Impact of updated Multipole Love numbers and f-Love Universal Relations in the context of Binary Neutron Stars Bikram Keshari Pradhan1Aditya Vijaykumar2 3and Debarati Chatterjee1

2025-05-08 0 0 6.57MB 21 页 10玖币
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Impact of updated Multipole Love numbers and f-Love Universal Relations in the
context of Binary Neutron Stars
Bikram Keshari Pradhan,1, Aditya Vijaykumar,2, 3 and Debarati Chatterjee1
1Inter-University Centre for Astronomy and Astrophysics,Pune University Campus,Pune,411007, India
2International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore 560089, India
3Department of Physics, The University of Chicago,
5640 South Ellis Avenue, Chicago, Illinois 60637, USA
(Dated: January 18, 2023)
Neutron star (NS) equation of state (EoS) insensitive relations or universal relations (UR) involv-
ing neutron star bulk properties play a crucial role in gravitational-wave astronomy. Considering a
wide range of equations of state originating from (i) phenomenological relativistic mean field models,
(ii) realistic EoS models based on different physical motivations, and (iii) polytropic EoSs described
by spectral decomposition method, we update the EoS-insensitive relations involving NS tidal de-
formability (Multipole Love relation) and the UR between f-mode frequency and tidal deformability
(f-Love relation). We analyze the binary neutron star (BNS) event GW170817 using the frequency
domain TaylorF2 waveform model with updated universal relations and find that the additional
contribution of the octupolar electric tidal parameter and quadrupolar magnetic tidal parameter
or the change of multipole Love relation has no significant impact on the inferred NS properties.
However, adding the f-mode dynamical phase lowers the 90% upper bound on ˜
Λ by 16-20% as well
as lowers the upper bound of NSs radii by 500m. The combined URs (multipole Love and f-Love)
developed in this work predict a higher median (also a higher 90% upper bound) for ˜
Λ by 6% and
also predict higher radii for the binary components of GW170817 by 200-300m compared to the
URs used previously in the literature. We further perform injection and recovery studies on simu-
lated events with different EoSs in A+ detector configuration as well as with third generation (3G)
Einstein telescope. In agreement with the literature, we find that neglecting f-mode dynamical tides
can significantly bias the inferred NS properties, especially for low mass NSs. However, we also find
that the impact of the URs is within statistical errors.
I. INTRODUCTION
The density in the core of a neutron star (NS) surpasses
the nuclear saturation density (n02×1014 g cm3)
and even the highest density that can be achieved in
terrestrial experiments. It is argued that strangeness
in the form of hyperons, meson condensates, or even
deconfined quark matter may appear at such high
densities, affecting several NS observable properties [1].
The behavior of ultra-dense NS matter is still largely
unknown, and therefore, NSs provide a natural labora-
tory to study nuclear matter under extreme conditions
such as high magnetic field and rotation [2].
The NS macroscopic observables relate to the mi-
croscopic NS physics through the pressure density
relationship or the equation of state (EoS). The two
main approaches used to model nuclear EoS are (i)
microscopic or ab-initio [3–7] and (ii) phenomeno-
logical (effective theories with parameters fitted to
reproduce saturation nuclear properties) [8, 9]. Yet
another approach is to construct empirical fits rather
than microphysics-based EoS. Where the high-density
behavior of EoS is described by polytropic models
(pressure is proportional to a power of density) either
using piece-wise polytropes [10] or using the spectral
bikramp@iucaa.in
decomposition method [11]. One can also infer the NS
EoS from observations using a nonparametric description
of NS EoS [12] or can even use a hybrid approach to
describe the NS EoS [13].
On the observational side, NSs are observed at mul-
tiple wavelengths in current generation electromagnetic
telescopes. The masses of these objects are typically
well measured in binary systems, whereas the measure-
ment of radius involves larger uncertainty. However,
the recently launched NICER (Neutron Star Interior
Composition Explorer) mission [14] has improved radius
measurement and can further improve the estimation
of the NS radius upto 5% [15, 16]. Additionally, binary
neutron star (BNS) mergers are sources of gravitational
waves (GW). In a BNS merger, tidal deformation of
the NSs carries information about NS radii and is
used to constrain the NS EoS in combination with
mass measurements from the same binary [17–21].
