Washington, Jan 3: New observations by astronomers has indicated that a white dwarf star in the Crab Nebula is emitting pulses of high energy X-rays as it whirls around on its axis, thus challenging the theory that white dwarfs are lifeless remains of stars that slowly cool and fade away.

The star, known as AE Aquarii, was observed from Suzaku, a joint Japanese Aerospace Exploration Agency (JAXA) and NASA X-ray observatory.

"We’re seeing behavior like the pulsar in the Crab Nebula, but we’re seeing it in a white dwarf," said Koji Mukai of NASA Goddard Space Flight Center in Greenbelt, Md.

White dwarfs and pulsars represent distinct classes of compact objects that are born in the wake of stellar death. Though a white dwarf forms when a star similar in mass to our sun runs out of nuclear fuel, a pulsar is a type of neutron star, a collapsed core of an extremely massive star that exploded in a supernova.

What the discovery team, led by Yukikatsu Terada of the Institute of Physical and Chemical Research (RIKEN) in Wako, Japan, was not expecting to find in their observations, was a white dwarf mimicking a pulsar.

Instead, the astronomers were hoping to find out if white dwarfs could accelerate charged subatomic particles to near-light speed, meaning they could be responsible for many of the cosmic rays that zip through our galaxy and occasionally strike Earth.

Some white dwarfs, including AE Aquarii, spin very rapidly and have magnetic fields millions of times stronger than Earth’s. These characteristics give them the energy to generate cosmic rays.

To find out if this is happening, Terada and his colleagues targeted AE Aquarii with Suzaku in October 2005 and October 2006.

The white dwarf resides in a binary system with a normal companion star. Gas from the star spirals toward the white dwarf and heats up, giving off a glow of low-energy
(soft) X-rays. But Suzaku also detected sharp pulses of hard X-rays.

After analyzing the data, the team realized that the hard X-ray pulses match the white dwarf’s spin period of once every 33 seconds.

"AE Aquarii seems to be a white dwarf equivalent of a pulsar," says Terada. "Since pulsars are known to be sources of cosmic rays, this means that white dwarfs should be quiet but numerous particle accelerators, contributing many of the low-energy cosmic rays in our galaxy," he added. (ANI)