Super-fast pulsar feeds on stellar companion
China’s Five-hundred-meter Aperture Spherical Telescope (FAST) has uncovered the first known pulsar in Messier 92, a globular star cluster roughly 27,000 light years away from Earth in the constellation of Hercules. The swiftly spinning and pulsating object, which goes by two names – PSR J1717+4307A and M92A – forms one part of an eclipsing binary system in which it is siphoning material from a stellar companion. A research team led by Zhichen Pan and Di Li from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), which operates FAST – the world’s largest radio telescope – has shown that M92A spins at a rapid speed of 316.5 rotations per second and co-orbits a star much lighter than our Sun, weighing in at 0.18 solar masses. Using FAST the researchers observed two eclipsing events in the binary system, when one object passed in front of the other from Earth’s point of view. One eclipse lasted around 5,000 seconds, and the second, which arrived between 1,000 and 2,000 seconds later, lasted for 500 seconds. M92A is known as a millisecond pulsar, a souped-up version of the slightly slower moving pulsar. Millisecond pulsars are highly magnetised neutron stars which pirouette rapidly at speeds less than 30 milliseconds. Ordinary pulsars spring into existence at the end of a massive star’s life, peaking as a supernova explosion that leaves behind a stellar corpse known as a neutron star in its dusty wake. The neutron stars left behind are small, weighing in at a mass that’s equivalent to anywhere from one to several Suns, crammed into a diameter of a mere 20 to 24 kilometres. Yet what they lack in size they make up for in speed, completing several rotations per second. If there is a magnetic field lurking around the neutron star, charged particles coming from the star itself become snagged, causing the star to blast electromagnetic radiation in a lighthouse-like beam every few seconds or less. These flashing, magnetic neutron stars are known as pulsars. Millisecond pulsars, however, move much more quickly, powering hundreds of rotations per second by chewing on gas from a companion star that survived the
Words by Gemma Lavender
supernova explosion, spilling material into a disc around the neutron star before falling onto it. During this process the system is visible as an X-ray binary. The neutron star then emerges as a millisecond radio pulsar when the accretion is over. In globular clusters like Messier 92, things work a bit differently – stars are so tightly packed together that it’s easier for ancient neutron stars to interact with other stars, allowing normal stellar binaries to be made. In the case of M92A, the pulsar hasn’t found it too difficult to siphon material from its stellar companion, earning itself the reputation of being likened to a ‘redback spider’ (Latrodectus hasseltii) – highly venomous Australian arachnids that often eat their male companions.