The ground-breaking BNS merger GW170817 [21–23]
also observed electromagnetic (EM) counterparts in
addition to GWs, and hence opened a new window in
multi-messenger astronomy.
The tidal deformation of the stars in a BNS contributes
to phase of the GW signal starting at the fifth Post
Newtonian (PN) order. In general, tidal deformability
(for electric type) of order ‘l’ (Λl) appears at 2l+ 1 Post
Newtonian (PN) order. The major contribution comes
arXiv:2210.09425v2 [astro-ph.HE] 17 Jan 2023
2
from the quadrupolar tides, followed by higher-order
octupolar (Λ3) and hexadecapolar (Λ4) terms [18, 24].
Recent waveform models have been updated to consider
the effect of the higher order tidal parameters Λ3, Λ4
and also the magnetic type deformation (quadrupolar
magnetic Σ2in the GW phase [24, 25]. In most
analyses, the tides are considered adiabatic, ie. the GW
frequency is much lower than the NS resonant oscillation
mode frequency. However, recent efforts largely support
the excitation of NS fundametal modes (f-modes) in the
late inspiral [26–31]. In a frequency domain model, the
dynamical tidal contribution to the GW phase due to the
excitation of f-modes appears at 8 PN order [18, 32] and
depends upon the f-mode frequency. Since the additional
tidal deformability parameters and the f-mode dynam-
ical phase contribute at high PN orders, they are often
dropped while constructing waveform approximants.
However, the f-mode dynamical tidal correction has been
shown to significantly affect the inference of NS proper-
ties from a binary due to its resonance behaviour [33, 34].
Previous works have suggested that there exist EoS-
insensitive universal relations (UR) between Λ2and each
of Λ3, Λ4, and Σ3( hereafter referred as ‘multipole Love’
relation) [35–37]. Similarly, URs exist between the
f-mode frequency and tidal parameters [38] ( hereafter
referred as ‘f-Love’ relation). This allows the additional
contribution due to higher order tidal parameters or
f-mode parameters to be considered without having to
actually sample over them in a parameter estimation
run, reducing the search parameter space. Additionally,
there exists UR involving stellar compactness and
quadrupole tidal parameters, which can be used to infer
NS radius from the measured mass and quadrupole tidal
parameters from a BNS event [39]. In this work, we will
only be interested in the URs mentioned above. There
also exist URs involving the combination of Λ2and mass
ratio qof the binaries, known as ‘binary Love relations,’
as well as URs involving NS spin which reduces effort in
measuring the NS tidal parameter (and NS radius) in a
BNS event [40–46] 1.
The existing URs are employed by considering a few
theoretical EoS models and some of the selective EoSs
are now incompatible with the current astrophysical
constraints [35, 37–39]. Also, the existing URs do not
consider the EoS uncertainties resulting from the nuclear
parameters. In this work, we improve the multipole
Love and f-Love URs relevant for GW astronomy and
investigate the impact of the updated URs by analyzing
the BNS event GW170817 and performing several
injection and recovery studies with the future GW
detector configurations. This work is organized in the
1Recently a new EoS insensitive approach involving the NS mass
and tidal deformability is proposed to constrain NS properties
from a GW event [47].
following way. In section II we discuss the choices of
Eos, followed by the description of the methodology to
solve for multipole tidal parameters and to solve for the
NS f-mode characteristics. We compile our results in
Section III and summarise our conclusions in Section IV .
II. METHOD
A. Choices of Equations of State
The EoS is essentially the relation between the pressure
(p) and density (ρ) (or energy density ), i.e., p=p().
As described in Section I, different physical motivations
develop a diverse family of EoSs depending upon the
physical descriptions of the NS matter. In this work,
we consider a wide range of EoSs based upon different
physical descriptions. They are discussed below:
a. Relativistic Mean Field (RMF) Models: RMF
models are phenomenological models where baryon-
baryon interaction is mediated via exchange of mesons.
