GRAVITATIONAL-WAVES FROM PULSARS - EMISSION BY THE MAGNETIC-FIELD-INDUCED DISTORTION

The gravitational wave emission by a distorted rotating fluid star is computed. The distortion is supposed to be symmetric around some axis inclined with respect to the rotation axis. In the general case, the g ravitational radiation is emitted at two frequencies: Omega and 2 Omeg a, where Omega is the rotation frequency. The obtained formula: are ap plied to the specific case of a neutron star distorted by its own magn etic field. Assuming that the period derivative P of pulsars is a meas ure of their magnetic dipole moment, the gravitational wave amplitude can be related to the observable parameters P and P and to a factor be ta which measures the efficiency of a given magnetic dipole moment in distorting the star. beta depends on the nuclear matter equation of ; state and on the magnetic field distribution. The amplitude at the fre quency 2 Omega, expressed in terms of P, P and beta, is independent of the angle a between the magnetic axis and the rotation axis, whereas at the frequency Omega, the amplitude increases as a decreases. The va lue of P for specific models of magnetic field distributions has been computed by means of a numerical code giving self-consistent models of magnetized neutron stars within general relativity. It is found that the distortion at fixed magnetic dipole moment is very dependent of th e mag netic field distribution; a stochastic magnetic field or a super -conductor stellar interior greatly increases beta with respect to the uniformly magnetized perfect conductor case and might lead to gravita tional waves detectable by the VIRGO or LIGO interferometers. The ampl itude modulation of the signal induced by the daily rotation of the Ea rth has been computed and specified to the case of the Crab pulsar and VIRGO detector.