Millisecond pulsar with dual identity discovered

Scientists using a fleet of orbiting X-ray telescopes, including NASA's Swift and Chandra X-ray Observatory have found a millisecond pulsar with a dual identity.

In a feat that has never before been observed, the star readily shifts back and forth between two mutually exclusive styles of pulsed emission-one in X-rays, the other in radio.

Co-author Sergio Campana, an astronomer at Brera Observatory in Merate, Italy, said that this transitional object took decades to find, and it provides them with a unique opportunity to observe a pulsar's intense magnetic field in action.

A team led by Alessandro Papitto of the Institute of Space Sciences in Barcelona, Spain, directed ESA's XMM-Newton satellite toward the object. It detected X-ray pulses, which indicated a neutron star spinning once every 3.9 milliseconds or at about 15,000 rpm. By analyzing changes in the arrival times of these pulses, the scientists established that the pulsar was joined by a small companion star less than one-fifth the mass of our Sun. The two stars orbit each other every 11 hours.

The team later detected variable radio emission with ATCA, but then, two days later, the object fired off an intense burst of X-rays carrying the telltale signature of a thermonuclear explosion on the surface of a neutron star.

With the pulsar's spin and orbital characteristics in hand, a team led by Alessandro Papitto of the Institute of Space Sciences in Barcelona, Spain, compared them to parameters for known radio pulsars in M28 and found a perfect match with PSR J1824-2452I.

The same pulsar showed clear evidence of accretion-powered X-ray emissions, as indicated by thermonuclear explosions, and rotation-powered signals at radio wavelengths.

Papitto and his team think the answer lies in the interplay between the pulsar's magnetic field and variations in the gas flow from the companion.

During periods when the mass flow is less intense, the magnetic field sweeps away the gas and prevents it from reaching the surface and creating X-ray emission. With the region around the neutron star relatively gas free, radio signals can easily escape and astronomers detect a radio pulsar.

The study has been published in the journal Nature.