New evidence from NASA's Chandra X-ray Observatory suggests that a
known pulsar is the present-day counterpart to a supernova that exploded
in 386 AD, a stellar explosion witnessed by Chinese astronomers. If
confirmed, this will be only the second known pulsar to be clearly associated
with a historic event.
In roughly the past 2,000 years, less than 10 reports of probable supernovae
have been archived, mostly by Asian astronomers. Until now, the Crab Nebula
has been the only pulsar whose birth is associated with a historic event, the
supernova of 1054 AD, making it the only neutron star with a firm age.
"Determining the true ages of astronomical objects is notoriously difficult,"
said Victoria Kaspi of the McGill University, Montreal, Canada, "and for this
reason, historical records of supernova are of great importance."
These results were presented today by Kaspi and Mallory Roberts, also of McGill
University, at the American Astronomical Society meeting in San Diego, Calif.
Also participating in the research were Gautum Vasisht from NASA's Jet Propulsion
Laboratory, Pasadena, Calif.; Eric Gotthelf from Columbia University, New York
City; Michael Pivovaroff from Thermawave, Inc., Fremont, Calif.; and Nobuyuki
Kawai from the Institute of Physical and Chemical Research, Japan.
Scientists used Chandra to locate the pulsar exactly at
the geometric center of the supernova remnant known as G11.2-0.3. This location
provides very strong evidence that the pulsar, a neutron star spinning 14 times
per second, was formed in the supernova of 386 AD, making it 1,615 years old.
Since pulsars move rapidly once they are formed, Chandra's
ability to pinpoint the pulsar at the remnant's center implies the system must
be very young. "We believe that the pulsar and the supernova remnant G11.2-0.3
are both likely to be left over from the explosion seen by the Chinese observers
over 1,600 years ago," said Roberts. "While this is exciting by itself, it also
raises new questions about what we know about pulsars, especially during their
These questions arose when the research team of the Japanese
Advanced Satellite for Cosmology and Astrophysics (ASCA) applied the present
spin rate to current models to determine the pulsar's estimated lifetime and
compared it to the age of G11.2-0.3. The result was an age of roughly 24,000
years -- far predating the birth year of 386 AD. To explain this contradiction,
the Chandra team argues that this pulsar may have had
Approximately the same spin rate today as it did at its
birth. If true, this could have important implications for conventional wisdom
regarding pulsars, which may be spinning more slowly than previously thought.
Between mid-April and mid-May in the year 386 AD, the sudden
appearance of a new star, presumably a supernova, was recorded by Chinese observers
in the direction of the sky now known as the constellation of Sagittarius. In
the 1970s, radio astronomers discovered an expanding nebula of gas and high-energy
particles, called G11.2-0.3, believed to be the remnant of that explosion. In
1997, a team of X-ray astronomers used ASCA to discover a pulsar in the same
area of the sky.
Chandra observed G11.2-0.3 with the Advanced CCD Imaging
Spectrometer at two points in time: Aug. 6, 2000, and Oct. 15, 2000, for approximately
20,000 and 15,000 seconds respectively.
NASA's Marshall Space Flight Center in Huntsville, Ala.,
manages the Chandra program. The Smithsonian's Chandra X-ray Center controls
science and flight operations from Cambridge, Mass. In addition to their appointments
at McGill, Kaspi is also affiliated with the Massachusetts Institute of Technology,
Cambridge, and Roberts is a Quebec Merit
Postdoctoral Fellow. The National Science Foundation and
NSERC (Canada) also provided funding for this work.
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