The document summarizes research on finding electromagnetic counterparts to gravitational wave sources detected by LIGO and Virgo. It discusses that neutron star mergers are a promising source of both gravitational waves and short gamma-ray bursts. Numerical simulations show neutron star mergers produce neutron-rich debris ejected at high velocities, which could power a luminous "kilonova" lasting several days. Future wide-field optical surveys like LSST could detect such kilonova emissions from neutron star mergers within the gravitational wave detection range of advanced LIGO and Virgo, helping associate gravitational wave sources with electromagnetic events.

1. The Biermann Lectures:
Adventures in Theoretical Astrophysics
III Searching for the Electromagnetic
Counterparts of Gravitational Wave Sources
Eliot Quataert (UC Berkeley)
w/ Brian Metzger, Tony Piro, Siva Darbha, Almudena Arcones,
Gabriel Martinez Pinedo, Dan Kasen,Todd Thompson, ...
NS-NS Merger
Rosswog 2007

2. Overview
• Context: kHz Gravitational Wave Astrophysics
Likely Astrophysical Sources
• Why worry about EM Counterparts?
• Compact Object Mergers: GWs & Gamma-Ray Bursts
• EM Counterparts & Transient Surveys
NS-NS Merger
Rosswog 2007

3. ~ kHz GWs: a New Frontier in
Compact Object Astrophysics
LIGO reached design sensitivity
in ~ 2006: h ~ ΔL/L ~ 10-21
(no detections; as expected)
• Direct detection of GWs: unique
insights into compact objects
• masses, spins, orientation to line of sight, ...
• no bias re. photons escaping to observer!
• probes of nuclear physics, relativity, ....
• Critical to connect these GW
detections to wealth of EM data
on similar (same??) sources

4. ~ kHz GWs: a New Frontier in
Compact Object Astrophysics
Advanced LIGO & Virgo in ~ 2015
~10x sensitivity →103 x volume/rate
worldwide effort: Geo600 (Germany),
LCGT (Japan), LIGO Australia (??), ...
• Direct detection of GWs: unique
insights into compact objects
• masses, spins, orientation to line of sight, ...
• no bias re. photons escaping to observer!
• probes of nuclear physics, relativity, ....
• Critical to connect these GW
detections to wealth of EM data
on similar (same??) sources

5. Astrophysical Sources of ~ kHz GWs
˙E
GW !
G
c5
!
d3Q
dt3
"2
Q ! MnaL2
na = non-axisymmetric
part of mass distribution
˙E
GW !
c5
G
!v
c
"6 !rs
L
"2 rs associated w/
non-axisymmetric
mass distribution
• NS-NS, NS-BH, BH-BH Mergers (~ M⊙)
• Asymmetric stellar collapse
• core-collapse supernovae, AIC of WD→NS
• Rapidly rotating NSs; max obs ~ 700 Hz

6. Astrophysical Sources of ~ kHz GWs
merge ! 10−5 − 3 × 10−4 yr−1 perMWgalaxy
Advanced LIGO/VIRGO:
NS-NS Mergers at ~ 300 Mpc
BH-BH Mergers at ~ Gpc
Taylor Nobel Prize Lecture
orbit decays due to
emission of grav. waves
‘Guaranteed’ Source:
3 known NS-NS binaries in our
galaxy will merge in a Hubble time
(no BH-NS systems known)
˙N
(Kalogera et al. 2004)
PSR 1913+16
Advanced LIGO : 20 − 103 yr−1 " 100 yr−1 best guess

7. Why Worry about EM Counterparts?
(i.e., can’t we do all this great science w/ GWs alone?)
• post Nobel Prize, LIGO/VIRGO are astronomical observatories
• With EM counterparts, astrophysicists can
• identify host galaxy (H0; constrain progenitors)
• connect to wealth of transient phenomenology (SNe, GRBs, new sources, ....)
• uniquely constrain models: know masses, spins, orientation, ...
Hawley
Accretion onto a Central BH
red = high density
blue = low density
•
e.g.,
3 M⊙ BH
a = 0.84

8. Why Worry about EM Counterparts?
(i.e., can’t we do all this great science w/ GWs alone?)
• post Nobel Prize, LIGO/VIRGO are astronomical observatories
• With EM counterparts, GW astrophysicists can
• improve parameter estimation on detections
• cross-correlate GW w/ EM searches
• gain factor of ~ 2 in sensitivity and ~ 10 in rate!
• if merger rate low: EM signal critical to significant # of GW detections
•

9. Why Finding EM Counterparts is Hard
Sky Localization: LIGO + VIRGO
LIGO+
VIRGO
Aylott+ 2011
+ Southern
Hemisphere
Telescope
area (deg2)
Problem: Poor positions ~ 3-100 deg2 from few-arm interferometer
Challenge: significant observational & theoretical work needed now

10. Astrophysical Sources of ~ kHz GWs
˙E
GW !
G
c5
!
d3Q
dt3
"2
Q ! MnaL2
na = non-axisymmetric
part of mass distribution
˙E
GW !
c5
G
!v
c
"6 !rs
L
"2 rs associated w/
non-axisymmetric
mass distribution
• NS-NS, NS-BH, BH-BH Mergers (~ M⊙)
• Asymmetric stellar collapse
• core-collapse supernovae, AIC of WD→NS
• Rapidly rotating NSs; max obs ~ 700 Hz
GWs & GRBs
Best Guess
Progenitors of
Gamma-ray Bursts

