How did the pulsar acquire its exotic companion? And how do we know it's made of diamond?

Pulsar J1719-1438 is a very fast-spinning pulsar-what's called a millisecond pulsar. Amazingly, it rotates more than 10,000 times per minute, has a mass of about 1.4 times that of our Sun but is only 20 km in radius. About 70% of millisecond pulsars have companions of some kind: astronomers think it is the companion that, as a star, transforms an old, dead pulsar into a millisecond pulsar by transferring matter and spinning it up to a very high speed. The result is a fast-spinning millisecond pulsar with a shrunken companion-most often a white dwarf.

"We know of a few other systems, called ultra-compact low-mass X-ray binaries, that are likely to be evolving according to the scenario above and may likely represent the progenitors of a pulsar like J1719-1438" said Dr. Andrea Possenti, of INAF-Osservatorio Astronomico di Cagliari.

But pulsar J1719-1438 and its companion are so close together that the companion could only be a very stripped-down white dwarf, one that has lost its outer layers and over 99.9% of its original mass. This remnant is likely to be largely carbon and oxygen, stars of lighter elements like hydrogen and helium just won't fit. The density means that this material is certain to be crystalline: that is, a large part of the star may be similar to a diamond.

"The ultimate fate of the binary is determined by the mass and orbital period of the donor star at the time of mass transfer. The rarity of millisecond pulsars with planet-mass companions means that producing such 'exotic planets' is the exception rather than the rule, and requires special circumstances", said Dr. Benjamin Stappers from the University of Manchester.

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