Gravitational waves from hyper-accretion on to nascent black holes

Abstract

We examine the possibility that hyper-accretion on to newly born black holes occurs in highly intermittent, non-asymmetric fashion favourable to gravitational-wave emission in a neutrino-cooled disc. This picture of near-hole accretion is motivated by magnetorotationally induced, ultrarelativistic disc dynamics in the region of the flow bounded from below by the marginally bound geodesic radius rmb. For high spin values, a largely coherent magnetic field in this region has the dynamical implication of compact mass segregation at the displacement nodes of the non-axisymmetric, magnetorotational instability modes. When neutrino stress competes favourably for the disc dynamical structure, the matter clumps may be rather dense and sufficiently long-lived to excite the quasi-normal ringing (QNR) modes of the Kerr geometry upon infall. We find that this accretion flow may drive bar-like, quadrupole (l, m = 2, 2) modes in nearly resonant fashion for spin parameters a ≥ 0.9. The ensuing build-up in strain amplitude of the undamped oscillations warrants a brisk rate of energy deposition into gravitational waves. A detectability assessment for the LIGO interferometers through the match filtering technique is given by integrating the energy flux over a one-second epoch of resonant hyper-accretion at 1 M_⊙ s^(−1). Thus, a 15-M_⊙ Kerr black hole spinning at a ≃ 0.98 (f_(QNR) ≃ 1677 Hz), and located at 27 Mpc (e.g. GRB 980425), will deliver a characteristic strain amplitude, h_(char) ≃ 2.2−21, large enough to be detectable by LIGO II. If resonant hyper-accretion were sustainable for a longer period (or at higher rates) possibly associated with a second broad hump in a gamma-ray burst light curve, these objects could be detected by LIGO I at very low redshifts.