# Gravitational waves from neutron star excitations in binary inspirals

###### Abstract

In the context of binary inspiral of mixed neutron star - black hole systems, we investigate the excitation of the neutron star oscillation modes by the orbital motion. We study generic eccentric orbits and show that tidal interaction can excite the -mode oscillations of the star by computing the amount of energy and angular momentum deposited into the star by the orbital motion tidal forces via closed form analytic expressions. We study the -mode oscillations of cold neutron stars using recent microscopic nuclear equations of state, and we compute their imprint into the emitted gravitational waves.

###### keywords:

Neutron stars, gravitational wave sources, relativistic star oscillations## 1 Introduction

After the historical detections of gravitational waves by binary black holes
Abbott
et al. (2016),
it is expected that mixed binaries composed of a neutron star (NS) and
a black hole (BH) may be the next, qualitatively different type of source to be
detected in the gravitational wave (GW) channel.
At first approximation mixed NS-BH can be treated in
General Relativity (GR) on equal footing as binary BH systems, however the
presence
of matter in the GW source may lead to new detectable astrophysical
effects in the GW signal that are not expected to appear in the binary BH case
like e.g.
NS tidal deformations leaving an imprint in the GW signal
Bildsten &
Cutler (1992); Flanagan &
Hinderer (2008) and
breaking of the NS giving origin to a gamma ray burst or more
general electromagnetic counterpart Lattimer &
Schramm (1976), to name
only the most studied effects.

Beside their direct phenomenological relevance, these effects carry information
on the highly uncertain equation of state of the NS, thus making GW detection an
invaluable probe of the internal structure of NSs.
In this work we focus on a specific effect in GW signals: NS can be tidally
deformed by the orbital motion in generic elliptic orbits, hence setting
oscillations of the NS normal modes. The orbit being elliptical can induce
resonant oscillations at a frequency much higher than the frequency
scale set by the inverse of the orbital period, since in general NS
oscillations are much higher than orbital frequency of inspiral binary systems.

Quantifying this phenomenon in light of the exciting prospect of a future
GW detection has been the subject of extensive investigations in literature in
a number of different contexts.
The theoretical setup for studied such tidally induced NS oscillations has been
provided in Thorne (1969); Press &
Teukolsky (1977). In Fabian
et al. (1975) it
was originally proposed that tidal encounters between a NS and a main-sequence
star might lead to the formation of X-ray binaries in globular clusters.
In Shibata (1994) the effects of the tidal resonances for a
circular orbital motion has been studied,
with the result if the companion of a NS is a
BH of mass , the -mode resonance is unimportant, while the
-mode resonance may affect the orbital evolution just before the merging.
Rathore
et al. (2005) considered the energy absorbed by tidal excitations in
eccentric orbit (but not their imprint in the GW-form).
Reisenegger &
Goldreich (1994) compute the effect on the emitted GW phase of resonant mode
excitation by the circular inspiral motion.
Rotating NS we considered by Ho & Lai (1999) (including g-modes and r-modes)
when the spin axis is aligned or anti-aligned with the orbital angular momentum
axis. Carter &
Luminet (1983) solved for the tidal deformation dynamics
of a NS in an external field of a massive object
and recently Chirenti
et al. (2017) presented a framework for the discussion of binary
NS and mixed NS-BH ones oscillation mode excitation and detection via the
GWs observed by future GW detector as Einstein Telescope or Cosmic Explorer.
Numerical results on the GW emission of tidally excited NS oscillations
in the last stages of a coalescence have been given in Gold et al. (2012),
and in Steinhoff et al. (2016)
the imprint of resonant tidal on the gravitational
waveform has been computed within the effective one body description of the
two body orbital motion.

In the present paper we consider non-rotating NS with four different equations
of states Akmal
et al. (1998); Douchin &
Haensel (2001); Walecka (1974); Bethe &
Johnson (1974) with
the goal of translating resonant excitations of various
-modes for NSs inspiraling binary NS-BH systems that move in an
elliptical orbit into quantitative prediction for the emitted GW-form.

Numerical simulations show that most of the energy released in gravitational
waves is indeed transferred into -modes, which are characterized by a
wave-function free of nodes along the radial direction.
We do not study the possibilities of exciting the g-modes because these modes
are related to the presence of density discontinuities in the outer envelopes
of NSs, see Finn (1987) and Strohmayer (1993),
density discontinuities in the inner core as a consequence of phase
transitions at high density, as studied in Sotani
et al. (2001),
and/or thermal gradients as for a proto-NS, see e.g. Ferrari
et al. (2003).
In this paper we do not consider the possibility of having discontinuities of
the density, moreover we focus on barotropic equations of state
where the pressure depends only on the energy density, implying
that all g-modes degenerate to zero frequency, hence we focus on the
excitations of -modes.
Our study is based on the following simplifying assumptions:

(i) we neglect BH rotation, thus we treat the BH as a point
particle with mass ;
(ii) the hydrodynamic stability of NS is computed using the Oppenheimer-Volkoff equations,
but we use Newtonian equations to calculate the oscillation modes, see
Appendix A;
(iii) the NS does not rotate and we neglect viscous effects.

