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Was GW190412 born from a hierarchical 3+1 quadruple configuration?

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Hamers,  Adrian S.
High Energy Astrophysics, MPI for Astrophysics, Max Planck Society;

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Hamers, A. S., & Safarzadeh, M. (2020). Was GW190412 born from a hierarchical 3+1 quadruple configuration? The Astrophysical Journal, 898(2): 99. doi:10.3847/1538-4357/ab9b27.


Cite as: http://hdl.handle.net/21.11116/0000-0007-1F65-6
Abstract
The gravitational wave source GW190412 is a binary black hole (BBH) merger with three unique properties: (i) its mass ratio is about 0.28, the lowest found so far, (ii) it has a relatively high positive effective spin parameter ceff = 0.25, and (iii) it is observed to be precessing due to in-plane projected spin of the binary with an in-plane precession parameter cp = 0.3. The two main formation channels of BBH formation fail to account for GW190412: field formation scenarios cannot explain the observed precession unless by invoking large natal kicks, and dynamical assembly in dense stellar systems is inefficient in producing such low-mass-ratio BBH mergers. Here, we investigate whether “double mergers” in wide hierarchical quadruple systems in the “3+1” configuration could explain the unique properties of GW190412. In this scenario, a compact object quadruple system experiences two mergers: first, two compact objects in the innermost orbit merge due to secular chaotic evolution. At a later time, the merged compact object coalesces with another compact object due to secular Lidov–Kozai oscillations. We find that our scenario is consistent with GW190412. In particular, we find a preferential projected spin around cp = 0.2. However, the likelihood of a double merger is small and the formation efficiency of these systems is uncertain. If GW190412 originated from a double merger in a 3+1 quadruple, we find a strong constraint that the first merger likely occurred between roughly equal-mass BHs in the innermost orbit, since the recoil velocity from unequal-mass BHs would otherwise have disrupted the system.