Nova Scorpii and Coalescing Low‐Mass Black Hole Binaries as LIGO Sources

Abstract

Double neutron star (NS-NS) binaries, analogous to thewell-known Hulse-Taylor pulsar PSR 1913+16 (documented by Hulse &Taylor in 1974), are guaranteed-to-exist sources of high-frequencygravitational radiation detectable by LIGO. There is considerableuncertainty in the estimated rate of coalescence of such systems(see the work of Phinney in 1991, Narayan and coworkers in 1991,and Kalogera and coworkers in 2001), with conservative estimatesof ~1 per 10 6 yr per galaxy, and optimistic theoreticalestimates 1 or more mag larger. Formation rates of low-mass black hole(BH)-neutron star binaries may be higher than those of NS-NS binariesand may dominate the detectable LIGO signal rate. Rate estimatesfor such binaries are plagued by severe model uncertainties. Recentestimates by Portegies Zwart & Yungelson in 1998 and De Donder &Vanbeveren in 1998 suggest that BH-BH binaries do not coalesce atsignificant rates despite being formed at high rates. We estimate theenhanced coalescence rate for BH-BH binaries due to weak asymmetrickicks during the formation of low-mass black holes like Nova Sco (seethe work of Brandt, Podsiadlowski, & Sigurdsson in 1995) and findthey may contribute significantly to the LIGO signal rate, possiblydominating the phase I detectable signals if the range of black holemasses for which there is significant kick is broad enough. For astandard Salpeter initial mass function, assuming mild natal kicks, weproject that the R 6 merger rate (the rate of mergers per10 6 yr 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 ofthese systems produces a signal nearly 4 times greater, on average,with a commensurate increase in search volume, hence, our claim thatBH-BH mergers (and, to a lesser extent, BH-NS coalescence) shouldcomprise a significant fraction of the signal seen by LIGO. The BH-BHcoalescence channel considered here also predicts that a substantialfraction of BH-BH systems should have at least one component withnear-maximal spin (a/M~1). This is from the spin-up provided by the fallbackmaterial after a supernova. If no mass transfer occurs between the twosupernovae, both components could be spinning rapidly. The waveformsproduced by the coalescence of such a system should produce a clear spinsignature, so this hypothesis could be directly tested by LIGO.