[PPT] - Gravitational wave signatures of exotic compact objects Elisa PowerPoint Presentation

SLIDE 1

Elisa Maggio Sapienza University of Rome

Gravitational wave signatures of exotic compact objects

Cortona Young 27-29 May 2020

SLIDE 2

Outline

Gravitational waves as probes of strong gravity
Exotic compact objects
Ergoregion instability
Gravitational wave echoes

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 1/14

SLIDE 3

Compact binary coalescence

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 2/14

On September 14th 2015 the first gravitational wave from a compact binary merger was detected.

LVC, PRL 116 (2016) 061102

Three stages:

Inspiral
Merger
Ringdown

SLIDE 4

Compact binary coalescence

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 2/14

On September 14th 2015 the first gravitational wave from a compact binary merger was detected.

LVC, PRL 116 (2016) 061102

Three stages:

Inspiral
Merger
Ringdown

SLIDE 5

Ringdown stage

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 3/14

The ringdown stage is dominated by the quasi-normal modes of the remnant which describe the response of the compact object to a perturbation.

LVC, PRL 116 (2016) 061102

ω = ωR + iωI

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τdamp = − 1 ωI

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fGW = ωR 2π

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The signal is a sum of exponentially damped sinusoids:

SLIDE 6

Gravitational wave detections

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 4/14

LVC, PRX 9 (2019) 031040

During the 1st and 2nd observing runs, LIGO and Virgo detected 1 quasi-normal mode compatible with a Kerr black hole: Solar Mass

GW150914

fQNM = 251+8

−8 Hz

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τQNM = 4.0+0.3

−0.3 ms

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LVC, PRL 116 (2016) 221101

SLIDE 7

Exotic compact objects

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 5/14

ECOs are theoretical compact objects without horizon which

can mimic black holes in terms of electromagnetic observations
can overcome paradoxes of BHs
thermodynamical instability
huge entropy

Mazur, Mottola, PNAS (2004) Abramowicz+, A&A 396, L31-L34 (2002) Cardoso, Pani, Nat. Astron. 1, 586 (2017)

SLIDE 8

Exotic compact objects

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 5/14

ECOs are theoretical compact objects without horizon which

can mimic black holes in terms of electromagnetic observations
can overcome paradoxes of BHs
thermodynamical instability
huge entropy

Cardoso, Pani, Nat. Astron. 1, 586 (2017) Mazur, Mottola, PNAS (2004) Abramowicz+, A&A 396, L31-L34 (2002)

SLIDE 9

Rotating exotic compact object

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 6/14

We analyze a compact object described by the Kerr metric with radius whose surface is parametrized by a complex reflectivity coefficient :

Perfectly reflecting surface
Partially absorbing surface

Black hole horizon Surface of exotic compact object

EM, Pani, Ferrari, PRD 96 (2017) 104047

R

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✏rH

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r0 = rH(1 + ✏), ✏ ⌧ 1

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SLIDE 10

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 7/14

Spinning compact objects with an ergoregion but without an event horizon might be unstable due to the ergoregion instability.

Event horizon Ergoregion

Brito, Cardoso, Pani, Lect. Notes Phys. 906 (2016)

The only way to prevent the instability is by absorbing negative-energy states through dissipation mechanisms at the surface of the object.

Ergoregion instability

Friedman, Commun. Math. Phys. 63 (1978) 243

Infinite redshift surface

Penrose, Nuovo Cimento.J. Serie 1 (1969) 252

SLIDE 11

Ringdown of exotic compact objects

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 8/14

Black hole Exotic compact

bject

No horizon Trapped Modes The quasi-normal modes are derived by

d2ψ dz2 + V (z)ψ = 0

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Different QNMs

SLIDE 12

Ringdown of exotic compact objects

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 8/14

Black hole Exotic compact

bject
Echoes

due to excitation of the light-ring

Same prompt ringdown

due to trapped modes

Cardoso, Pani, Nat. Astron. 1, 586 (2017)

The quasi-normal modes are derived by

Exotic compact

bject

No horizon Trapped Modes

d2ψ dz2 + V (z)ψ = 0

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Different QNMs

SLIDE 13

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 9/14

Quasi-normal mode spectrum

acrit

stable unstable

Exotic compact objects with a perfectly reflecting surface turn unstable above a critical value of the spin due to the ergoregion instability.

