Sagittarius A*
Credit: NASA/CXC/APC/Université Paris Diderot/M.Clavel et al
Researchers using NASA's Chandra X-ray Observatory have found evidence that the normally dim region very close to the supermassive black hole at the center of the Milky Way Galaxy flared up with at least two luminous outbursts in the past few hundred years.
This discovery comes from a new study of rapid variations in the X-ray
emission from gas clouds surrounding the supermassive black hole,
a.k.a. Sagittarius A*, or Sgr A* for short. The scientists show that
the most probable interpretation of these variations is that they are
caused by light echoes.
The echoes from Sgr A* were likely produced when large clumps 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. Just as echoes of sound reverberate
long after the original noise was created, so too do light echoes in
space replay the original event.
While light echoes
from Sgr A* have been seen before in X-rays by Chandra and other
observatories, this is the first time that evidence for two distinct
flares has been seen within a single set of data.
More than just a cosmic parlor trick, light echoes provide
astronomers an opportunity to piece together what objects like Sgr A*
were doing long before there were X-ray telescopes
to observe them. The X-ray echoes 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 (as viewed in Earth's time frame) that
followed a straight path would have arrived at Earth at that time.
However, the reflected X-rays in the light echoes took a longer path as
they bounced off the gas clouds and only reached Chandra in the last few
years.
A new animation shows Chandra images that have been combined from
data taken between 1999 and 2011. This sequence of images, where the
position of Sgr A* is marked with a cross, show how the light echoes
behave. As the sequence plays, the X-ray emission appears to be moving
away from the black hole in some regions. In other regions it gets
dimmer or brighter, as the X-rays pass into or away from reflecting
material.
The X-ray emission shown here is from a process called fluorescence.
Iron atoms in these clouds have been bombarded by X-rays, knocking out
electrons close to the nucleus and causing electrons further out to fill
the hole, emitting X-rays in the process. Other types of X-ray emission
exist in this region but are not shown here, explaining the dark areas.
This is the first time that astronomers have seen both increasing and
decreasing X-ray emission in the same structures. 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 flares were
responsible for the light echoes observed from Sgr A*.
There are several possible causes of the flares: a short-lived jet
produced by the partial disruption of a star by Sgr A*; the ripping
apart of a planet by Sgr A*; 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*. Further studies of the variations are needed to
decide between these options.
The researchers also examined the possibility that a magnetar - a
neutron star with a very strong magnetic field - recently discovered
near Sgr A* might be responsible for these variations. However, this
would require an outburst that is much brighter than the brightest
magnetar flare ever observed.
A paper describing these results has been published in the October 2013 issue of the journal Astronomy and Astrophysics and is available online.
The first author is Maïca Clavel from AstroParticule et Cosmologie
(APC) in Paris, France. The co-authors are Régis Terrier and Andrea
Goldwurm from APC; Mark Morris from University of California, Los
Angeles, CA; Gabriele Ponti from Max-Planck Institute for
Extraterrestrial Physics, Garching, Germany; Simona Soldi from APC and
Guillaume Trap from Palais de la découverte - Universcience, Paris,
France.
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra Program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory controls Chandra's science and
flight operations from Cambridge, Mass.
Scale Image: is about 18.5 arcmin across (about 140 light years)
Category: Black Holes, Milky Way Galaxy
Coordinates (J2000): RA 17h 45m 40s | Dec -29° 00' 28.00"
Constellation: Sagittarius
Observation Date: 54 pointings between 21 Sep 1999 and 29 Jul 2011
Observation Time: 477 hours 21 min (19 days 21 hours 21 min).
Obs. ID: 242, 945, 1561, 2273, 2276, 2282, 2284, 2943, 2951-2954, 3392, 3393, 3549, 3663, 3665, 4500, 4683, 4684, 5360, 5950-5954, 6113, 6363, 6639-6646, 7048, 7554-7559, 9169-9174, 10556, 11843, 12949, 13438, 13508
Instrument: ACIS
Also Known As: Galactic Center
References: Clavel, M. et al, 2013, A&A 558, A32; arXiv:1307.3954
Color Code: X-ray (Blue)
Distance Estimate: About 26,000 light years