The Lagrangian density describes the interaction be-
tween baryons through the exchange of mesons. The
complete description of the Lagrangian density for
nucleonic (npeµ) and nucleon-hyperon (npeµY ) mat-
tered NS, and hence the EoS within the RMF model
considered in this work can be found in [48]. The
parameters of the RMF model are calibrated to the
nuclear and hypernuclear parameters at saturation:
nuclear saturation density (n0), the binding energy
per nucleon (E/A or Esat), incompressibility (K), the
effective nucleon mass (m), symmetry energy (J) and
slope of symmetry energy (L) at saturation. Hyperon
coupling constants are fixed using hyperon nucleon
potential depths (UY) or using symmetry properties. We
consider the total uncertainty ranges in the saturation
parameters resulting from nuclear and hypernuclear
experiments, summarized in Table I [49].
b. Selective EoSs: Along with the RMF mod-
els with nucleonic and hyperonic mattered EoSs, we
consider many realistic EoSs. The realistic EoSs are
taken either from CompOSE database [51–53] or
from LALSimulation [54]. The considered realistic
EoSs are APR4, APR3 [6, 55], SLy4 [56], BL [57],
DD2(GPPVA) [58], SRO-APR [59], BSk22 [60, 61],
WFF1 [62], MPA1 [63]. We also consider the Soft and
Stiff EoS from [5]. In Figure 1a, though the Stiff-EoS
terminates at earlier energy density, it is sufficient to
reach the maximum stable NS mass the EoS model
can reproduce [5]. To capture the hypothesis of a
deconfined quark phase in the interior of the NS core,
we account for some realistic hybrid EoSs containing the
quark phase in the interior. The selective hybrid EoS
models are BFH-D [64, 65], KBH (QHC21 AT) [66],
JJ-VQCD [67] and OOS-DD2(FRG) [68]. We do not
consider the EoSs regarding quark stars only, as they
3
Model n0Esat K J L m/mNUΣUΞ
(fm3) (MeV) (MeV) (MeV) (MeV) MeV MeV
RMF [48] [0.14, 0.17] [-16.5, -15.5] [200, 300] [28, 34] [40, 70] [0.55, 0.75] [0, +40] [-40,0]
TABLE I: Range of nuclear and hypernuclear saturation parameters considered in this work. Meson and nucleon
masses are fixed at mσ= 550 MeV, mω= 783 MeV, mρ= 770 MeV, mσ= 975 MeV, mφ= 1020 MeV and
mN= 939 MeV. Masses of the hyperons are fixed from [50]. Note that we fix the Λ hyperon potential depth
(UΛ=-30 MeV).
might deviate from the universal behavior and leave
them for a separate investigation [35, 69].
c. Spectral Decomposition: To span the EoSs with
empirical fit formalism, we consider the four parameters
spectral decomposition method developed in [11]. In
spectral decomposition, the adiabatic index (Γ) of the
EoS is spectrally decomposed onto a set of polynomial
basis functions and expressed as,
Γ(p) = exp X
k
γk[ln(p/p0)]k!(1)
where γkis the expansion coefficient and p0is the ref-
erence pressure where the high-density EoS is stitched to
the low-density crustal EoS. The EoS can then be gener-
ated by integrating the relation,
d
dp =+p
pΓ(p)(2)
which can be reduced to,
(p) = (p0) + 1
µ(p)Zp
p0
µ(p0)
Γ(p0)dp0(3)
where,
µ(p) = exp Zp
p0
dp0
p0Γ(p0)
.
We fix the low-density EoS to SLy EoS [70] and stitch
the high-density EoS at a density below half of the sat-
uration density such that the NS macroscopic properties
will not be affected significantly. We generate spectral
decomposed EoSs as implemented in LALSimulation [54]
and consider the ranges for spectral indices (γk) from
[20, 71].
