By Ron Cowen
New images taken with the Chandra X-Ray Observatory have confirmed recent evidence that a known pulsar, a rapidly spinning neutron star, was born in a supernova explosion that Chinese astronomers witnessed in A.D. 386. The finding, which indicates the pulsar is much younger than earlier calculations suggested, calls into question how astronomers traditionally compute the ages of these rapidly rotating, ultracompact stars.
The finding also represents only the second time that a pulsar has been definitively linked to a supernova observed centuries ago, notes Victoria M. Kaspi of McGill University in Montreal. Scientists had previously established a clear link between the Crab nebula pulsar and a supernova sighting in China in A.D. 1054.
Observations in 1997 with Japan’s ASCA satellite indicated that a known pulsar coincides with the position of the A.D. 386 supernova remnant G11.2-0.3. But it took the sharper eye of Chandra to reveal that the pulsar lies at the exact core of the remnant. “This makes the association between the pulsar and the supernova remnant essentially inescapable,” says Kaspi.
It also pegs the pulsar’s age at just over 1,400 years. From the pulsar’s current rotation rate and a model of how rapidly it slows over the years, astronomers had calculated its age as 24,000 years. If the pulsar were really that old, it would have had enough time to migrate far from the supernova’s core, Kaspi adds.
To explain the discrepancy between the actual and calculated ages, the pulsar must have started with a slower rotation than the standard model assumes, Kaspi says. The new findings, along with a similar age discrepancy on another pulsar found by a different team, suggest that pulsar ages derived from their assumed rate of spin down “can be even more deceptive than we had previously thought,” says Andrew S. Fruchter of the Space Telescope Science Institute in Baltimore. “It is the difficulty of determining true pulsar ages . . . [that] makes these historical associations [with supernova remnants] so valuable,” he notes.
