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Black hole mergers induced by tidal encounters with a galactic centre black hole

Fernandez, JJ and Kobayashi, S (2019) Black hole mergers induced by tidal encounters with a galactic centre black hole. Monthly Notices of the Royal Astronomical Society, 487 (1). pp. 1200-1209. ISSN 0035-8711

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We discuss the properties of stellar mass black hole (BH) mergers induced by tidal encounters with a massive BH at galactic centres or potentially in dense star clusters. The tidal disruption of stellar binaries by a massive BH is known to produce hypervelocity stars. However, such a tidal encounter does not always lead to the break-up of binaries. Since surviving binaries tend to become hard and eccentric, this process can produce BH mergers in principle. For initially circular binaries, we show that the gravitational wave (GW) merger times become shorter by a factor of more than 102 (105) in 10 per cent (1 per cent) of the surviving cases. This reduction is primarily due to the growth in binary’s eccentricity at the tidal encounter.We also investigate the effective spins of the survivors, assuming that BH spins are initially aligned with the binary orbital angular momentum. We find that binary orientations can flip in the opposite direction at the tidal encounter. For the survivors with large merger time reduction factors, the effective spin distribution is rather flat. We estimate the merger rate due to the tidal encounter channel to be ∼0.6Gpc−3 yr−1. This mechanism is unlikely to be the dominant formation channel of BH mergers. However, the current and near-future GW observatories are expected to detect an enormous number of BH mergers. If mergers are found in the vicinity of massive BHs (e.g. the detection of GW lensing echoes or preceding extreme-mass-ratio bursts), this mechanism would provide a possible explanation for their origin.

Item Type: Article
Additional Information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2019 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Uncontrolled Keywords: 0201 Astronomical and Space Sciences
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: Oxford University Press
Related URLs:
Date Deposited: 30 Jul 2019 08:25
Last Modified: 04 Sep 2021 09:04
DOI or ID number: 10.1093/mnras/stz1353
URI: https://researchonline.ljmu.ac.uk/id/eprint/11134
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