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�RECENT RESULTS AND FUTURE CHALLENGES FOR ISOLATED CONTINUOUS GRAVITATIONAL WAVE SEARCHES WITH A NETWORK OF TERRESTRIAL GRAVITATIONAL WAVE DETECTORS

Pia Astone, INFN Roma

LIGO, Livingston (LSU)

Virgo, Cascina

LIGO, Hanford (Washington State)

GRASPS2023 Pisa Thursday, October 26, 2023

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O4 run started on May 24^th

^{Will last 18 months}

around 2027. 330 Mpc

https://observing.docs.ligo.org/plan/

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GW sources classification, using signal characteristics

Modeled waveform

Unmodeled waveform

The sources represented in the pictures do not cover all the possibilities

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A classification based on signal characteristics

Modeled waveform

Unmodeled waveform

<~10^-26

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CBC: Coalescence of binary system of neutron stars and/or stellar-mass black-holes

Burst: Core-collapse of

massive stars

Unmodeled searches

SOURCES, SIGNALS and SEARCH METHODS

CW: Isolated or binary neutron stars

Matched-filter (MF) Modeled searches

MF/FFT based and/or hierarchical searches

Stochastic Background

Cross-correlations

90 detections to date GWTC-3

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Continuous wave searches:

My talk has to be seen as part of the two following talks,

already given during this conference

I will not talk of searches in binary systems

I will not talk of long transient nor DarkMatter searches

Tuesday 24

Wednesday 25

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The basic problem of detecting CW sources

CBC signals are of limited duration, well modeled and visible given the actual sensitivities , even if ”far” from us. GW170817 distance was ~ 40 Mpc from earth.
Continuous signals, like those emitted by compact objects isolated or in binary systems, typically have long duration but strain amplitudes are much weaker , o(10^-26) compared to o(10^-21)

Spinning NS

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Expected signals are not monochromatic at the detector. Frequency (and phase) are modified by various effects:

Doppler effect due to the detector motion;
Orbital motion for sources in binary systems;
Source spin-down (rotation frequency decreases due to energy loss; relativistic effects; antenna pattern).

10^-26

³⁰⁰

CONTINUOUS WAVES EMITTED BY NEUTRON STARS

deformation due to elastic or magnetic stresses: f_GW= 2 f _spin

deformation due to (anisotropic ) matter accretion (e.g., LMXB);

excitation of long-lived oscillations (e.g., r-modes): f_GW= 4/3 f _spin

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~ 10^-5for a ‘standard’ NS (fluid core, normal nuclear matter)

~ 10^-3 for ‘hybrid’ stars (hadron-quark core)

~ 10^-3 (B/10¹⁶G)²deformation from the volume averaged magnetic field.

~ 10^-1 for quark star.

10^-5corresponds to a ‘mountain’ ~10 cm high!

For isolated NS the maximum foreseen ellipticity depends on the star crust physics, the matter equation of state at supra-nuclear density and on the deformation mechanism.

Lasky, Glampedakis, MNRAS 458 2016

N. Jonhson-McDaniel, B. Owen

PRD 86 063600 , PRD 87 129903

max

ma x

max

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CW signals: What will a detections tell us?

NS internal structure (EOS, viscosity, superfluidity)
Maximum spin allowed for a NS
Intensity and geometry of interior magnetic field
Accretion physics
NS demography (including a possible population of ``exotic” stars) and implications for a stochastic background.
Testing GR

Even a null result can be used to constrain NS parameters, like ellipticity or the internal magnetic field, and at least some of the EOS’s.

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0 days 180

10^-27

The continuous wave

signal at the detector

Values here are just to mark order of magnitues of the effects

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The continuous wave signal

in the detector noise

0 days 180

10^-21

Values here are just to mark order of magnitudes of the effects

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Neutron stars signal model

Long-term frequency evolution of a CW signal

PicturecCredit:

K. Wette

For isolated NSs

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The basic problem of detecting CW sources

To detect these signals, we need to integrate over long times.

