Neutron Star Merger Afterglows: Population Prospects for the Gravitational Wave Era

R. Duque, F. Daigne & R. Mochkovitch

March. 26th 2019 –

New Era of Multi-Messenger Astrophysics,Groningen, The Netherlands

On August 17th 2017…

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• Confirmed NS-NS mergers as progenitors for short GRBs• Inauguration of the era of multi-messenger

astronomy with GW• Other fundamental (astro-)physics: GR, NS EOS,

Hubble constant measurement, r-process nucleosynthesis, etc.

Afterglows and kilonovae:What should we expect for O3?

Vill

aret

al.

2018

Abbot

t et

al.

2017b

Moo

ley

et a

l. 2018

Context

3

• Which kilonovae and afterglows to expect and what will they look like?• How will they help to study the environments of NS binaries?• What insight will they bring on the origin of the jet structure?• What will they tell us on GRBs and their dissipation mechanisms?

O3 is coming (April 2019)àMore GW with afterglow

and kilonova!

Afterglow, kilonova= great wealth of information!

ü Localizationü External medium densityü Jet kinetic energyü Jet geometryü Viewing angleü Magnetic fieldü And more!

Ghir

landa

et a

l. 2018

Gravitational wave

Θv

Kinetic energy E

external densitynext

1540+3200 BNS/Gpc3/yr (Abbott+2018)-1220

GW h

orizo

n

Afterglow

Detection depends on D, Θv

… and GW horizon

O3: 380 MpcDesign: 430 Mpc

Detection depends on D, Θv , E, next, … and radio sensitivity

VLA: 15 µJy @ 3GHzSKA1-Mid: 3 µJy

Population model with ingredients: D, Θv , E, next , …+ Detection criterionà Deduce GW+AG observed population of mergers

Method

Kilonova

Detection depends on D, Θv

… and Vis.-IR follow-up depth

D

5

Population model distributions:

Energy Density

intrinsic populationGW+Radio observed

intrinsic populationGW+Radio observed

Reference model:Ø Energy: BPL, break energy 2.1052 erg, slopes +0.5 and -2 (Ghirlanda et al. 2016)

Ø Density: Log-normal centered on 10-3 cm-3

selection bias towards larger energy and density

Log(

d

)

(Detectable) Event rates for NS-NS

Detector conf. #GW #(GW+AG) #(GW+KN)

O3 + VLA 9 2

Design + VLA 21 4

Design + SKA 21 6

700 MpcHorizon + SKA

92 20

6

• In general: 10-20% events have detectable AG (depending on energy distribution)

• Large deviation from this = constraints on population!

Uncertainties: +200% (intrinsic rate from LIGO-Virgo O2/O3)+ uncertainty on population model-73%

GW+GRB ~ 1-10% (O3) (Beniamini et al. 2018)

?

100%

Can we detectall detectable

events?

� Most events seen off axis!

� Mean angle ~20-30°

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0 20 40 60 80

✓v (deg)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

dN/d

✓ v

Joint (O3+VLA)

GW only

All events

RD

, Dai

gne,

Moc

hko

vitc

h(in p

rep.)

+ Other distributions:distance, peak flux, proper motion, …

Properties of joint events: viewing angle

0 20 40 60 80

✓v (deg)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

dN/d

✓ v

Joint (O3+VLA)

GW only

All events

Properties of joint events: viewing angle

� Most events seen off axis!

� Mean angle ~20-30°

� New insight on GRB physics

à Jet geometry? Origin of lateral structure?

à GRB dissipation mechanisms (thermal tail?)

8

170817

RD

, Dai

gne,

Moc

hko

vitc

h(in p

rep.)

+ Other distributions:distance, peak flux, proper motion, …

Merger medium density (low)

Formation medium density (high)

10�2 10�1 100

fHD

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

f HDp(

f HD|N

HD,N

TO

T)

HD/Tot = 0/10

HD/Tot = 1/10

HD/Tot = 3/10

9

Binaries in high density media

Ben

iam

ini,

RD

, Moc

hko

vitc

h, D

aign

e(in p

rep.)

