A photo of our galactic neighbour Andromeda has revealed a surprise subject – an X-ray source initially thought to be inside Andromeda, but which turned out to be 1,000 times farther away.
Using data from NASA’s Chandra X-ray observatory and from ground-based optical telescopes, astronomers have discovered a binary supermassive black hole that may be the most tightly paired ever discovered.
It’s called J0045+41, and astronomers initially thought it was a star in the Andromeda galaxy, which at 2.5 million light years away is relatively close to us, and will probably merge with the Milky Way in about 4 billion years.
“We were looking for a special type of star in M31 and thought we had found one,” said lead researcher Trevor Dorn-Wallenstein, a graduate student at the University of Washington in Seattle. “We were surprised and excited to find something far stranger!”
When it was thought to be inside Andromeda, researchers classified J0045+41 as a pair of stars that orbit each other once every 76 days. But data from Chandra showed that the intensity of the X-ray signal was too powerful to fit this classification.
Dorn-Wallenstein thought it may have been a binary black hole and neutron star – so he started studying it to find out.
But then spectral data from the Gemini-North telescope in Hawaii showed that J0045+41 had to contain at least one supermassive black hole – which then allowed the researchers to calculate the object’s distance.
The powerful X-ray source turned out to be 2.6 billion light-years away – and the most likely cause is a pair of supermassive black holes wrapped in a tight binary orbit.
Combined, the two black holes have a total mass 200 million times that of the Sun. The black hole in the centre of our Milky Way galaxy, Sagittarius A*, has a mass around 4 million times the mass of the Sun.
And they’re orbiting each other super close, too – a distance just a few hundred times the distance between the Earth and the Sun, less than one hundredth of a light-year.
The team believes that the two black holes could have each been at the centre of a galaxy, coming together when the two galaxies merged. They’re slowly being pulled together by their gravity and will, at some point, collide.