ar X iv : a st ro - p h / 02 02 38 5 v 1 2 1 Fe b 20 02 Nova Sco and coalescing low mass black hole binaries as LIGO sources

Double neutron star (NS-NS ) binaries, analogous to the well known Hulse–Taylor pulsar PSR 1913+16 (Hulse & Taylor 1975), are guaranteed-to-exist sources of high frequency gravitational radiation detectable by LIGO. There is considerable uncertainty in the estimated rate of coalescence of such systems (Phinney 1991; Narayan et al 1991; Kalogera et al 2001), with conservative estimates of ∼ 1 per million years per galaxy, and optimistic theoretical estimates one or more magnitude larger. Formation rates of low-mass black hole-neutron star binaries may be higher than those of NS-NS binaries, and may dominate the detectable LIGO signal rate. Rate estimates for such binaries are plagued by severe model uncertainties. Recent estimates suggest that BH-BH binaries do not coalesce at significant rates despite being formed at high rates (Portegies Zwart & Yungelson 1998; De Donder & Vanbeveren 1998). We estimate the enhanced coalescence rate for BH-BH binaries due to weak asymmetric kicks during the formation of low mass black holes like Nova Sco (Brandt, Podsiadlowski & Sigurdsson 1995), and find they may contribute significantly to the LIGO signal rate, possibly dominating the phase I detectable signals if the range of BH masses for which there is significant kick is broad enough. For a standard Salpeter IMF, assuming mild natal kicks, we project that the R6 merger rate (the rate of mergers per million years in a Milky Way-like galaxy) of BH-BH systems is ∼ 0.5, smaller than that of NS-NS systems. However, the higher chirp mass of these systems produces a signal nearly four times greater, on average, with a commensurate increase in search volume. Hence, our claim that BH-BH mergers (and, to a lesser extent, BH-NS coalescence) should comprise a significant fraction of the signal seen by LIGO. The BH-BH coalescence channel considered here also predicts that a substantial fraction of BH-BH systems should have at least one component with near-maximal spin (a/M ∼ 1). This is from the spin-up provided by the fallback material after a supernova. If no mass transfer occurs between the two supernovae, both components could be spinning rapidly. The waveforms produced by the coalescence of such a system should produce a clear spin signature, so this hypothesis could be directly tested by LIGO. Subject headings: black hole physics — binaries: close — stars — gravitational radiation. [email protected] [email protected]