By implementing the formalism presented in Thorne (1969); Press &
Teukolsky (1977) we find
generic analytic expressions for the energy and angular momentum
deposited into NS oscillations during the elliptic orbital motion, allowing to
compute the mass quadrupole which is sourcing GW emission, and eventually
comparing it with the orbital quadrupole.

The outline of this paper is as follows: in Sec. 2 we
present the setup of the physical system under consideration,
and we provide new analytic expressions for the dynamics of tidally induced
NS oscillations, which are the main result of this paper.
In Sec. 3 we analyze quantitatively their GW emission.
Finally, conclusions for future detectability of NS oscillations in the GW
channel are drawn in Sec. 4.
We set the speed of light throughout this paper.

## 2 Coupling of neutron star oscillation modes to orbital motion

In this section we study the tidal excitation of NS oscillation
modes in non-rotating stars in an elliptical orbit. Our analysis will be general,
but the astrophysical case we have in mind is that of a binary NS-BH system.
The idea to compute the energy deposited in stellar oscillations by the tidal
gravitational field is first described by Turner (1977)
and Press &
Teukolsky (1977).

In this paper we use Newtonian linearized equations to calculate the
oscillation modes. The use of Newtonian equations is consistent with our
Newtonian description of tidal interactions. For the -mode, general
relativistic effects are expected to modify our results of oscillation
frequencies by not more than per cent, see Lai (1994),
where and are the mass and radius of the NS. We also neglect the spin
of the NS.
When , the normal modes of the star get more complicated,
especially when becomes comparable to the mode frequencies
Gaertig &
Kokkotas (2008).
For the eigenmodes can be adequately approximated by those
of a non-rotating spherical star, the basic equations that governing the
oscillations of stars are discussed in more detail in Appendix A.

The NS oscillations are excited by tidal forces while the NS is bound in a
binary system with black hole in an eccentric orbit
whose evolution is driven by gravitational radiation.
The distance between two objects in an elliptic orbit can be
parametrized by, see e.g. eq. (4.54) of
Maggiore (2008),

(1) |

being the semi-major axis and the eccentricity (with corresponding to the periastron), and the true anomaly is related to the eccentric anomaly and time via, see e.g. eqs. (4.57,58) of Maggiore (2008),

(2) |

being the orbital period, with the following relationships holding among orbital parameters

(3) |

(where is the total mass of the binary system and the Newton constant) and the standard definition of the relativistic orbital parameter

(4) |

the last equality holding only at Newtonian level.

In order to study quantitatively the effect of the
gravitational force inducing oscillations into the NS
and following the procedure outlined in Press &
Teukolsky (1977), it is useful
to expand the Newtonian potential in spherical harmonics, see e.g. eq. (3.70) of
Jackson (1998), centered at the star as per

(5) |

being coordinates of the mass elements of the NS, are the spherical harmonic indices and the orbital motion is assumed to be planar (no spin-induced precession). Using eq. (5) for elliptic orbit, it will be useful to expand for generic into a Fourier series of the type

(6) |

The detailed calculation of the Fourier coefficients
and their analytic expressions are presented
in Appendix B.

In order to perform an analytic quantitative analysis we borrow here the
framework of Rathore
et al. (2005), where NS oscillations are modeled as a
series of damped harmonic oscillator displacements driven by external
force, that we can take purely monocromatic:

(7) |

where is the stellar mode frequency, its damping time^{1}^{1}1As a possible mechanism for the damping of non-radial NS oscillations we take the gravitational emission, we do not consider neutrino losses, radiative heat leakage, and magnetic damping.,
is the -th harmonic of the main orbital angular frequency , and
the exciting force amplitude.^{2}^{2}2Note that the time scale of
variation is set by the GW radiation and via the Einstein quadrupole formula
(as , with
), hence we neglect the time
variation of the frequency of “forcing” term in eq. (7).
Eq. (7) admits the exact analytic solution

(8) |

the solution to the homogeneous equation being

(9) |

leading to an average absorbed energy per unit of mass per unit of time

(10) |

The NS oscillation vectors satisfy an equation of the type see Kosovichev & Novikov (1992)

(11) |

where is an operator characterizing the internal restoring force of the star. In order to apply this toy model of a damped harmonic oscillator to the tidally excited NS oscillation, we decompose the oscillation field into normal modes with factorized time and space dependence:

(12) |

where we have added the spherical harmonics labels and the spatial mode eigenfunctions satisfy

(13) |

allowing the identification of with the stellar frequency of
the eigenmode.
The differential equations the oscillation modes fields satisfy are summarized in Appendix A,
which are solved for 4 different equations of
state and 4 values of the central density of the NS, with the resulting
mass, radius, frequency and damping times (the last two depending on )
are reported in Appendix C for .