Before proceeding with any further calculations, we
ensure that each EoS satisfies the required physical con-
ditions, such as thermodynamic stability (dp/d > 0),
causality (pdp/d 1) and the monotonic behaviour of
pressure (dp/dρ > 0 and d/dρ > 0). We additionally im-
pose the constraint that the EoS must be able to produce
a 2Mstable NS and the tidal deformability of a 1.4M
is less than 800 (i.e, Λ1.4M800) [20, 21]. EoSs and
the corresponding mass-radius relations used in this work
are displayed in Figure 1a and Figure 1b respectively.
B. Macroscopic Structure and Tidal
Deformabilities
For a given EoS, NS mass (M) and radius (R) are
obtained by solving the Tolman-Oppenheimer-Volkoff
(TOV) equations. The vanishing of pressure at the sur-
face of NS (p(R) = 0) provides the stellar radius Rand
the stellar mass M=m(R). TOV equations correspond-
ing to a static and spherically symmetric metric (4) are
summarised in Eq. (5) [74, 75].
ds2=e2Φ(r)dt2+e2λ(r)dr2+r22+r2sin2θ2(4)
dm(r)
dr = 4πr2(r),
dp(r)
dr =[p(r) + (r)] m(r)+4πr3p(r)
r(r2m(r)) ,
dΦ(r)
dr =1
(r) + p(r)
dp
dr ,
e2λ(r)=r
r2m(r). . (5)
The NS can be tidally deformed in a binary system due
to mutual gravitational interaction. The tidal field can
be decomposed to an electric (Eij ) and magnetic (Mij )
component, leading to the induction of mass multipole
moment (Qij ) and current multipole moment (Sij ). The
gravitoelectric tidal deformability (λl) and gravitomag-
netic tidal deformability (σl) of order lcan be defined
as [76],
Qij =λlEij ,
Sij =σlMij (6)
kl=(2l1)!!
2λl,
jl= 4(2l1)!! σl.(7)
The electric Love number (kl) and magnetic Love num-
ber (jl) relate to the tidal deformability parameters as
given in Eq. (7). The valuable parameters that can be
determined from a GW signal are dimensionless tidal de-
formability. The dimensionless gravitoelectric tidal de-
formability parameter (Λl) and gravitomagnetic tidal de-
formability parameter (Σl) can be expressed using the
corresponding tidal Love numbers and stellar compact-
ness (C=M/R) as [77]:
4
500 1000 1500 2000
( MeV fm3)
200
400
600
800
1000
p( MeV fm3)
RMF(npeµ)
RMF(npeµY)
Spectral
APR3
APR4
BFH-D
BL
DD2
JJ-VQCD
KBH
MPA1
OOS-DD2
SLy4
SRO-APR
Stiff-EoS
WFF1
BSk22
Soft-EoS
(a)
10 12 14 16
R(km)
0.5
1.0
1.5
2.0
2.5
3.0
M(M)
PSR J0740+6620
PSR J0348+0432
GW170817 M1
GW170817 M2
Causality
Rotation
RMF(npeµ)
RMF(npeµY)
Spectral
APR3
APR4
BFH-D
BL
DD2
JJ-VQCD
KBH
MPA1
OOS-DD2
SLy4
SRO-APR
Soft-EoS
Stiff-EoS
WFF1
BSk22
(b)
FIG. 1: (a) EoSs used in this work. (b) Mass radius relation corresponding to the EoSs used in this work. For each
EoS, the corresponding M-R relationship is shown up to the maximum possible stable mass. Horizontal bands
correspond to masses M= 2.072+0.067
0.066Mof PSR J0740+6620 [72] and M= 2.01+0.04
0.04Mof PSR J0348+0432
[73]. The mass radius estimates of the two companion neutron stars in the merger event GW170817 [20] are shown
by the shaded area labeled with GW170817 M1 (M2) a.
ahttps://dcc.ligo.org/LIGO-P1800115/public
Λl=2
(2l1)!!
kl
C2l+1 ,
Σl=1
4(2l1)!!
jl
C2l+1 .(8)
For computing the electric and magnetic Love numbers,
one needs to integrate additional set of differential equa-
tions [77] along with the TOV equations. We follow the
methodology developed in [77] to solve for the Love num-
bers. We test our numerical scheme by reproducing the
Love numbers corresponding to selective EoSs used in
[77]. We provide the electric tidal deformability up to
order ‘`= 4’ and magnetic tidal deformability up to or-
der ‘`= 3’.
