SAN DIEGO, Jan 10 (Reuters) -- Pulsars never tell their age, but scientists said on Wednesday that cosmic X-rays and a 1,600-year-old observation by Chinese astronomers show one of these spinning space searchlights may be far younger than it seemed.
Previous estimates had put this particular pulsar, which was shooting out radiation after a star exploded, at a ripe old age of 24,000 years or so. But new data suggests it is about 1,615 years old, and call into question earlier methods for determining the age of these reticent celestial objects.
Astronomers from McGill University in Montreal, Canada trained NASA's orbiting Chandra X-ray Observatory at a pulsar in the constellation Sagittarius and found that it was probably born when a massive star blew up in the year 386 AD.
They knew the date of the explosion, called a supernova, because Chinese observers recorded a "guest star" between mid April and mid May of that year in that part of the sky.
Nearly 1600 years later, in the 1970s, astronomers tracking radio waves in space detected an expanding nebula of gas and high-energy particles known to scientists as G11.2-0.3 that they believed was a remnant of the star's explosion.
Then, in 1997, Japanese X-ray astronomers found a pulsar at roughly the same location.
When Chandra observed the pulsar last year, it was seen at the precise center of the supernova remnant.
This was important, McGill astronomer Victoria Kaspi and others told reporters at a news conference, because pulsars tend to flee their birthplaces soon after they are created. This one had stayed close to home, so scientists inferred it was still a relative infant.
Small and symmetrical, not big and deformed
It was also small and symmetrical, another sign of youth: older pulsars tend to spread out and become deformed.
"Determining the true ages of astronomical objects is notoriously difficult," Kaspi said in a statement released at a meeting of the American Astronomical Society in San Diego. "And for this reason, historical records of supernova are of great importance."
If confirmed, this would be only the second known pulsar to be clearly associated with an historic event.
There have been less than 10 reports of probable supernovae in the last 2,000 years and, until now, the Crab Nebula has been the only pulsar whose birth is associated with a historic event, the supernova of 1054, according to the National Aeronautics and Space Administration.
The 1600-year-old pulsar is about 15,000 light-years away from Earth, fairly close in cosmic terms, and that is another clue that the Chinese observations were accurate, according to researcher Mallory Roberts, also of McGill. A light-year is about 6 trillion miles (10 trillion kilometers), the distance light travels in a year.
"If it was much farther than that, it could not be seen by the naked eye by the Chinese," Roberts said.
The Chinese observations of the "guest star" were bolstered further by concurrent records of astronomical movements of planets in the night sky, which 21st century scientists could confirm.
The earlier calculation of the pulsar's age was based on how fast it was spinning -- about 14 times per second -- and the assumption that it had slowed down considerably since its birth. Scientists now theorize that this pulsar was just born slow and has not slowed down much since the year 386 AD.
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