Black Hole Sheds Light on a Galaxy

An international team has observed the light echo of an enormous X-ray flare, which was apparently produced when a single star was disrupted by a super-massive black hole. The importance of this observation is that it offers researchers a new method for mapping galactic nuclei.

For the first time, scientists were able to observe the light echo of a rare and highly dramatic event in great detail. A light echo occurs when interstellar gas is heated by radiation and reacts by emitting light. The team, led by Stefanie Komossa from the Max Planck Institute (MPE) for Extraterrestrial Physics in Garching, Germany, observed the light echo which revealed the stellar disruption process.

When a star is disrupted by a black hole in the nucleus of a galaxy, its debris are inevitably attracted and absorbed by the black hole. The accretion rate is suddenly increased; since the gas from the disrupted star becomes very hot, the result is an abrupt burst of ultraviolet and X-ray light. The probing of the regions of the galaxy that would otherwise be unobservable is made possible due to the high-energy radiation, which travels through the core of the galaxy and illuminates the surrounding matter.

“To study the core of a normal galaxy is like looking at the New York skyline at night during a power failure: You can’t learn much about the buildings, roads and parks”, says Stefanie Komossa. “The situation changes, for example, during a fireworks display. It’s exactly the same when a sudden burst of high-energy radiation illuminates a galaxy.” However, the astronomers did not have much time to observe the galaxy because X-ray bursts do not last very long.

Using different parameters – the strength, the degree of ionization, and the deduced velocities of the rapidly varying emission lines – the physicists can tell in which part of the galaxy they are emitted. Using these emission lines the researchers were able to identify the atoms in the hot gases, which were heated by the flare.

The galaxy that was in the center of this study has the catalog name SDSSJ0952+2143 and was detected in December 2007 by Komossa and her team. The reason this specific galaxy caught their attention is that it has a very strong iron lines. In fact, it has the strongest (relative to oxygen emission) lines that were ever observed in a galaxy. The researchers consider these lines to be indicators of a molecular torus which plays an important part in so-called unified models of active galaxies.

The unified model postulates that all active galaxies are composed of identical components and that the perceived differences are just due to the varied directions from which we view the galaxies. An important element of this model is the molecular torus, which surrounds the black hole and its accretion disk, and which ‘covers’ them when viewed from certain angles. If the expectations of this team will be confirmed, it will be the first time that scientists have seen such a strong time-variable signal from a molecular torus. The light echo enables the mapping of the torus and its geometry could be inferred, a goal that has yet to be accomplished.

Besides the remarkable strong iron lines, the scientists observed another phenomenon: a very peculiar shape of the hydrogen emission lines, which had never been seen before. This line suggests activities of the disk of matter around the black hole, which consists mainly of hydrogen. “Probably we are seeing the debris of the disrupted star here which is just being accreted by the black hole”, explains Hongyan Zhou, co-author of the research paper. “Reverberation-mapping of light echoes opens up new possibilities to study galaxies”, concludes Komossa. According to the team, their next goal is using this method to explore the physical conditions in the circumnuclear material in active and non-active galaxies.

TFOT recently covered the theory about the involvement of smaller stars in the formation of big stars, and the imaging of the Triangulum Galaxy, which took over 11 hours. Other related stories are the discovery of life’s building blocks in space and the Red Novae, a new class of stellar explosions.

More information on the X-ray flare can be found on the Max Planck Institute website.

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