Scientists discover star circling closest known orbit around likely black hole!

Image courtesy: X-ray - NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration - NASA/CXC/M.Weiss

New Delhi: Black holes and their behaviour have long since been on the list of mysterious space objects that scientists crave to decipher.

Found in abundance in the universe, these gravity-pulling black holes have always been known to exist in distant galaxies.

Scientists have time and again revealed discoveries of various balck holes – sometimes wandering, at times still and some which rip stars into shreds before devouring them.

This time, it is the behaviour of a star that has scientists baffled. Scientists have come upon evidence of a star that has been giving a black hole its company.

The star has been seen whipping around the black hole about twice an hour and this may be the tightest orbital dance ever witnessed for a likely black hole and a companion star.

This discovery was made using NASA’s Chandra X-ray Observatory as well as NASA’s NuSTAR and CSIRO’s Australia Telescope Compact Array (ATCA).

Although, the binary has been under observation for many years, radio observations with the ATCA in 2015 revealed that the pair likely contains a black hole pulling material from a companion star called a white dwarf, a low-mass star that has exhausted most or all of its nuclear fuel.

According to NASA, new Chandra data of this system, known as X9, show that it changes in X-ray brightness in the same manner every 28 minutes, which is likely the length of time it takes the companion star to make one complete orbit around the black hole. Chandra data also shows evidence for large amounts of oxygen in the system, a characteristic feature of white dwarfs. A strong case can, therefore, be made that the companion star is a white dwarf, which would then be orbiting the black hole at only about 2.5 times the separation between the Earth and the Moon.

“This white dwarf is so close to the black hole that material is being pulled away from the star and dumped onto a disk of matter around the black hole before falling in,” said first author Arash Bahramian of the University of Alberta in Edmonton, Canada, and Michigan State University in East Lansing. “Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.”

Even though the black hole doesn't seem like a threat for the white dwarf, the star has an uncertain fate, says NASA.

“Eventually so much matter may be pulled away from the white dwarf that it ends up only having the mass of a planet,” said co-author Craig Heinke, also of the University of Alberta. “If it keeps losing mass, the white dwarf may completely evaporate.”

How did the black hole get such a close companion? One possibility is that the black hole smashed into a red giant star, and then gas from the outer regions of the star was ejected from the binary. The remaining core of the red giant would form into a white dwarf, which becomes a binary companion to the black hole. The orbit of the binary would then have shrunk as gravitational waves were emitted, until the black hole started pulling material from the white dwarf, NASA reported.

An alternative explanation for the observations is that the white dwarf is partnered with a neutron star, rather than a black hole.

“We’re going to watch this binary closely in the future, since we know little about how such an extreme system should behave”, said co-author Vlad Tudor of Curtin University and the International Centre for Radio Astronomy Research in Perth, Australia. “We’re also going to keep studying globular clusters in our galaxy to see if more evidence for very tight black hole binaries can be found.”