At the actual sensitivities our main target for continuous searches are galactic non-axisymmetric spinning NS, isolated or in binary systems, such that the frequency of the emitted GW, is in the band of our detectors ~[20-2000] Hz.

We know that potential sources of CW exist: 2500+ NS are observed (mostly pulsars) and O(10⁸ - 10⁹) are expected to exist in the Galaxy.

In some cases, hierarchical procedures are needed

(compromise between sensitivity and computational problems)

Multimessenger astronomy plays a very important role

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The computational problem

Credit C. Palomba

To give an example:

Order of magnitude estimation for the Frequency Hough hierarchical All Sky search (isolated NS) is: 1 year, 3 detectors. ~ 80 million core-hours

All Sky searches

✓Sky position

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LVK Continuous Wave group��CWs

SOME RECENT Results

On isolated pulsars

searches

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LVK coll: https://arxiv.org/pdf/2111.13106.pdf

ε ~ 3* 10^-9

236 pulsars

For the Crab and Vela pulsars limits are factors of

∼ 100 and ∼ 20 more constraining than spin-down limits

ε ~ 7* 10^-6

Single harmonic result

TARGETED

Searches for CWs from known pulsars at two harmonics in the O2 and O3 LIGO/Virgo runs

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Pagina 18

236 pulsars analyzed

LVK coll: https://arxiv.org/pdf/2111.13106.pdf

pink contour: lines of equal characteristic age tau assuming GW�emission alone is causing spin-down

Single harmonic result

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Spindown limit beaten for 23 pulsars��For 9 pulsars, their spin-down limits have been surpassed for the first time��

Lowest upper limit on ellipticity:

J071+6830 ellipticity < ≈ 3* 10^-9

For the Crab pulsar, the GW upper limit is less

than 0.009% (previously ∼0.02%) of spin-down

limit. With an ellipticity of 7.2x10^-6

(maximum mountain height of ~2 cm)

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Narrowband searches for CWs from known pulsars in O3, the third LIGO/Virgo observing run

Relax the assumption that GW emission is phase-locked to EM emission, allowing the GW frequency to vary from EM expectation in a narrow band. 18 known pulsars

Search for long-duration (hours–months) transient GWs after pulsar glitches for 6 targets (9 glitches during O3)

As usual, different analysis methods used

O3 HLV, arXiv:2112.10990 (2022)

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Narrowband searches for CWs from known pulsars in O3, the third LIGO/Virgo observing run

Surpassed spin-down limits for 7 pulsars (2 for first time)

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Energetic young pulsar PSR J0537-6910

(“The Big Glitcher”). (1-5) kyear. LMC.

O2 and O3 data

ApJL 913, L27 (2021)

X-ray pulsar, largest spin-down luminosity, frequent and strong glitches

In the analysis,

we have used NICER timing ephemeris

f_rot= 62 Hz

NICER:

Neutron star Interior

Composition Explorer

TARGETED SEARCHES

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Search for continuous gravitational wave emission from the Milky Way center in the O3 LIGO–Virgo run

Sgr A*

https://arxiv.org/pdf/2204.04523.pdf

Image Credit NASA

DIRECTED SEARCHES

Compelling evidence shows a large population of neutron stars in the GC

Search frequency band [10, 2000] Hz

Assume a distance of 8 kpc

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Search for continuous gravitational wave emission from the Milky Way center in O3 LIGO–Virgo data

Sgr A*.

Assuming distance 8 kpc

Image Credit NASA

I_fid=

10³⁸ kg*m²

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Search for continuous gravitational wave emission from the Milky Way center in the O3 LIGO–Virgo run� R-mode searches

Sgr A*.

