'Mystery object' in space may be smallest black hole

Pablo Tucker
June 25, 2020

Scientists have discovered an astronomical object that has never been observed before.

The study suggested that the object was either the lightest black hole, or the most massive neutron star ever discovered.

In August 2019, LIGO and Virgo detected a gravitational wave signal called 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.

"We can't rule out any possibilities", he told BBC News.

Phillipe Landry
Philippe Landry Cal State Fullerton postdoctoral research associate

"What is really exciting is that this is just the start", he said.

Currently, the heaviest known neutron star is 2.5 times the mass of our sun and the lightest black hole is five times the mass of our sun.

Data at the LIGO and Virgo observatories are compared to confirm gravitational waves and help determine where the waves originated. "We thought the Universe would be kind of lazy in producing binaries of objects with such different masses, if it did so at all". The object was found on August 14, 2019, 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.

That makes the lighter object more massive than the heaviest type of dead star, or neutron star, previously observed - of just over two solar masses.

In theory, black holes could be lighter than five suns.

It's the 2.6-solar-mass object that raises eyebrows because it falls squarely in the mass gap, says Vicky Kalogera, an astrophysicist and LIGO team member from Northwestern University.

"We use fancy parallelized algorithms which can run our analyses on a supercomputer cluster containing many hundreds, or thousands of individual computers", he said.

This object exceeds that, making it too large to fit neatly with other neutron stars that astronomers know about.

Unfortunately, while the mass of the object is well defined, there is no way to tell its nature.

Black holes are the ultimate limit of gravitational collapse. His ideas are receiving increased attention following the new discovery.

Usually, epic interactions in the cosmos leave clues as to what has occurred; clues for example, like high energy radiation births in the form of light, gamma and infrared or X-rays arising from the universe's most energetic stellar death events. Neutron star collisions are messy affairs with matter flung outward in all directions and are thus expected to shine with light. Blue are the black holes detected through gravitational waves, purple are those detected through electromagnetic observations, yellow are neutron stars detected through electromagnetic observations, and orange are those detected through gravitational waves.

So, either the researchers just spotted the smallest black hole (by a long shot) or the heaviest neutron star ever (by a smaller but still significant margin). It might be a more exotic object known as a quark star. 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.

Still, we now know some objects are very close to black hole limit.

STFC funds and supports research in particle and nuclear physics, astronomy, gravitational research and astrophysics, and space science and also operates a network of five national laboratories, including the Rutherford Appleton Laboratory and the Daresbury Laboratory, as well as supporting United Kingdom research at a number of global research facilities including CERN, FERMILAB, the ESO telescopes in Chile and many more.

"We don't know how nuclear forces operate under the extreme conditions you need inside a neutron star. On the other hand, if the collision involved two black holes, it's not likely that it would have shone with any light".

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