NASA's Chandra X-ray Observatory has detected the largest ever X-ray flare from the supermassive black hole at the centre of the Milky Way galaxy.
This event raises questions about the behaviour of this giant black hole and its surrounding environment, researchers said.
The supermassive black hole at the centre of our galaxy, called Sagittarius A, or Sgr A, 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," said lead researcher Daryl Haggard of Amherst College in Massachusetts.
"However, nature often surprises us and we saw something else that was really exciting," said Haggard.
Haggard and her team detected an X-ray flare from Sgr A 400 times brighter than its usual, quiet state.
This "megaflare" was nearly three times brighter than the previous brightest X-ray flare from Sgr A in early 2012.
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.
The debris from such a tidal disruption became very hot and produced X-rays before disappearing forever across the black hole's point of no return, or event horizon.
"If an asteroid was torn apart, it would go around the black hole for a couple of hours - like water circling an open drain - before falling in," said co-author Fred Baganoff of the Massachusetts Institute of Technology.
If this theory holds up, it means astronomers may have found evidence for the largest asteroid to produce an observed X-ray flare after being torn apart by Sgr A.
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.
