NASA's Chandra X-ray Observatory has observed the largest flare detected from a supermassive black hole at the centre of the Milky Way.
The supermassive black hole is called Sagittarius A* (Sgr A*) and is estimated to contain about 4.5 million times the mass of our sun.
Astronomers made the unexpected discovery while using Chandra to observe how Sgr A* would react to a nearby cloud of gas known as G2.
"Unfortunately, the G2 gas cloud didn't produce the fireworks we were hoping for when it got close to Sgr A*. However, nature often surprises us and we saw something else that was really exciting," said lead researcher Daryl Haggard from Amherst College in Massachusetts.
Astronomers estimate that G2 was closest to the black hole in the spring of 2014 - 15 billion miles away.
The Chandra flare observed in September 2013 was about a hundred times closer to the black hole, making the event unlikely related to G2.
The researchers have two main theories about what caused Sgr A* to erupt in this extreme way.
The first is that an asteroid came too close to the supermassive black hole and was torn apart by gravity.
A second theory is that the magnetic field lines within the gas flowing towards Sgr A* could be tightly packed and become tangled.
These field lines may occasionally reconfigure themselves and produce a bright outburst of X-rays.
These types of magnetic flares are seen on the sun, and the Sgr A* flares have similar patterns of intensity.
"Such rare and extreme events give us a unique chance to use a mere trickle of infalling matter to understand the physics of one of the most bizarre objects in our galaxy," said co-author Gabriele Ponti from the Max Planck Institute for Astrophysics in Garching, Germany.