It is also useful to expand the eigenmodes into a radial () and a poloidal
() component

(14) |

and impose the normalization condition^{3}^{3}3Note that with the
normalization chosen have dimension of length, is
dimension-less. However the normalization can be arbitrarily chosen without
affecting physical results, our choice has the advantage of making following
formulae simpler.

(15) |

where are respectively the density, central density and radius of the NS, and we used

(16) |

and the integral of products of spherical harmonics with unequal number of
derivatives vanish for any , .

By multiplying both members of eq.(11) by ,
substituting the expansion in eq. (5),
and integrating over the NS volume the mode is singled out and it
satisfies an equation of the type (7):

(17) |

where

(18) |

is the black hole mass. Note that the r.h.s of eq.(17) is complex, but given the symmetries of the coefficients: (and if have different parity), , the sum of returns a real quantity. The modes thus satisfy an equation of the type (7) with the coefficients replaced by

(19) |

These expressions will be needed in sec. 3 to compute the
time varying quadrupole associated to these oscillations, source of
GWs.

The rate of energy (per unit of mass, per unit NS radius) absorbed by each
oscillation modes can be read from eq. (10) by inserting the above
values of , summing over and , the
rate of absorbed energy via tidal mechanism being

(20) |

The contribution from individual modes to the rate of energy absorption is plotted in fig. 1 after being divided by the factor

(21) |

where for , . Factorizing the absorbed energy rate by the quantity has the virtue of making dimension-less and independent on the relativistic parameter (as long as the orbital frequency does not hit a resonance with ) and mildly dependent on , .

In fig. 2 we report the absorbed energy rate normalized by

(22) |

(being the reduced mass of the orbital system), which is the expression of the leading order in of the GW emission rate at zero eccentricity from a binary inspiral, making visually easier the comparison between GW radiated energy and . For we use the 3PN formula taken from Arun et al. (2008), see also sec. 10.3 of Blanchet (2014).

The absorbed angular momentum can be computed in a similar way, following Lai (1994), where it is noted that the variation of angular momentum

(23) |

we can derive in our setup

(24) |

where in the last passage we have inserted the expansion of eqs. (5,6) and derived by parts inside the integral. In this form the angular momentum absorption rate by NS oscillations can be rewritten as:

(25) |

In fig. 3 the absorbed angular momentum rate normalized by the leading order expression in of is reported for various values of the relativistic parameter and the eccentricity . The values of are negligible with respect to and given the typical moment of inertia of a NS ( gr cm, see book of Haensel et al. (2007)), the induced rotation on the NS is also negligibly small.

## 3 Gravitational Wave emission

We have seen in the previous section that the energy absorbed in by the NS
is very small compared to the orbital energy at moderate eccentricity values
(), hence such absorption will not
alter in any significant way the chirping signal.
However the energy absorbed will set oscillations in the neutron star that
gives rise to a time varying quadrupole, which will in turn generate GWs with
a significantly different pattern that the GWs associated to the decaying
orbital motion.

The general expression for the GW in the TT gauge is given by,
see e.g. eq. (3.275) of Maggiore (2008),

(26) |

where () is linearly related to the -th time derivative of the mass (momentum) multipole moments. The leading-order contribution to radiation reaction comes from the mass quadrupole term, for which it is (see e.g. sec. 3 of Maggiore (2008))

(27) |

being the tensor spherical harmonics and is the standard quadrupole mass moment in Cartesian coordinates. It will be convenient to express the leading order GW amplitude in terms of the spherical components of the quadrupole, related to their Cartesian counterpart via

(28) |

leading to (explicit expressions of tensor spherical harmonics are reported in app. D)

(29) |

We now have all the ingredients to relate the leading GW source to the NS tidal oscillations via

(30) |

that in terms of the displacement vector introduced in eqs. (11,12) can be expressed as, see Ushomirsky et al. (2000), by

(31) |

where an integration by parts has been performed in the last step, the explicit expression of has been inserted and only the contribution has been considered. since we analyzed only the -mode. Observing that the boundary term is numerically smaller than the integral term, substituting the solution of eq. (17) and considering only the resonant contribution for the NS average quadrupole value can be written as

(32) |

The quantity directly related to GW emission, , follows straightforwardly via eq. (29). In fig. 4 we report the contribution to the second time derivative of the quadrupole (divided by the reduced mass of the binary system) and as a comparison the (magnified) second derivative of the quadrupole associated to , NS oscillations during an ordinary binary inspiral in which the orbit shrinks due to GW back reactions.

