NASA Chandra Produces Image of Exploded Star Cassiopeia A
A spectacular new image of Cassiopeia A released today
from NASA’s Chandra X-ray Observatory has nearly 200 times
more data than the “First Light” Chandra image of this object
made five years ago. The new image reveals clues that the
initial explosion, caused by the collapse of a massive star,
was far more complicated than suspected.
“Although this young supernova remnant has been intensely
studied for years, this deep observation is the most detailed
ever made of the remains of an exploded star,” said Martin
Laming of the Naval Research Laboratory, Washington. Laming
is part of a team of scientists led by Una Hwang of NASA’s
Goddard Space Flight Center, Greenbelt, Md. “It is a gold
mine of data that astronomers will be panning through for
years to come,” he added.
The 1 million-second (about 11.5-day) observation of
Cassiopeia A uncovered two large, opposed jet-like structures
that extend to about 10 light-years from the center of the
remnant. Clouds of iron that have remained nearly pure for
the approximately 340 years since the explosion also were
detected.
“The presence of the bipolar jets suggests that jets could be
more common in relatively normal supernova explosions than
supposed by astronomers,” said Hwang. A paper by Hwang,
Laming and others on the Cassiopeia A observation will appear
in an upcoming issue of The Astrophysical Journal Letters.
X-ray spectra show that the jets are rich in silicon atoms
and relatively poor in iron atoms. In contrast, fingers of
almost-pure iron gas extend in a direction nearly
perpendicular to the jets. This iron was produced in the
central, hottest regions of the star.
The high silicon and low iron abundances in the jets indicate
that massive, matter-dominated jets were not the immediate
cause of the explosion, as these should have carried out
large quantities of iron from the central regions of the
star.
A working hypothesis is that the explosion produced high-
speed jets similar to those in hypernovae that produce gamma-
ray bursts, but in this case, with much lower energies. The
explosion also left a faint neutron star at the center of the
remnant.
Unlike the rapidly rotating neutron stars in the Crab Nebula
and Vela supernova remnants that are surrounded by dynamic
magnetized clouds of electrons, this neutron star is quiet
and faint. Nor has pulsed radiation been detected from it. It
may have a very strong magnetic field generated during the
explosion that helped to accelerate the jets, and today
resembles other strong-field neutron stars (a.k.a.
“magnetars”) in lacking a wind nebula.
Chandra was launched July 23, 1999, aboard the Space Shuttle
Columbia. Less than a month later, it was able to start
taking science measurements along with its calibration data.
The original Cassiopeia A observation was taken August 19,
1999, and released to the scientific community and the public
one week later. At launch, Chandra’s original mission was
intended to be five years. Last August NASA announced that
the mission, having successfully completed that objective,
would be extended for another five years.
The data for this new Cassiopeia A image were obtained by
Chandra’s Advanced Charged Coupled Device Imaging
Spectrometer (ACIS) instrument during the first half of 2004.
Due to its value to the astronomical community, this rich
dataset was made available immediately to the public.
NASA’s Marshall Space Flight Center, Huntsville, Ala.,
manages the Chandra program for the NASA Science Mission
Directorate, Washington. Northrop Grumman of Redondo Beach,
Calif., formerly TRW, Inc., was the prime development
contractor for the observatory. The Smithsonian Astrophysical
Observatory controls science and flight operations from the
Chandra X-ray Center in Cambridge, Mass.
For additional information and images on the Internet, visit: http://chandra.harvard.edu and http://chandra.nasa.gov
