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Light
Echo at Galactic Center: Chandra Discovers Light Echo from the
Milky Way's Black Hole
01/10/07
This
set of Chandra images shows evidence for a light echo generated
by the Milky Way's supermassive black hole, a.k.a. Sagittarius A*
(pronounced "A-star"). Astronomers believe a mass
equivalent to the planet Mercury was devoured by the black hole
about 50 years earlier, causing an X-ray outburst which then
reflected off gas clouds near Sagittarius A*.
The large
image shows a Chandra view of the middle of the Milky Way, with
Sagittarius A* labeled. The smaller images show close-ups of the
region marked with ellipses. Clear changes in the shapes and
brightness of the gas clouds are seen between the 3 different
observations in 2002, 2004 and 2005. This behavior agrees with
theoretical predictions for a light echo produced by Sagittarius
A* and helps rule out other interpretations.
While the
primary X-rays from the outburst would have reached Earth about
50 years ago, before X-ray observatories were in place to see it,
the reflected X-rays took a longer path and arrived in time to be
recorded by Chandra.
The clouds of gas featured in the
image are glowing by a process called fluorescence. Iron in these
clouds has been bombarded either by X-rays from a source that had
an outburst in the past or by very energetic electrons. The
electrons or photons hit the iron atoms, knocking out electrons
close to the nucleus, causing electrons further out to fill the
hole, emitting X-rays in the process.
The detection of
variability in these fluorescing gas clouds rules out the
possibility that they were bombarded by energetic electrons. It
also helps rule out other explanations for the X-ray emission,
including the possibility that the gas clouds are the remnants of
exploded stars or that the light echo came not from Sagittarius
A* but from a neutron star or black hole pulling matter away from
a binary companion.
Studying this light echo gives a
crucial history of activity from Sagittarius A*, and it also
illuminates and probes the poorly understood gas clouds near the
center of the galaxy.
Source
/ Credit: NASA / Chandra X-Ray
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