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.

This Chandra image clearly locates a pulsar exactly at the geometric center of the supernova remnant known as G11.2-0.3. Chandra provides very strong evidence that the pulsar was formed in the supernova of 386 AD, which was witnessed by Chinese astronomers. Photo: NASA/McGill/V. Kaspi et al.

In roughly the past 2,000 years, less than 10 reports of probable supernovae have been archived, mostly by Asian astronomers.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 by Kaspi and Mallory Roberts, also of McGill University, at the American Astronomical Society meeting in San Diego, CA. Also participating in the research were Gautum Vasisht from NASA's Jet Propulsion Laboratory, Pasadena, CA; Eric Gotthelf from Columbia University, New York City; Michael Pivovaroff from Thermawave, Inc., Fremont, CA; 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 infancies."

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, AL, manages the Chandra program. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, MA. 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.