The fastest spinning White Dwarf?
28 January 2016
The fastest spinning known pulsar rotates at a rate of 716 Hz (i.e. 716 rotations per second), the slowest takes 8.5 seconds to complete one rotation. Rotating White Dwarfs are much slower, AE Aqr being the fastest and taking 33 seconds for one rotation, while the rest are even slower. Then there is RX J0648.0-4418, a nearby (~650 parsec) compact star in a 1.5-day binary with a rather unremarkable sub-dwarf star. It is unclear what RX J0648.0-4418 is but it is an intriguing and extreme case. The star is seen as an X-ray source with a temperature of about 450,000 Kelvin and black-body radius of only about 20 km. The X-ray emission varies periodically as the star rotates, with a period of 13.2 seconds.
The temperature and size tell us that it is either a massive (and therefore small) White Dwarf or a Neutron Star but the spin period does not point to a clear answer: it is either the fastest spinning White Dwarf known or a very slow Neutron Star. Studying the orbital dynamics, its mass was determined as 1.3 solar masses – which, again, does not solve the mystery. Neutron Stars can be stable well above 2 solar masses, whereas White Dwarfs are unstable to gravitational collapse beyond the Chandrasekhar limit of 1.4 solar masses. So if RX J0648.0-4418 is a White Dwarf, it is very close to the limit, which is uncommon (most are ten times lighter). If RX J0648.0-4418 is a Neutron Star, its temperature suggest that it is young and, therefore, that its pulsar mechanism is active.
With this in mind, CAASTRO Affiliate Dr Evan Keane (SKA Office, Manchester; previously postdoc at Swinburne University) devised an observational study to resolve the issue: is there pulsar emission or not? Australia’s prolific pulsar hunting telescope, the 64-m CSIRO Parkes Radio Telescope, was employed in one of the deepest searches for pulsar emission ever performed, sensitive to both steady periodic emission and sporadic bursts, but the study came up empty. So, it appears as if RX J0648.0-4418 is not a pulsar but rather the heaviest White Dwarf known and also the fastest spinning. It would not take much to push it over the edge towards supernova. But for now it seems stable and offers a unique chance to study this most extreme version of this extreme type of star.