Tidal deformations of a spinning compact object

TL;DR

This paper extends perturbative techniques to compute the tidal Love numbers of spinning compact objects, revealing how spin influences their deformations and providing analytical solutions for slowly rotating, tidally deformed Kerr black holes.

Contribution

It introduces a method to calculate the exterior geometry and Love numbers of spinning objects under tidal fields to second order in spin, including new couplings and moments.

Findings

Love numbers of Kerr black holes remain zero to second order in spin.

Analytical solutions for slowly rotating, tidally-deformed Kerr black holes are provided.

Spinning objects acquire various multipole moments due to tidal interactions.

Abstract

The deformability of a compact object induced by a perturbing tidal field is encoded in the tidal Love numbers, which depend sensibly on the object's internal structure. These numbers are known only for static, spherically-symmetric objects. As a first step to compute the tidal Love numbers of a spinning compact star, here we extend powerful perturbative techniques to compute the exterior geometry of a spinning object distorted by an axisymmetric tidal field to second order in the angular momentum. The spin of the object introduces couplings between electric and magnetic deformations and new classes of induced Love numbers emerge. For example, a spinning object immersed in a quadrupolar, electric tidal field can acquire some induced mass, spin, quadrupole, octupole and hexadecapole moments to second order in the spin. The deformations are encoded in a set of inhomogeneous differential equations which, remarkably, can be solved analytically in vacuum. We discuss certain subtleties in defining the multipole moments of the central object, which are due to the difficulty in separating the tidal field from the linear response of the object in the solution. By extending the standard procedure to identify the linear response in the static case, we prove analytically that the Love numbers of a Kerr black hole remain zero to second order in the spin. As a by-product, we provide the explicit form for a slowly-rotating, tidally-deformed Kerr black hole to quadratic order in the spin, and discuss its geodesic and geometrical properties.