C. Finding f-mode oscillation characteristics
A neutron star can have several quasi-normal modes
depending upon the restoring force. Mode characteris-
tics (frequency and damping time) contain information
about the NS interior. Hence, the determination of mode
parameters can be employed to constrain the NS inte-
rior or EoS. In our previous work [78], we had shown
that depending upon the EoS, the Cowling approxima-
tion can overestimate the quadrupolar f-mode frequency
up to about 30%. As recent efforts are going on to im-
prove the gravitational waveform models with considera-
tion of excitation of f-modes which depends on the mode
frequency [32], one should consider the general relativis-
tic formalism to find the mode characteristics. We use the
direct integration method developed in [78–81] to find
the NS f-mode frequency. In short, the coupled equations
for perturbed metric and fluid variables are integrated
within the NS interior with appropriate boundary con-
ditions [80]. Outside of the NS, fluid variables are set
to zero, and then the Zerilli’s wave equation [82] is inte-
grated to far away from the star. Then a search is carried
out for the complex f-mode frequency (ω= 2πf +i
τf) for
which one has only outgoing wave solution to the Zer-
illi’s equation at infinity. The real part of ωrepresents
the f-mode angular frequency, and the imaginary part
represents the damping time. For finding the mode char-
acteristics, we use the numerical methods developed in
our previous work [78].
III. RESULTS
A. Multipole Love and f-Love Relations
The universality among higher-order electric tidal de-
formability parameters (Λ`3) or the magnetic de-
formability parameter (Σ`2) with the quadrupolar tidal
deformability parameter (Λ`=2) were first discussed in
[35, 40] with few selective EoSs. The universality of com-
pactness (C) with Λ2initially introduced in [39] and the
UR involving f-mode frequency and tidal deformability
parameters are introduced in [38]. Original URs from
5
[35, 37–39] are expressed using a polynomial fit of the
following form (the order of the polynomial differs in dif-
ferent works).
P=
6
X
k=0
ak[ln (Λ2)]k,(9)
where,
P={ln(Λ3),ln(Λ4),ln(|Σ2|),ln(|Σ3|), C,
Mω2, Mω3, M ω4}
In recent works [43, 83], the universal relations for elec-
tric type deformability were updated by considering the
phenomenological piece-wise polytropic EoSs. In differ-
ent waveform models, the correction on the tidal phase
includes the electric tidal parameter up to order `4
and for magnetic deformation up to order `3. Hence,
we provide the universal relations for Λ3and Λ4with
Λ2for electric type, whereas for magnetic type, we pro-
vide the universal relation of Σ2and Σ3with Λ2. From
the tidal deformability and mass obtained from a bi-
nary system, one can infer the NS radius (R) by using
the EoS-independent relation that exists between stellar
compactness (C=M/R), and tidal deformability param-
eter [39, 43, 47, 84–86]2. There are EoS-independent
relations, which involve the tidal deformability of both
binary NSs and binary parameters ( like q=m1/m2).
The universal relation involving the binary parameters
and the CΛ2relation is also used to infer the NS
radii [40, 41, 43, 87]. Though binary relations reduce the
parameter space of BNS search parameters, the inferred
NS parameters are model dependent [88].
