
Closed-form solutions of spinning, eccentric binary black holes at 1.5 post-Newtonian
order
Rickmoy Samanta ,
1, 2
Sashwat Tanay ,
3, 4, ∗
and Leo C. Stein
4
1
Indian Statistical Institute, 203 B. T. Road, Kolkata 700108, India
2
Birla Institute of Technology and Science Pilani, Hyderabad 500078, India
3
LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 92190 Meudon, France
4
Department of Physics and Astronomy, The University of Mississippi, University, MS 38677, USA
The closed-form solution of the 1.5 post-Newtonian (PN) accurate binary black hole (BBH)
Hamiltonian system has proven to be evasive for a long time since the introduction of the system
in 1966. Solutions of the PN BBH systems with arbitrary parameters (masses, spins, eccentricity)
are required for modeling the gravitational waves (GWs) emitted by them. Accurate models of
GWs are crucial for their detection by LIGO/Virgo and LISA. Only recently, two solution methods
for solving the BBH dynamics were proposed in Ref. [Phys. Rev. D 100, 044046 (2019)] (without
using action-angle variables), and Refs. [Phys. Rev. D 103, 064066 (2021), Phys. Rev. D 107, 103040
(2023)] (action-angle based). This paper combines the ideas laid out in the above articles, fills the
missing gaps and compiles the two solutions which are fully 1.5PN accurate. We also present a
public Mathematica package
BBHpnToolkit
which implements these two solutions and compares
them with the result of numerical integration of the evolution equations. The level of agreement
between these solutions provides a numerical verification for all the five action variables constructed
in Refs. [Phys. Rev. D 103, 064066 (2021), Phys. Rev. D 107, 103040 (2023)]. This paper hence
serves as a stepping stone for pushing the action-angle-based solution to 2PN order via canonical
perturbation theory.
I. INTRODUCTION
Construction of accurate gravitational wave (GW) tem-
plates (or models) has been crucial to the the GW detec-
tions that have taken place so far since 2015 [
1
–
3
]. This
is so because the method of matched filtering for GW
detection requires as one of the inputs, the theoretical tem-
plates of the GW signal to be detected. Post-Newtonian
(PN) theory serves as a useful framework within which
GWs from binary black holes (BBHs) are modeled when
the system is in its initial and longest-lived inspiral stage
[
4
]. At this stage, the two black holes (BHs) of the BBH
are far apart and move slowly around a common center.
This is also referred to as the PN regime. In the PN
framework, quantities of interest are presented in a PN
power series in the small PN paramter (ratio of the typical
speed of the system and that of light). As is typical of
power series, higher-order corrections are of smaller mag-
nitudes and carry higher PN orders. Since GWs from a
BBH are functions of the positions-momenta of the source,
modeling the positions-momenta of the BBH system is
crucial for constructing the GW templates. This paper
deals with the construction of 1.5PN accurate closed-form
solutions of the BBH system.
Since we restrict ourselves to 1.5PN order, the dissipa-
tive effects on the BBH dynamics due to GW emission
don’t enter the picture; they show up at 2.5PN. The
conservative dynamics of the system can be described
with the PN Hamiltonian framework, wherein the system
possesses a Hamiltonian that is a function of the system’s
∗sashwat.tanay@obspm.fr
canonical coordinates [
5
]. The leading PN order effect is
simply that of two point masses moving under mutual
Newtonian gravitational attraction whose Hamiltonian
treatment is a textbook subject matter. Such systems,
move on a closed ellipse if they are bound. The next level
of sophistication is at 1PN order wherein 1PN effects are
added to the above Newtonian system. At this level, spin
effects are ignored (they enter at 1.5PN). Ref. [
6
] provided
the quasi-Keplerian parametric solution for this system;
the trajectory is no longer a closed ellipse and the system
features the advance of periastron. The orbit still remains
confined in a plane due to the constancy of the angular
momentum vector.
Moving further up the PN ladder, we encounter the
1.5PN system whose Hamiltonian was proposed in Ref. [
7
].
At 1.5PN order, spin effects come into play for the first
time via a spin-orbit interaction (linear-in-spin), while the
spin-spin interaction terms enter at 2PN. Via numerical
integration of the resulting equations of motion (EOMs),
it is seen that the orbital plane precesses; the orbital
angular momentum is constant only in magnitude, but
not in direction. This system displays the rich interplay
of non-zero BH spins, periastron precession, along with
spin and orbital-plane precession. We concern ourselves
with this system in this paper.
Over the past decades, solutions to the dynamics of the
spinning BBH system (at 1.5PN order or higher) have
been constructed by many groups [
8
–
16
], but most of
them worked under some simplifying specialization like
only one BH spinning, equal masses, small eccentricity,
orbit-averaging, etc. Two recent breakthroughs have oc-
curred on the front of finding solutions to the most general
1.5PN BBH system without any simplifying assumptions
where the qualifier “most general” indicates a system with
arXiv:2210.01605v4 [gr-qc] 18 Feb 2025