light-show produced by the merger. We were able to show that if any light was released, it must have been extremely faint to avoid detection. This means that if the lighter companion was a neutron star, its more massive black hole partner may have simply swallowed it whole! On the other hand, if the collision involved two black holes, it's not likely that it would have shone with any light."

Related Links
Virgo at EU
The Physics of Time and Space

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