The center of the Milky Way galaxy, with the supermassive black hole Sagittarius A* (Sgr A*), located in the middle. NASA

A team of researchers have used NASA’s Chandra X-ray Observatory to discover that the normally darkish region close to the supermassive black hole at the heart of the Milky Way galaxy exploded with at least two bright outbursts hundreds of years ago. The discovery is based on a new study of fast variations in the X-ray emission from gas clouds surrounding the black hole, also known as Sagittarius A*, or Sgr A*.

The study, published in the October 2013 issue of the journal, Astronomy and Astrophysics, said that the most likely interpretation of these variations is that they are caused by light echoes from Sgr A*, which were probably produced when large blocks of material, possibly from a disrupted star or planet, fell into the black hole.

“Some of the X-rays produced by these episodes then bounced off gas clouds about thirty to a hundred light years away from the black hole, similar to how the sound from a person’s voice can bounce off canyon walls,” NASA said in a statement. “Just as echoes of sound reverberate long after the original noise was created, so too do light echoes in space replay the original event.”

According to scientists, this is the first time that evidence for two distinct outbursts has been seen within a single set of data. Scientists believe that the light echoes can help astronomers determine what objects such as Sgr A* were doing long before the arrival of X-ray telescopes to observe them.

The X-ray echoes suggest that the dimly lit area very close to Sgr A* was at least a million times brighter as recently as within the past few hundred years, and that the X-rays from the outbursts, which followed a straight path, would have arrived at Earth at that time.

However, scientists said that the reflected X-rays in the light echoes reached Chandra only in the last few years as they took a longer path after bouncing off the gas clouds.

The researchers have created an animation, which shows Chandra images compiled from data recorded between 1999 and 2011. The animation is a sequence of images that shows how the light echoes behave. As the sequence plays, the X-ray emissions move away from the black hole in some regions. In other regions, the emissions appear dimmer or brighter, as the X-rays pass away from reflecting material.

“Because the change in X-rays lasts for only two years in one region and over ten years in others, this new study indicates that at least two separate outbursts were responsible for the light echoes observed from Sgr A*,” NASA said.