The rotating neutron star is believed to be at the centre of this supernova remnant. Picture by Nasa/Chandra X-ray Observatory

New Delhi, Sept. 13: After two hours of scanning the ashes of a dead, overweight star in the galaxy, astronomer Yashwant Gupta spotted a celestial child that seems older than its parent. But he’s elated. The age problem, he says, can be resolved.

Using an array of radio telescopes nestled in a valley 80 km from Pune, Gupta and his colleagues have discovered a fast-rotating neutron star, the leftover of a massive star that died in a supernova explosion a thousand years ago midway towards the centre of the galaxy.

Their discovery closes a two-decade search by international scientists to locate a neutron star, also called a pulsar, hidden in the core of the remnants of the supernova named G21.5.

A supernova is a star that undergoes a catastrophic explosion, becoming suddenly very much brighter. All pulsars are believed to have been created at the centres of massive stars following supernova explosions. The explosions eject the star’s outer layers and leave behind dense cores packed with neutrons. The pulsar’s rotation makes it emit radio signals and behave a bit like a beam from a lighthouse.

Using the Giant Metrewave Radio Telescope (GMRT), Gupta and his colleagues at the National Centre for Radio Astronomy (NCRA) in Pune picked up radio signals from the pulsar at the core of G21.5. The pulsar rotates 16 times a second.

Astronomers have catalogued more than 1,000 pulsars since the first was discovered in 1967. All are rotating neutron stars, but their rotation rates vary ? the fastest spins 600 times a second while the slowest might rotate just once in eight seconds.

“We don’t have a mechanism to explain the birth of pulsars except through supernova explosions,” said Gupta, principal investigator in the team that detected the pulsar in G21.5. Although the Milky Way has 230 known supernova remnants, astronomers have found pulsars in less than 20.

Scientists have spent years hunting for pulsars at the cores of supernova remnants to prove themselves right. “This discovery significantly improves the meagre statistics of association between supernova remnants and pulsars,” said Rajaram Nityananda, NCRA director.

The G21.5 supernova is believed to have exploded about a thousand years ago. But its pulsar, initial calculations indicate, might be older.

“Such age discrepancies are common in astronomy ? we’ve made certain assumptions about the pulsar which are wrong. It’s obviously younger,” Gupta said.

The credit for discovering the pulsar is shared by NCRA astronomer Dipanjan Mitra and Amit Acharya, a student from the National Institute of Technology, Durgapur, who detected its unique signals just one day before his project work at the NCRA ended. The NCRA team has reported its discovery in the latest issue of the journal Current Science.

The NCRA collaborated with Cambridge University astronomer Dave Green in the pulsar search.