Bizarre planet-mass object orbiting neutron star detected

NASA astronomers have found a bizarre Planet-Mass orbiting a neutron star.

Washington, Sept 13 : NASA astronomers have found a bizarre Planet-Mass orbiting a neutron star.

The object's minimum mass is only about seven times that of Jupiter, but instead of orbiting a normal star, this low-mass body orbits a rapidly spinning pulsar.

It orbits the pulsar every 54.7 minutes at an average distance of only about 230,000 miles (slightly less than the Earth-Moon distance).

"This object is merely the skeleton of a star. The pulsar has eaten away the star's outer envelope, and all the remains is its helium-rich core," said co-discoverer Craig Markwardt of NASA's Goddard Space Flight Center in Greenbelt, Md.

Hans Krimm of NASA Goddard Center discovered the system on June 7, when Swift's Burst Alert Telescope picked up an outburst of X rays and gamma rays in the direction of the galactic centre.

The source was named SWIFT J1756.9-2508 for its sky coordinates in the constellation Sagittarius.

The agency's Rossi X-ray Timing Explorer (RXTE) then began observing SWIFT J1756.9 on June 13 with its Proportional Counter Array (PCA).

After analyzing the PCA data, Markwardt realized that the object was pulsing in X rays 182.07 times per second, which told him that it was a rapidly spinning pulsar.

Markwardt said these so-called millisecond pulsars are neutron stars that spin hundreds of times per second, faster than a kitchen blender.

"Normally, the spin rate of neutron stars slows down as they age, but much like we can pull a string to "spin up" a top, gas spiraling onto a neutron star from its companion can maintain or even increase its fast spin," Markwardt said.

In the case of SWIFT J1756.9-2508, the scientists detected subtle modulations in the X-ray timing data that revealed a low-mass companion tugging the pulsar toward and away from Earth.

Calculations showed that the companion had a minimum mass about seven times that of Jupiter. arkwardt said since they don't know the orbital inclination of the system, the companion's actual mass is unknown, but it is extremely unlikely to exceed 30 Jupiters.

MIT astronomers led by Deepto Chakrabarty also observed the system with RXTE, before it faded to invisibility on June 21. Chakrabarty's group reached identical conclusions, and the two teams have co-authored a paper, scheduled for publication in the Astrophysical Journal Letters.

ANI