Gravitational waves from pulsars in binary systems

Rotating neutron stars are remnants of old imploded stars. They are called pulsars, as they emit an electromagnetic pulse for each rotational period. Pulsars are known for their extreme timing accuracy and continous emission.

Various neutron star models allow for small deformities in the crust which are not aligned with the axis of symmetry. This implies a quadrupole moment and thus such a system will emit gravitational waves. These waves are thought to be very weak, in the order of 10^-26 in amplitude. This amplitude is much lower than what one would expect when looking at for instance coalescing binary systems. On the other hand the expected gravitational wave signal will be a continuous one. Since continuous signals can be integrated over long time periods a large gain in signal to noise ratio can be obtained.

In oder to utilize the long observation times fully the gravitational wave signal has to be stationary. However, there are a number of effects which introduce various frequency modulations in the signal. First of all the energy which is emitted in gravitational waves must come from the rotational energy of the pulsar introducing a so-called spindown in the frequency of the signal. Secondly the earth's movement with respect to the solar system barycenter introduces Doppler shifts in the signal with periods of one day and one year respectively. Finally the pulsar itself may have a motion of its own which will introduce another Doppler effect in the signal.

When considering pulsars in binary systems this last source of frequency modulation is the strongest effect. This effect comes directly from the Keplerian orbit of the pulsar and is larger for pulsars with very eccentric orbitals or smaller periods. It is estimated that there are in the order of 10⁵ pulsars in our galaxy alone. Because many of these pulsars are located in binary systems gravitational waves from these pulsars are a natural thing to search for.

The continuous waves group at Nikhef is developing new algorithms in to perform an all-sky search for gravitational waves from pulsars in binary systems. These algorithms take long stretches of data of the VIRGO (and LIGO) detectors and utilize the continuous property of the waves in order to increase the sensitivity beyond the detector noise level.

The limiting factor in this analysis is the available computing power. So besides developing smarter algorithms which utilize the CPUs more efficiently, we also take advantage of the available GRID facilities at Nikhef.