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Light Echo at Galactic Center: Chandra Discovers Light Echo from the Milky Way's Black Hole
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.
| Fast Facts for Light Echo at Galactic Center: |
| Credit |
NASA/CXC/Caltech/M.Muno et al. |
| Scale |
Close up images are 5 arcmin across; Full-field is 12.5 arcmin across |
| Category |
Milky Way Galaxy |
| Coordinates (J2000) |
RA 17h 45m 58.013s | Dec -28º 56' 33.11" |
| Constellation |
Sagittarius |
| Observation Dates |
22 pointings from 1999 to 2005
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| Observation Time |
9.3 days |
| Obs. IDs |
0242, 1561, 2284, 2549, 2662, 2943, 2951-54, 3392-93, 3665, 4683-84, 5950-52, 5954
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| Color Code |
Intensity |
| Instrument |
ACIS |
| References | Iron Fluorescence from Reflection Nebulae in the Galactic Center. Muno, M.P., et al. ApJL in press. |
| Distance Estimate |
About 26,000 light years |
| Release Date |
January 10, 2007 |
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