Observation of 3 high-density out of 10 à log(fHD) = - 1 ± 0.6High density ⬄ Large viewing angle

� NS binaries with high eccentricity or efficient common envelope phase merge in high density media after short delay time (Beniamini+2016)

� Mergers occurring in dense media produce brighter AG and are more likely detected (F ~ n4/5)

à Tight constraints on binary environment from only a few events

0 20 40 60 80

✓v/deg

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

dN/d

✓ v

Joint, n = 1 cm�3

Joint, n = 10�3 cm�3

Ben

iam

ini,

RD

, Moc

hko

vitc

h, D

aign

e(in p

rep.)

?

intrinsic fraction

obs. fraction of HD events

0/101/103/10

Expectations for kilonovae

10

Moc

hko

vitc

h, R

D, D

aign

e, Z

itou

ni(

in p

rep.)

b filterr filter

170817

• Vis.-IR signal depends on viewing angle because of ejecta contrasts

For O3:ü Magnitude OK a prioriü Volume to explore potentially

100x larger than for 170817ü Contrast with host galaxy

Lanthanide-poorBlue (low !)

Lanthanide-richRed (high !)

For O3…

Moves with increasing GW horizon(more weak events)

à Finding the OT challenging!KN model and population details in poster: Prospects for KN signals in the GW era

Conclusion� Afterglows and KN are important to understand the local and

viewing conditions of NS-NS mergers

� O3 is coming: several NS-NS GW events, a few with afterglow, all with detectable KN

� Actual fraction of AG/GW will constrain population of NS-NS merger population (jet parameters, external density, etc.)

� Most events are seen off-axis, allowing to probe the jet geometry and emission therein

� High-density mergers will allow to study fast-merging binaries. Only a few events are necessary to constrain this particular binary NS evolution channel.

� What to expect from NS-BH mergers?

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Interpretation tools for observations of GRBs in the multi-messenger context:① Modeling of EM counterparts of CO fusions: sGRBs and

afterglows Context: observations by LIGO–Virgo (~2019)

② Modeling of the general population of GRBs and afterglows

Context: present and future observations:Swift, Fermi, INTEGRAL, SVOM

Long runLi

nk p

op. c

oale

scen

ce

—po

p. s

GRB

Determining viewing angle and density from multi-messenger observations

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10 15 20 25 30 35 40 45

✓v/deg

10�5

10�4

10�3

10�2

n/c

m�

3

KNAG

AG

VLBI

GW

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• Distinguish NS-NS and BH-NS?• Nature of final object? Link with ring-

down signal?• Systematic

fusion/GW/sGRB/kilonova/afterglow association?

• GW/GRB delay?

1: GRBs & CO fusions

2: General population of GRBs

Rates: (Wei, Cordier et al. 2017a):• SVOM: 60-70 yr-1

• Swift, Fermi, INTEGRAL: ~100 yr-1

• Radiative processes in GRB (shocks/magnetic reconnection)?

• Ejecta magnetization?• Other afterglow observables

(polarization, imaging)?

Wei

, Cor

die

r et

al.

2017a

Gamma-ray bursts

Duration

Longs (ccSNe):• > 2s• Soft• High SFR galaxies

Short (compact object mergers):• < 2s• Hard• Early-type galaxies

Light curves:• Strong variability• Shape diversity• Variation time-scale diversity

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Pac

iesa

s et

al.

1996

Pac

iesa

s et

al.

1996

Gamma-ray burstslog(R/m)

compact central engine

radiation: photosphere + IC?

dissipation:• shocks?• reconnection?

radiation:synchrotron

prompt phase (X-ray, gamma) afterglow (X, UV, O, IR, Radio)

afterglow:• deceleration• jet opening• synchrotron

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