For the first time, the light echo of such a rare and highly dramatic event could be observed in great detail. The light echo not only revealed the stellar disruption process, but it also provides a powerful new method for mapping galactic nuclei (
Astrophysical Journal Letters, Mai 2008).
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.
"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 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 their attention because of its superstrong iron lines: the strongest (relative to oxygen emission) 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.
Along the same lines is the detection of variable emission in the infrared: It can be interpreted as the "last cry for help" of the heated dusty torus matter before the dust is destroyed by the flash.
In addition to the remarkably strong iron lines, the scientists also noticed a very peculiar shape of the hydrogen emission lines which had never been seen before. This line hints at 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 from the MPE, co-author of the research paper.
The recently-discovered light echo still continues and is being traced with powerful telescopes. The burst itself has faded away. The first observations with the X-ray satellite Chandra show measurable but already faint X-ray light from the galactic nucleus.
"Reverberation-mapping of light echoes opens up new possibilities to study galaxies", concludes Komossa. The team now wants to use this method to explore the physical conditions in the circumnuclear material in active and non-active galaxies.
Original paper:
arXiv:0804.2670v1 [astro-ph]
Source: Max Planck Institute for Astrophysics
Related stories:
Most Black Holes Might Come in Only Small and Large
(PhysOrg.com) -- Black holes are sometimes huge cosmic beasts, billions of times the mass of our sun, and sometimes petite with just a few times the sun's mass. But do black holes also come in size medium? A new study suggests that, for the most part, the answer is no.
Hubble sees magnetic monster in erupting galaxy
NGC 1275 is one of the closest giant elliptical galaxies and lies at the centre of the Perseus Cluster of galaxies. It is an active galaxy, hosting a supermassive black hole at its core, which blows bubbles of radio-wave emitting material into the surrounding cluster gas. Its most spectacular feature is the lacy filigree of gaseous filaments reaching out beyond the galaxy into the multi-million degree X-ray emitting gas that fills the cluster.
'Cosmic ghost' discovered by volunteer astronomer
When Yale astrophysicist Kevin Schawinski and his colleagues at Oxford University enlisted public support in cataloguing galaxies, they never envisioned the strange object Hanny van Arkel found in archived images of the night sky.
Caltech astronomers describe the bar scene at the beginning of the universe
Bars abound in spiral galaxies today, but this was not always the case. A group of 16 astronomers, led by Kartik Sheth of NASA's Spitzer Science Center at the California Institute of Technology, has found that bars tripled in number over the past seven billion years, indicating that spiral galaxies evolve in shape.
The Quiet Explosion: Object intermediate between normal supernovae and gamma-ray bursts found
A European-led team of astronomers are providing hints that a recent supernova may not be as normal as initially thought. Instead, the star that exploded is now understood to have collapsed into a black hole, producing a weak jet, typical of much more violent events, the so-called gamma-ray bursts. The object, SN 2008D, is thus probably among the weakest explosions that produce very fast moving jets. This discovery represents a crucial milestone in the understanding of the most violent phenomena observed in the Universe.
A new way to weigh giant black holes
How do you weigh the biggest black holes in the universe? One answer now comes from a completely new and independent technique that astronomers have developed using data from NASA's Chandra X-ray Observatory.
Einstein was right: Unique stellar system provides 'laboratory' for testing relativity
Researchers at McGill University's Department of Physics – along with colleagues from several countries – have confirmed a long-held prediction of Albert Einstein's theory of general relativity, via observations of a binary-pulsar star system. Their results will be published July 3 in the journal
Science.
Radio telescopes reveal unseen galactic cannibalism
Mystery of black hole 'feeding' resolved
Radio-telescope images have revealed previously-unseen galactic cannibalism -- a triggering event that leads to feeding frenzies by gigantic black holes at the cores of galaxies. Astronomers have long suspected that the extra-bright cores of spiral galaxies called Seyfert galaxies are powered by supermassive black holes consuming material. However, they could not see how the material is started on its journey toward the black hole.
