Even by the extreme standards of neutron stars, the Pulsar PSR J1023+0038 is exceptional. It is also, according to new research, almost – but not quite – perfectly spherical.
One scientist has now used changes in the rate at which it turns to calculate just how far it is from a perfect sphere and concluded it is a few micrometers longer in one direction than the other. That is approximately the size of a single bacterium, something too small to see without a microscope. Yet if he's right, Professor Sudip Bhattacharyya of India’s Tata Institute of Fundamental Research has inferred that deviation in an object a thousand times further away than the nearest star. Besides being an astonishing achievement, a correct assessment would make J1023+0038 a target to find the first continuous gravitational wave.
Like all neutron stars, pulsars are phenomenally dense, with a mass greater than the Sun packed into an area the size of a middling city. Pulsars have the extra feature of shooting out beams of radiation from their poles that turn across the sky like a lighthouse. PSR J1023+0038 achieved fame in astronomical circles because it produces a jet almost as strong as a black hole. Only two other pulsars have been found to do the same thing. All three have companion stars and alternate between periods where they draw material off their less dense neighbor and when they don't.
Like other pulsars, PSR J1023+0038 is slowing down, but where most have a single “spin-down rate”, the three transitional pulsars have two, depending on whether or not they are accreting mass. PSR J1023+0038 is the only one where both rates have been measured.
Bhattacharyya realized that the comparison between these two spin-down rates could be used to calculate any elongation at the pulsar’s equator. In Monthly Notices of the Royal Astronomical Society, Bhattacharyya calculates that PSR J1023+0038’s circumference deviates from perfect symmetry by 0.48-0.93 x 10-9. With a radius a little over 20 kilometers (12 miles), that means it bulges by 1-2 micrometers, considerably less than the width of a hair.
The detection of gravitational waves from colliding black holes and neutron stars has been one of the scientific breakthroughs of recent years. However, we are yet to detect continuous gravitational waves, which would generally be much weaker but longer lasting. Bhattacharyya points out a rapidly spinning neutron star elongated at its equator would produce a continuous gravitational wave, an effect of the deformation moving through such an immense gravitational field. This wave’s emission contributes to the loss of energy that is slowing PSR J1023+0038 down.
Such a gravitational wave should be at the limits of our capacity to observe, making it an exciting challenge for the LIGO network, in the process creating a test of Bhattacharyya’s calculations