(ed.). Gravitational waves (IOP, 2001)(422s).

46 Astrophysics of gravitational-wave sourcesTable 4.2. The range for detecting a 10M ⊙ black-hole binary. Conventions as in table 4.1.Detector: GEO600 LIGO I VIRGO LIGO IIRange (S/N = 5) 75 Mpc 160 Mpc 190 Mpc 2.6 Gpcelectromagnetically. So we can only make theoretical estimates, and there arebig uncertainties.Some evolution calculations [14] suggest that the coalescence rate of BH–BH systems may be of the same order as the NS–NS rate. Other models [15]suggest it could even be zero, because stellar-wind mass loss (significant in verymassive stars) could drive the stars far apart before the second BH forms, leadingto coalescence times longer than the age of the universe. A recent proposalidentifies globular clusters as ‘factories’ for binary black holes, forming binariesby three-body collisions and then expelling them [16]. Gamma-ray bursts mayalso come from black-hole/neutron-star coalescences. If the more optimistic eventrates are correct, then black-hole coalescences may be among the first sourcesdetected by ground-based detectors (table 4.2).4.1.4 Pulsars and other spinning neutron starsThere are a number of ways in which a spinning neutron star may give off acontinuous stream of gravitational waves. They will be weak, so they will requirelong continuous observation times, up to many months. Here are some possibleemission mechanisms for neutron stars.The r-modes. Neutron stars are born hot and probably rapidly rotating.Before they cool (during their first year) they have a family of unstable normalmodes, the r-modes. These modes are excited to instability by the emissionof gravitational radiation, as predicted originally by Andersson [17]. They areparticularly interesting theoretically because the radiation is gravitomagnetic,generated by mass currents rather than mass asymmetries. We will study thetheory of this radiation in chapter 6. In chapter 7 we will discuss how the emissionof this radiation excites the instability (the CFS instability mechanism).Being unstable, young neutron stars will presumably radiate away enoughangular momentum to reduce their spin and become stable. This could lower thespin of a neutron star to ∼100 Hz within one year after its formation [18]. Theenergy emitted in this way should be a good fraction of the star’s binding energy,so in principle this radiation could be detected from the Virgo Cluster by LIGOII, provided matched filtering can be used effectively.We discuss a possible stochastic background of gravitational waves from ther-modes below.Accreting neutron stars (figure 4.1) are the central objects of most of the