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

Sources detectable from ground and from space 47Figure 4.1. Accreting neutron star in a low-mass x-ray binary system.binary x-ray sources in the Galaxy. Astronomers divide them into two distinctgroups: the low-mass and high-mass binaries, according to the mass of thecompanion star. In these systems mass is pulled from the low- or high-mass giantby the tidal forces exerted by its neutron star companion. In low-mass x-raybinaries (LMXBs) the accretion lasts long enough to spin the neutron star up tothe rotation rates of millisecond pulsars. Astronomers have therefore supposedfor some time that the neutron stars in LMXBs would have a range of spins, fromnear zero (young systems) to near 500 or 600 Hz (at the end of the accretionphase). Until the launch of the Rossi X-ray Timing Explorer (RXTE), there wasno observational evidence for the neutron star spins. However, in the last twoyears there has been an accumulation of evidence that most, if not all, of thesestars have angular velocities in a narrow range around 300 Hz [19]. It is not knownyet what mechanism regulates this spin, but a strong candidate is the emission ofgravitational radiation.A novel proposal by Bildsten [20] suggests that the temperature gradientacross a neutron star that is accreting preferentially at its magnetic poles shouldlead to a composition and hence a density gradient in the deep crust. Spinning at300 Hz, such a star could radiate as much as it accretes. It would then be a steadysource for as long as accretion lasts, which could be millions of years.In this model the gravitational-wave energy flux is proportional to theobserved x-ray energy flux. The strongest source in this model is Sco X-1,which could be detected by GEO600 in a two-year-long narrow-band mode ifthe appropriate matched filtering can be done. LIGO II would have no difficultyin detecting this source.Older stars may also be lumpy. For known pulsars, this is constrained by therate of spindown: the energy radiated in gravitational waves cannot exceed the