Berlin, April 18 : Astronomers have for the first time detected the light echo of a high-energy flash from a black hole.
An international team led by Stefanie Komossa from the Max Planck Institute for extraterrestrial Physics in Garching, Germany, made the observations.
The team observed the light echo of an enormous X-ray flare, which was almost certainly produced when a single star was disrupted by a supermassive black hole.
A light echo occurs when interstellar gas is heated by radiation and reacts by emission of light.
When a star is disrupted by a black hole in the nucleus of a galaxy, its debris is inevitably attracted and absorbed by the black hole. This sudden increase in the accretion rate causes an abrupt burst of ultraviolet and X-ray light because the gas from the disrupted star becomes very hot.
As the high-energy radiation travels through the core of the galaxy, it illuminates surrounding matter and so makes it possible to probe regions of the galaxy that would otherwise be unobservable.
However, the astronomers had to hurry up and look through the telescope at the right moment, because X-ray bursts don't last very long.
From 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. The emission lines represent the "fingerprints" of the atoms in the hot gases heated by the flare.
The galaxy with catalog name SDSSJ0952+2143, which was detected in December 2007 by Komossa and her team in the Sloan Digital Sky Survey archive, caught the research team's attention because of its superstrong iron lines: the strongest that were ever observed in a galaxy.
In them, the authors see an evidence for 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 made of identical components and that the perceived differences are just due to the different 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 covers them when viewed from certain directions. Also, the breadth of the spectral lines which the scientists measure is influenced by the viewing direction and that means by the molecular torus.
Should the expectations of Komossa and her colleagues be confirmed, this will be the first time that scientists have seen such a strong time-variable signal from a molecular torus.
From the light echo, the torus can be mapped and its geometry inferred, something which has not been possible up to now.
According to Komossa, "Reverberation-mapping of light echoes opens up new possibilities to study galaxies."