EM, Pani, Ferrari, PRD 96 (2017) 104047

✏ = 10−10

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|R|2 = 1

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0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

a/M M R

0.0 0.2 0.4 0.6 0.8 1.0 10-10 10-8 10-6 10-4

a/M |M I|

SLIDE 14

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 9/14

Quasi-normal mode spectrum

EM, Pani, Ferrari, PRD 96 (2017) 104047

5
10
15
20
25
30
35
40

0.2 0.4 0.6

Log10() acrit/M

For , slowly spinning ECOs are unstable.
The timescale of instability is short compared to

astrophysical timescales

✏ → 0

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τinst ∼ 50 ✓ M 10M ◆ s

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EM, Cardoso, Dolan, Pani, PRD 99 (2019) 064007

acrit

stable unstable

✏ = 10−10

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|R|2 = 1

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0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

a/M M R

0.0 0.2 0.4 0.6 0.8 1.0 10-10 10-8 10-6 10-4

a/M |M I|

Exotic compact objects with a perfectly reflecting surface turn unstable above a critical value of the spin due to the ergoregion instability.

SLIDE 15

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 10/14

How to quench ergoregion instability

Partial absorption at the surface destroys the instability. The minimum absorption rate to quench the instability is the maximum amplification factor of superradiance of black holes.

Spin Absorption 0.7 0.9 any 0.3% 6% ~60%

EM, Cardoso, Dolan, Pani, PRD 99 (2019) 064007 Cardoso, Pani, LRR (2019) 22:4 Vilenkin, Phys. Lett. 78B (1978) 301

SLIDE 16

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 11/14

Template for echoes

Transfer function Parameters: The signal emitted by an ECO at infinity can be written in terms of the signal emitted by a BH:

Standard BH ringdown:
2 of the ECO:

M, χ, A+,×, φ+,×, t0

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˜ Z+

ECO(ω) = ˜

Z+

BH(ω) + K ˜

Z−

BH(ω)

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Mark+, PRD 96 (2017) 084002

SLIDE 17

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 11/14

Template for echoes

EM, Testa, Bhagwat, Pani, PRD 100 (2019) 064056

Analytical template Transfer function Parameters: The signal emitted by an ECO at infinity can be written in terms of the signal emitted by a BH: Mark+, PRD 96 (2017) 084002

Standard BH ringdown:
2 of the ECO:

M, χ, A+,×, φ+,×, t0

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˜ Z+

ECO(ω) = ˜

Z+

BH(ω) + K ˜

Z−

BH(ω)

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SLIDE 18

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 12/14

Ringdown+echo waveforms

Plus polarization Cross polarization

EM, Testa, Bhagwat, Pani, PRD 100 (2019) 064056

SLIDE 19

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 12/14

Ringdown+echo waveforms

EM, Testa, Bhagwat, Pani, PRD 100 (2019) 064056

Plus polarization Cross polarization

The mixing of the

polarizations occurs for a spinning ECO

r a complex .

SLIDE 20

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 12/14

Ringdown+echo waveforms

EM, Testa, Bhagwat, Pani, PRD 100 (2019) 064056

The mixing of the

polarizations occurs for a spinning ECO

r a complex .
The phase in

modifies the echo pattern. Plus polarization Cross polarization

SLIDE 21

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 13/14

Detection of exotic compact objects?

EM, Testa, Bhagwat, Pani, PRD 100 (2019) 064056

Third generation and space based detectors will allow us to detect or

exclude exotic compact objects with a partially absorbing surface.

Abedi+, PRD 96 (2017) 082004, Conklin+, PRD 98 (2018) 044021

Debate on the evidence of echoes in LIGO/Virgo data

Westerweck+, PRD 97 (2018) 124037, Tsang+, PRD 101 (2020) 064012

SLIDE 22

Elisa Maggio - Exotic Compact Objects Cortona Young 2020 14/14

Conclusions and future prospects

We can understand the nature of compact objects and look for new

physics at the horizon scale through gravitational waves.

We analyzed the stability of a rotating exotic compact object:
We derived an analytical gravitational-wave template for the postmerger

ringdown and the echo signal.

Accurate matched-filter searches for gravitational wave echoes