The first generation of long-baseline laser interferometric detectors of gravitational waves will start collecting data in [2001][2002][2003]. We carefully analyse their planned performance and compare it with the expected strengths of astrophysical sources. The scientific importance of the anticipated discovery of various gravitatinal wave signals and the reliability of theoretical predictions are taken into account in our analysis. We try to be conservative both in evaluating the theoretical uncertainties about a source and the prospects of its detection. After having considered many possible sources, we place our emphasis on (1) inspiraling binaries consisting of stellar mass black holes and (2) relic gravitational waves. We draw the conclusion that inspiraling binary black holes are likely to be detected first by the initial ground-based interferometers. We estimate that the initial interferometers will see 2-3 events per year from black hole binaries with component masses 10-15 M ⊙ , with a signalto-noise ratio of around 2-3, in each of a network of detectors consisting of GEO, VIRGO and the two LIGOs. It appears that other possible sources, including coalescing neutron stars, are unlikely to be detected by the initial instruments. We also argue that relic gravitational waves may be discovered by the space-based interferometers in the frequency interval 2 × 10 −3 Hz-10 −2 Hz, at the signal-to-noise ratio level around 3.
The paucity of old isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic Gaussian distribution with mean velocity 0 ≤ V ≤ 550 km s −1 . The spin-down, induced by dipole losses and the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All Sky Survey an upper limit of ∼ 10 accreting neutron stars within ∼ 140 pc from the Sun, we infer a lower bound for the mean kick velocity, V ∼ > 200 − 300 km s −1 , corresponding to a velocity dispersion σ V ∼ > 125−190 km s −1 . The same conclusion is reached for both a constant magnetic field (B ∼ 10 12 G) and a magnetic field decaying exponentially with a timescale ∼ 10 9 yr. Such high velocities are consistent with those derived from radio pulsar observations. Present results, moreover, constrain the fraction of low velocity stars, which could have escaped pulsar statistics, to less than 1%.
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