Wednesday, December 13, 2006

Beating spin-down

The results of one of the pieces of research that I've been spending the majority of my time working on were presented at the Texas Symposium on Monday by the head of the LIGO lab, Jay Marx - the slides of his presentation can be found here. What I've been working on is searching for gravitational waves from pulsars using data from the LIGO and GEO600 detectors. You can read more about pulsars in the wikipedia link I just gave, but here's a brief description: A pulsar is a neutron star, which is the ultra dense end state in the life of a massive star - one that have a mass of a few solar masses. Once it is has exhausted its fuel for fusion burning the star's gravity will collapse its core down to a ball of mainly neutrons, whilst the outer layers are blown off in a supernova explosion. This neutron ball will have a radius of only about 10 km, but will contain about one and a half solar masses of material! Due to conservation of magnetic field, the field of the massive progenitor star will be compressed down to the scale of the neutron star, leaving very tight field lines and a very strong magnetic field. Conservation of angular momentum will also mean that the neutron star will initially be very rapidly spinning. A pulsar is seen when radiation beamed out the poles of the neutron star's magnetic field intersects with the Earth. Providing the star's magnetic axis and rotation axis aren't aligned this beam will be seen as a pulse once per rotation - like a lighthouse. Most pulsars are seen by radio observations of these pulses, which allow the determination of the pulsar's position and rotation rate. The rotation rate of pulsars is seen to slow over time due to a loss of rotational kinetic energy. The main process thought to be dissipating this energy is magnetic dipole radiation caused by the motion of the magnetic field about the rotation axis. However, if the pulsar has some for of non-axisymmetry i.e. it has a bump on it distorting it from spherical symmetry, then as the star spins that bump will provide a non-spherical acceleration of mass - exactly what is needed to produce gravitational waves. So I've been trying to look for gravitational waves from exactly this mechanism, by targeting the precise frequency and position of a number of known pulsars using data collected by LIGO and GEO600 - see the ATNF online catalogue for a list of known pulsars. Using this data we've not managed to detect gravitational waves from any of these known potential sources, we've only been able to set upper limits on the amplitude of the radiation they could be emitting. Not seeing anything is however not a surprise to us. By looking at the rate at which the pulsar slows down you can infer how much energy it's losing. If you assume that all this energy were radiated away via gravitational waves (not a completely valid assumption as some will be lost via magnetic dipole radiation and particle acceleration, but...) then you can convert this energy into a amplitude of gravitational waves. For the vast majority of pulsars this spin-down upper limit, as we call it, on the amplitude is much, much lower than the upper limits we can set using our detectors. What makes the results presented on Monday more exciting is that for one particular pulsar, the Crab pulsar, we have produced an upper limit using our data, which for the first time beats the existing spin-down upper limit. This means that we're in the realm of doing meaningful astrophysics with our null results. I can't say how much we've beaten the upper limit by as we're leaving that for the publication we hope to have out in a few months time, but if you look on the plot in Jay's talk you can figure it out. I'll be presenting the result with a bit more explanation at GWDAW 11 next week.