Light Echoes From The Black Hole At The Heart Of The Milky Way
October 25, 2013

Chandra Observations Reveal Black Hole’s Violent Past

[ Watch the Video: Time-Lapse of Supermassive Black Hole Sagittarius A* ]

April Flowers for - Your Universe Online

During the last few hundred years, the dim region very close to the supermassive black hole near the center of the Milky Way has flared up with at least two luminous outbursts, according to a team of researchers using NASA's Chandra X-ray Observatory.

A new study of rapid variations in the X-ray emissions from gas clouds surrounding Sagittarius A* (Sgr A*) - the supermassive black hole - led to the new discovery. The research team demonstrates that the most probable interpretation of these variations is that they were caused by light echoes.

The echoes from Sgr A* probably originated when large clumps of material, most likely from a disrupted star or planet, fell into the black hole. Similarly to how a person's voice can bounce off canyon walls, some of the X-rays from these episodes bounced off gas clouds around 30 to 100 light years away from the black hole. Light echoes in space replay the original event much like a sound echo reverberates long after the original noise abates.

Scientists have seen light echoes from Sgr A* before in X-rays from Chandra and other observatories. However, this is the first time that a single set of data has revealed evidence for two distinct outbursts.

These light echoes are more than just a cosmic parlor trick. They allow astronomers an opportunity to piece together what objects like Sgr A* were doing long before there were X-ray telescopes to observe them. The results also suggest that the area very close to Sgr A* was at least a million times brighter within the past few hundred years. X-rays from the outbursts that followed a straight path would have arrived at Earth during the time of that brightness. The reflected X-rays, however, took a longer path as they bounced off the gas clouds. These X-rays only reached Chandra within the last few years.

The researchers created an animation that shows Chandra images which have been combined from data collected between 1999 and 2011. In the animation, Sgr A* is marked with a cross. The sequence of images show how the light echoes behave, with X-ray emissions moving away from the black hole in some regions, while in other regions it gets dimmer or brighter, as the X-rays pass into or away from reflecting material.

A process called fluorescence created the X-ray emission shown in the animation. X-rays bombarded the iron atoms in these gas clouds, knocking out electrons close to the nucleus and causing electrons further out to fill the hole, emitting X-rays in the process. The dark areas are explained by other types of X-ray emissions that exist but were not included in the animation.

This is the first time that astronomers have observed both increasing and decreasing X-ray emissions in the same structures. The new study indicates that at least two separate outbursts were responsible for the light echoes observed from Sgr A* because the change in X-rays lasts for only two years in one region and over ten years in others.

The researchers propose several possible causes for the outbursts. One theory involves a short-lived jet produced by the partial disruption of a star by Sgr A*. Other theories include the collection by Sgr A* of debris from close encounters between two stars and an increase in the consumption of material by Sgr A* because of clumps in the gas ejected by massive stars orbiting Sgr A*. The team says that further studies of the variations are needed to decide between these options.

The team also investigated the possibility that a recently discovered magnetar - a neutron star with a very strong magnetic field - near Sgr A* might be responsible for these variations. This would require an outburst, however, that is much brighter than the brightest magnetar outburst ever observed.

The results of this study were published in the journal Astronomy and Astrophysics.