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Scientists finds mystery object in 'mass gap'

LIGO image

Scientists have announced the discovery of a record-breaking mystery object around two and a half times the mass of the sun.

Strathclyde is part of an international network of scientists, the LIGO Scientific Collaboration (LSC), who made the observation through gravitational waves.

For decades, astronomers have been puzzled by a gap that lies between neutron stars and black holes. The heaviest known neutron star is no more than two and a half times the mass of the sun, or two and a half solar masses, and the lightest known black hole is about five solar masses. The question has been whether anything lies within this mass gap.

Mass gap

Now a new study from LIGO and the Virgo detector in Europe, has announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap. The object was found on August 14, 2019, as it merged with a black hole of 23 solar masses.

Professor Stuart Reid, from the Department of Biomedical Engineering at Strathclyde and Chair of the Optics Working Group of the LIGO Scientific Collaboration, said: “This is a remarkable discovery, either being the heaviest neutron star or the lightest black hole ever observed.

“It will leave many astronomers scratching their heads, as we seek to understand better the Universe we live in.”

When the most massive stars die, they collapse under their own gravity and leave behind black holes. When stars that are a bit less massive die, they explode in supernovas and leave behind dense, dead remnants of stars called neutron stars.

Gravitational waves

The object was detected as it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo. A paper about the detection has been accepted for publication in The Astrophysical Journal Letters.

The cosmic merger described in the study resulted in a final black hole about 25 times the mass of the sun – some of the merged mass was converted to a blast of energy in the form of gravitational waves. The newly-formed black hole lies about 800 million light-years away from Earth.

Before the two objects merged, their masses differed by a factor of nine, making this the most extreme mass ratio known for a gravitational-wave event.

So far, such light counterparts to gravitational-wave signals have been seen only once, in an event called GW170817. The event, discovered by the LIGO-Virgo network in August of 2017, involved a fiery collision between two neutron stars.

Interesting puzzle

Pedro Marronetti, program director for gravitational physics at the National Science Foundation (NSF), said: "The mass gap has been an interesting puzzle for decades, and now we've detected an object that fits just inside it.

"That cannot be explained without defying our understanding of extremely dense matter or what we know about the evolution of stars. This observation is yet another example of the transformative potential of the field of gravitational-wave astronomy, which brings novel insights with every new detection."

LIGO is formed of around 1,300 scientists from around the world, while the Virgo Collaboration has around 550 members from 106 institutes in 12 different countries including Belgium, France, Germany, Poland and Spain.
Two LIGO observatories in the USA are the first instruments to observe gravitational waves, associated with the distant collision of black holes, and the subject of the 2017 Nobel Prize in Physics.