For the first time, astronomers have caught a pulsar in a crucial transitional phase that explains the origin of the mysterious millisecond pulsars. The newly found pulsar swings back and forth between accretion-powered X-ray emission and rotation-driven radio emission, bringing conclusive evidence for a 30-year-old model that explains the high spin rate of millisecond pulsars as a result of matter accretion from a companion star.
Pulsars are the highly magnetized, spinning remnants of supernova explosions of massive stars and are primarily observed as pulsating sources of radio waves. The radio emission is powered by the rotating magnetic field and focused in two beams that stem from the magnetic poles. Similarly to a rotating lighthouse beacon, the rotation of the pulsar swings the emission cone through space, resulting in distant observers seeing regular pulses of radio waves (CERN Courier March 2013 p12). It is actually the kinetic rotational energy of the neutron star that is radiated away, leading to a gradual slow down of the rotation. While pulsars spin rapidly at birth, they tend to rotate more slowly – with periods of up to a few seconds – as they age. For this reason, astronomers in the 1980s were puzzled by the discovery of millisecond pulsars – old but extremely quickly rotating pulsars with periods of a few thousandths of a second.
The mysterious millisecond pulsars can be explained through a theoretical model known as the “recycling” scenario. If a pulsar is part of a binary system and is accreting matter from a stellar companion via an accretion disc, then it might also gain angular momentum. This process can “rejuvenate” old pulsars, boosting their rotation and making their periods as short as a few milliseconds. This scenario relies on the existence of accreting pulsars in binary systems, which can be detected through the X rays that are emitted in the accretion process. The discovery in the 1990s of the first X-ray millisecond pulsars was first evidence for this model but, until now, the search for a direct link between X-ray bright millisecond pulsars in binary systems and the radio-emitting millisecond pulsars has been in vain.
Now, the missing link to prove the validity of the scenario has finally been discovered by the wide-field IBIS/ISGRI imager on board ESA’s INTEGRAL satellite. A new X-ray source appeared in images taken on 28 March 2013 at the position of the globular cluster M28. Subsequent observations by the XMM-Newton satellite found a modulation of its X-ray emission at a period of 3.9 ms, revealing the incredibly fast spin of the neutron star of more than 250 rotations per second. A very clear modulation of the delay in the pulse arrival time further showed that a low-mass companion star orbits the pulsar every 11 hours.
These results obtained by an international team led by Alessandro Papitto from the Institute of Space Sciences in Barcelona were then compared with properties of a series of known radio pulsars in M28 and – luckily – they found one with precisely the same values. There is therefore no doubt that the radio and X-ray sources are the same pulsar, providing the missing link that validates the recycling scenario of millisecond pulsars. Follow-up radio observations by several antennae in Australia, the Netherlands and the US showed that the source does not exhibit radio pulsations when active in X-rays and vice-versa. It was only at the end of the X-ray outburst, on 2 May, that radio pulsations resumed.
This bouncing behaviour is caused by the interplay between the pulsar’s magnetic field and the pressure of accreted matter. When the accretion dominates, the source emits X rays and radio emission is inhibited by the presence of the accretion disc closing the magnetic field lines.