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 ( 10⁻¹¹ M yr⁻¹) weakly magnetic (B10¹¹ G) neutron stars in the Galaxy are rotating at spin frequencies ν_s 250 Hz. Remarkably, they all rotate in a narrow range of frequencies (no more than a factor of 2, with many within 20% of 300 Hz). 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 ν_s-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 ( ~10⁻⁷MR²) to reach this limiting situation at ν_s ≈ 300 Hz. This quadrupole is only present during accretion and makes it difficult to form radio pulsars with ν_s > (600-800) Hz by accreting at 10⁻¹⁰ M yr⁻¹. The gravity wave strength is h_c ~(0.5-1) × 10⁻²⁶ from many of these neutron stars and greater than 2 × 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.