Gravitational Radiation and Rotation of Accreting Neutron Stars

Abstract

Recent discoveries by the Rossi X-Ray Timing Explorer indicate that most of the rapidly accreting and weakly magnetic neutron stars in the Galaxy are rotating at spin frequencies greater than 250 Hz. Remarkably, they all rotate in a narrow range of frequencies. I suggest that these stars rotate fast enough so that, on average, the angular momentum added by accretion is lost to gravitational radiation. The strong spin frequency dependence of the angular momentum loss rate from gravitational radiation then provides a natural reason for similar spin frequencies. Provided that the interior temperature has a large scale asymmetry misaligned from the spin axis, then the temperature sensitive electron captures in the deep crust can provide the quadrupole needed to reach this limiting situation at 300 Hz. This quadrupole is only present during accretion and makes it difficult to form radio pulsars spinning more rapidly than 600-800 Hz via rapid accretion. The gravity wave strength is <10^{-26} from most neutron stars and >2 X 10^{-26} for Sco X-1. Prior knowledge of the position, spin frequency and orbital periods will allow for deep searches for these periodic signals with gravitational wave interferometers (LIGO, VIRGO and the dual-recycled GEO 600 detector) and experimenters need to take such sources into account. Sco X-1 will most likely be detected first.