As discussed in Section II A, we consider tentatively
6000 EoSs originating from different physical motiva-
tions. We fix the lower mass for an EoS by imposing the
constraint resulting from the maximum rotation pulsar
PSR J1748-2446 [89, 90] (also notice the rotation lim-
iting curve in Figure 1b). The maximum mass for an
EoS is fixed at the maximum stable NS mass the EoS
can produce. We then use the method described in Sec-
tion II B to obtain the multipole tidal parameters (Λ`and
Σ`). We obtain the multipole Love relations by solving
the NS properties (with tidal parameters) for 1.2×106
neutron stars. We produce the universal relation for mul-
tipole Love relations as a polynomial fit (9) as introduced
in [35]. Note that the original fit from Yagi [35] was a
4th order polynomial which was then updated to a 6th or-
der polynomial for a larger data set in [43]. In our data
set, we notice that by updating the polynomial from a
quartic polynomial to a 5th order polynomial, the result-
ing goodness of fit improved significantly and then did
2The systematic errors that could be occurred in the inferred NS
properties due to the choice of binary Love relation or CΛ2
UR are recently discussed in [43, 86].
not improve significantly after increasing the order of the
polynomial. However, we provide the universal relations
with a 6th order polynomial (9) to be consistent with the
updated relations. Universal relations Λ3Λ2, Λ4Λ2,
Σ2Λ2and Σ3Λ2are displayed in Figures 2a, 2b, 3a
and 3b respectively. The fit parameters for the multipole
Love relations (9) involving tidal parameters obtained
from this work and other works are tabulated in Ta-
ble II. Similarly, for CΛ2relation we provide the fit
parameters in Table III and displayed the relation in
Figure 4. Our multipole universal relations are valid for
2.3Λ24×104which is the essential range for GW
astronomy ( mostly in the range 104) and for CΛ2,
the relation is valid for 0.08 C0.33.
Although the detection of f-modes from BNS would
only be possible with third generation detectors [34],
the impact on the inferred NS parameters can be seen
in the A+ detector configurations [31, 33]. In the fre-
quency domain, the phase correction due to the dynami-
cal excitation of f-modes depends solely on the mode fre-
quency [18, 26, 32, 91]. The universality of mass scaled
f-mode angular frequency (ie.,Mω) with the tidal pa-
rameter (Λ) was first studied in the work of Chan et al.
[38], where the universality was explained by using the
universal behavior of f-mode frequency with the moment
of inertia (I), and the ILoveQrelations [38, 44]. To
avoid the numerical instabilities, we fix the lower mass
1Mfor each EoS despite the rotation limit while solv-
ing for the f-mode frequency. The proposed quartic poly-
nomial fit of [38] is updated to a 5th order fit in [78, 92].
We provide the polynomial fit (9) up to 6th order to be
consistent with other multipole universal relations. Our
f-Love relations are valid for 2.3Λ24×104.
B. Error Analysis
We analyze the errors and compare the URs in the
range Λ2104as required for GW astronomy. Our
Λ3Λ2relation holds a maximum error of 11%, with 90%
of the errors are below 7%. In comparison, the original fit
from Yagi [35] holds a maximum error of 16% with 90%
of the errors below 10%, and the updated Λ3Λ2UR
from Godzieba et al. [43] holds a maximum error of 23%
with 90% of the errors below 16%. On similar lines, the
Λ4Λ2relation developed in this work holds a maximum
error of 24% with 90% of the errors below 13%, whereas,
the original fit from Yagi holds a maximum error of 35%
with 90% of the errors below 20%, and the UR from [43]
holds a maximum error 44% with 90% of the errors
below 30%. For 50 Λ2104, the URs developed in
this work behave quite similarly to the URs developed
in [43] and also in this range, the updated URs, as well
as the URs from [43] account less error compared to the
original fits from Yagi [35]. However, in the complete
range of Λ2, our relation have lower error bounds ( see
Figure 9 in Appendix A or Table II).
For Λ2104, our Σ2Λ2fit holds a maximum error
摘要:

ImpactofupdatedMultipoleLovenumbersandf-LoveUniversalRelationsinthecontextofBinaryNeutronStarsBikramKeshariPradhan,1,AdityaVijaykumar,2,3andDebaratiChatterjee11Inter-UniversityCentreforAstronomyandAstrophysics,PuneUniversityCampus,Pune,411007,India2InternationalCentreforTheoreticalSciences,TataInst...

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