Assuming distance 8 kpc

https://arxiv.org/pdf/2204.04523.pdf

R mode emission frequency and amplitude

Image Credit NASA

Long-lasting oscillations in the fluid that makes up most of the star 🡺

a fluid wave travelling around the star and driven by the Coriolis force due to rotation

R-mode amplitude

DIRECTED SEARCHES

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Search for continuous gravitational wave emission from the Milky Way center in the O3 LIGO–Virgo run� R-mode searches

Sgr A*.

Assuming distance 8 kpc

https://arxiv.org/pdf/2204.04523.pdf

Under some assumptions for parameters of the NS

Estimates of the 95 % C.L. r-mode amplitude upper limits for neutron stars in the GC region assuming d = 8 kpc.

Image Credit NASA

DIRECTED SEARCHES

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Energetic young pulsar PSR J0537-6910

(“R-mode searches). O3 data

The Astrophysical Journal, 922:71, 2021 November

Inter-glitch braking index of the pulsar suggests that GW emission due to r-mode oscillations may play an important role in the spin evolution of this pulsar

Stiff EOS

Soft EOS

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Search for 15 young supernova remnants in frequency bands within [10, 2000] Hz

O3a HLV, ApJ 921, 80 (2021)

O3a HL, PRD 105, 082005 (2022)

Directed SEARCHES

Cassiopeia A

[Credit: NASA/JPL-Caltech]

• Take into consideration spin-wandering and dual-harmonic emission

No detections (h_0,min~ 7.7x10^-26for G65.7+1.2 )

• Ellipticity <10^-6 for most of the sources;

reaching below the rough theoretical upper limit for normal neutron stars.

Minimal ellipticity ~ 6x10^-8for G266.2-1.2/Vela Jr.

• r-mode amplitude < 10^-3, reaching below the theoretical prediction

level expected for the non l-inear saturation mechanisms

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Search for 15 young supernova remnants

in frequency bands within [10, 2000] Hz

A previous deeper O3a search in band [20, 976] Hz achieved

the best sensitivities for Cassiopeia A and Vela Jr., setting limits ~30% lower below 600 Hz.

Cassiopeia A

[Credit: NASA/JPL-Caltech]

Vela Jr.

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All-Sky searches:

The “Frequency Hough” procedure

An example of hierarchical

procedure used in All-Sky searches.

This is one of the

LIGO/Virgo/KAGRA official pipelines

ALL – Sky SEARCHES

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All-Sky searches:

The “Frequency Hough” procedure

Astone et al,

Phys. Rev. D 90, 042002

Serious issues

have to be faced

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All-sky Search for Continuous Gravitational Waves from isolated Neutron Stars in the Early O3 LIGO Data

https://arxiv.org/pdf/2107.00600.pdf

Upper limits on

gravitational

strain amplitude

for O3a analysis

and for previous

O2 analyses.

ALL – Sky SEARCHES

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PRD 100, 024004

(2019)

All-sky search for continuous gravitational waves from isolated neutron stars in the O3 LIGO/Virgo run

https://arxiv.org/pdf/2201.00697.pdf

4 pipelines

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PRD 100, 024004

(2019)

All-sky search for continuous gravitational waves from isolated neutron stars in the O3 LIGO/Virgo run

4 pipelines

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PRD 100, 024004

(2019)

All-sky search for continuous gravitational waves from isolated neutron stars in the O3 LIGO/Virgo run

https://arxiv.org/pdf/2201.00697.pdf

Assuming all rotational

energy goes into GW

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Interesting procedure in development,�see Lorenzo Mirasola poster�(Virgo Cagliari and Roma 1)

Semi-coeherent approach for known pulsars in Binary systems

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Credit NASA

CW searches during the O4 run will start in a while

Some CW analysis might run after 6 months, producing

results during 2024

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Conclusions

We live in highly exciting times; many interesting progresses and results have been made in the past years.
A strong effort is ongoing to unveil sources whose fingerprint is hidden in the data, and this is based on joint experimental, data analysis and theoretical work

Multi messenger astronomy has an important role for CW searches in many cases

Spinning NS

SN remnant

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