Gravitational Radiation and Rotation of Accreting Neutron Stars

Recent discoveries by the Rossi X-Ray Timing Explorer indicate that most of the rapidly accreting (${u{M}{"705F}}$ -->${u{M}{"705F}}$ --> 10 -->−11 M${s}$ -->$t SUBgt {s}t/SUBgt $ --> yr -->−1) weakly magnetic (B10 -->11 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 νs-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 -->−7MR -->2) 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 ${u{M}{"705F}}$ -->${u{M}{"705F}}$ --> 10 -->−10 M${s}$ -->$t SUBgt {s}t/SUBgt $ --> 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.