Astronomers see 'mystery object' in space that could solve black hole puzzle

Scientists have seen a "mystery object" in space that could solve a puzzle about black holes.

The object sits right in the "mass gap" – a class of object that researchers have found seems to be missing as they search through the universe. Researchers have been unable to know whether dying stars leave anything behind that lies in that mysterious gap, and why it has proven so difficult to find any object that sits in it.

“We’ve been waiting decades to solve this mystery,” said Vicky Kalogera, a professor at Northwestern University. “We don’t know if this object is the heaviest known neutron star, or the lightest known black hole, but either way it breaks a record.”

It was detected after researchers picked up a "highly unusual" signal using the Virgo and Ligo observatories, which made the first gravitational wave discovery.

“This is going to change how scientists talk about neutron stars and black holes,” said co-author Patrick Brady, a professor at the University of Wisconsin, Milwaukee, and the LIGO Scientific Collaboration spokesperson. “The mass gap may in fact not exist at all but may have been due to limitations in observational capabilities. Time and more observations will tell.”

When the most massive stars die out, they collapse under their own gravity and turn into black holes. Less massive ones explode into supernovas and leave behind their remains in the form of neutron stars.

The problem of the mass gap arose because the heaviest neutron star is 2.5 times the size of the Sun, and the lightest known black hole is about five times our Sun. Researchers were puzzled by the fact that there appeared to be nothing in that strange gap between.

Now they have detected an object that appears to be either the heaviest neutron star ever found, or the lightest black hole.

The object is 2.6 solar masses, putting it right in the middle of that gap, and was discovered in August last year but reported in a new paper published today. It was discovered when it merged with a black hole of 23 solar masses, an event that sent gravitational waves rippling through space so they could be picked up from observatories on Earth.

That discovery has already led to questions about our understanding of how stars die and how they pair up into binary systems, the astronomers who found it said.

“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.”

As well as sitting in the middle of the mass gap, the object throws up other questions. The vast disparity in mass of the object and that of the companion black hole it merged with – which is nine times more massive – seems to challenge theories about how black holes and neutron stars are formed.

It is the most extreme disparity ever seen in a gravitational wave event, scientists said.

“It’s a challenge for current theoretical models to form merging pairs of compact objects with such a large mass ratio in which the low-mass partner resides in the mass gap. This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object,” said Professor Kalogera.

“The mystery object may be a neutron star merging with a black hole, an exciting possibility expected theoretically but not yet confirmed observationally. However, at 2.6 times the mass of our Sun, it exceeds modern predictions for the maximum mass of neutron stars, and may instead be the lightest black hole ever detected.”

Despite scrambling to see the object and its merging as soon as the gravitational wave was detected, researchers were unable to pick up any evidence of the event through light-based telescopes. The difference in size may have meant that the large black hole swallowed up the mystery object hole, before it could send out any light that would be tracked from Earth, astronomers said.

The dramatic, destructive nature of the event also means that there is no way for researchers to better see what the object was before it was destroyed. But they hope to conduct further observations that will hopefully find yet more objects in the mass gap, and could provide insight into how they are formed – and what they actually are.

“This is the first glimpse of what could be a whole new population of compact binary objects,” said Charlie Hoy, a member of the LIGO Scientific Collaboration and a graduate student at Cardiff University. “What is really exciting is that this is just the start. As the detectors get more and more sensitive, we will observe even more of these signals, and we will be able to pinpoint the populations of neutron stars and black holes in the universe.”