For comparison, we also report in fig. 5 the time evolution of the displacement along the inspiral phase.

## 4 Conclusions

In this paper we have developed and presented a framework able to perform analytic and quantitative study of the excitations of a neutron star in an inspiralling binary system of arbitrary eccentricity. We have computed the energy and the angular momentum deposited into stellar mode oscillations by the tidal field via closed form analytic formulae. The amount of energy absorbed by the neutron star in a given mode depends on the overlap of the tidal force field with the displacement field of the mode, hence it requires solving the equilibrium equations of a neutron star, done here in the Newtonian approximation. We focused our analysis on the fundamental -mode of a non-relativistic star, finding the rate of energy absorbed and angular momentum as a function of eccentricity and of the period of the inspiral orbital, when -mode can be in resonance with higher harmonics of the main orbital frequency.

As a future development of this work, we intend to extend our analysis to the General Relativistic equilibrium equations of a rotating neutron star, with the inclusion of -mode and -modes, and considering a not barotropic equation of state: such modes have lower frequency values than the -mode, and can therefore be excited at resonance in an elliptical orbit earlier in the inspiral phase. The phenomenological impact of the computations presented here relies on the signature that neutron star oscillations will imprint onto the gravitational signals of an inspiral binary system. Despite being sub-dominant with respect to the gravitational wave sourced by the orbital motion, the detailed features of the star oscillation bears invaluable information on its equation of state and density, allowing to make a bridge to the nuclear physics ruling its equilibrium. Since it is expected in the near future that third generation gravitational wave detector could observe signals from binary systems involving neutron star at signal-to-noise ratio of order or more, see e.g. Punturo et al. (2010), and that such detection will involve the observation of hundred of thousand gravitational wave cycles during the inspiral of a binary system for a time stretch of order of several days, the quantitative prediction of the modification of the inspiral signal, even at very low level, will have an impact on the physics outcome of the detection.

## Acknowledgments

The authors wish to thank C. Chirenti for useful discussions. The work of AP has been supported by the FAPESP grant 2016/00096-6, RS has been supported by FAPESP grant 2012/14132-3.

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## Appendix A Four first-order linear differential equations of non-radial oscillations

The normal modes of a spherical star can be labeled by spherical harmonic indices and , and by a “radial quantum number" . In spherical coordinates the Lagrangian displacement of a fluid element is given by

(33) |

where denotes a spherical harmonic; and denotes the pulsation angular frequency. The oscillation is assumed to be adiabatic, we ignore the thermal evolution of the NS, for simplicity we use the Newtonian description in the Dziembowski (1971) formulation, in this case the equations reduce to a system of four first-order differential equations with four dimensionless variables, given by:

(34) |

(35) |

Here, the meanings of the symbols are as follows: and are the radial part of the Eulerian perturbation to the pressure and the gravitational potential , respectively; is the distance from the center of the star, is the density, and is the local acceleration due to gravity. The system of differential equations that governs the linear adiabatic oscillations of stars is then given by:

(36) | |||||

(37) | |||||

(38) | |||||

(39) |

Where

(40) |

(41) |

Here is the first adiabatic exponent, is the sound speed, is the concentric mass, and are the total mass and radius of the star, respectively, and is the gravitational constant. There are four boundary conditions, the inner boundary conditions at are:

the outer boundary conditions at are:

The two central boundary conditions require that the two divergences
involved, , , remain
finite.
At the surface we require to be finite and , the
gravitational force per unit mass, to be continuous across the perturbed
surfaces.
The above equations and boundary conditions constitute an eigenvalue problem
for the eigenvalue .

The expression for the damping time due to emission of gravitational waves in the Newtonian case see Thorne (1969); Balbinski &
Schutz (1982) ) is given by:

(42) |

where .

## Appendix B Expansion of the Fourier coefficients

Expanding in eq. (6) we have

(43) |

for , where we used that is an odd function of
time,
hence ()is an even (odd) function of time, for
, and .

In order to expand into sums of terms of the type
it is useful to express it in terms of powers of
via M.Abramowitz &
Stegun (1964)

(44) |

where is the Chebyshev polynomial of order and it has the form

(45) |

begin the integer part of . Using the standard relationships between eccentric anomaly , true anomaly and time , see sec. 2, one finds

(46) |

to obtain

(47) |

In order to perform this integral we use the standard Taylor-expansions

(48) |

to write

(49) |

and then we use the DeMoivre formula