11. Gamma-Ray Bursts
• Bursts of ~ 0.1-10 MeV γ-rays (non-thermal)
• “Long” (~ 30 s) & “Short” (~ 0.3 s)
• Isotropic & Cosmological: z ~ 0.1-8.3
• Very Energetic: ~ 1048-55 ergs (isotropic)
• Highly Relativistic: Γ ~ 102-3
• Rare: GRB Rate ~ 10-6/yr/galaxy ~ 10-4 SN rate
“Short”
~ 0.3 s
“Long”
~ 30 s
Distribution on the Sky
Flux
Time (s)

12. Long-Duration GRBs
Hjorth et al; Stanek et al.
As GRB fades, a supernova appears
Associated with massive star
formation and Type 1bc supernovae
→ Birth of a BH or NS
during core-collapse
Distinguished from typical SNe
by rapid rotation but ...
Level of Rotation Required in
Long GRBs ⇏ GW Emission
(can be ~ axisymmetric)

13. Short GRBs Hosts
GRB Here
Bloom et al. 2006
Elliptical @ z = 0.22
SFR < 0.1 M⊙ yr-1
E @ z=0.257
Berger et al. 2005
No Coincident SNe
Older Stellar Population
Distinct Progenitor

14. NS-NS & NS-BH Mergers
(Paczynski 1986; Goodman 1986; Eichler et al. 1989; Narayan et al. 1992 ....)
NS-NS Merger
t = 0.7 ms
t = 3 ms
Shabata & Taniguchi 2006
Merger Leaves Behind
Disk ~ 10-3-0.1 M⊙
(mostly free neutrons initially)
density contours & velocity vectors
consistent w/ short GRB durations

15. NS-NS & NS-BH Mergers
(Paczynski 1986; Goodman 1986; Eichler et al. 1989; Narayan et al. 1992 ....)
NS-NS Merger
t = 0.7 ms
t = 3 ms
Shabata & Taniguchi 2006
density contours & velocity vectors
3 known NS-NS binaries in MW
will merge in a Hubble time
(Kalogera et al. 2004)
short grb rate ~ 10-6 yr-1per MW
˙N
merge ! 10−5 − 3 × 10−4 yr−1 perMWgalaxy
⇒ emission beamed
(or not all mergers ⇒ GRB)
EM counterpart to GW detection
unlikely GRB; need ~ isotropic emission

16. The Evolution of the Remnant Disk
ang momentum conservation → disk spreads (& cools)
Local Disk Mass (M⊙)
Radius (cm)
1D time-dependent Models
(α-viscosity; realistic ν-microphysics)
➝ only neutrino
cooling impt
Lphoton ≲ LEdd
(undetectable at ~ 100 Mpc!)

17. The Little Bang: Late-time Disk Winds
Initially T ~ few MeV; disk mostly free neutrons
After ~ sec, R ~ 500 km & T ≲ 0.5 MeV
free n & p recombine to He
fusion (~ 7 Mev/nucl) unbinds disk
Ejected Mass ~ 1/2 Initial Disk ~ 10-2 M⊙, at v ~ 0.1 c
Neutron-rich matter (Ye ~ 0.3)
Metzger et al. 2008

18. Dynamical Ejecta in NS Tidal Tails
1.1 & 1.6 M⊙ NS merger
Rosswog 2007
Tidal Tails 10-3-10-2 M⊙ unbound during
early dynamical phases of merger
eg., Rosswog 2007; Goriely+ 2011 ...
Luminosity of Ejecta (Dynamical & Disk)
Depends on Heating
Initial thermal energy
lost to adiabatic expansion
emission peaks when tdiff ≲ texp
t ~ 1 day for NS ejecta

19. Nucleosynthesis in NS Debris
Natural Abundance of Elements
Big Bang: Ye = 0.88
Supernovae
Ye ~ 0.5
~ 3x103 Msun
n-rich elements
in the Galaxy
Neutron Star
produces 56Ni whose
~ 6 day 1/2 life is ideal
for heating SNe ejecta
Atomic Mass
Debris
r-process
(rapid neutron capture)
Ye ~ 0.1-0.3
End State of Nucleosynthesis
set by Electron Fraction: Ye

20. Binding
Energy
(per nucleon)
r-process:
free n’s + seed nuclei →
n-rich elements
ΔEr ~ 1-3 MeV/nucl:
beta-decays + fission
Atomic Mass

21. Heating of NS Debris in
Compact Object Mergers
Heating Rate (log)
NS Debris
R-process produces
significant heating
(~ Ni) at ≲ day
largely β-decays & fission
(some ɣ-rays)
thermalization ~ 50%
(Coulomb scattering)
R-process calcs by
Almudena Arcones & Gabriel
Martinez-Pinedo
Ni decay
(for comparison)
Late-time
R-process
heating
~2 hrs 1 day 10 days
Power-law htg ∝t-1.2 ~ identical
to that of radioactive waste
from fission reactors
(Cottingham & Greenwood 2001)

22. R-process Powered Transient
NS Debris
Bolometric Luminosity
Observational Diagnostics
few day “kilonova”:
L ~ 3 1041 ergs s-1
(MV ~ -15)
T ~ 104 K at peak: optical
spectroscopic: all n-rich elements
(no Ni, Fe, C, O, He, Si, H, Ca, ...)
colors, etc. hard to predict bec.
insufficient atomic line info
for relevant nuclei!
spherical RT w/ SEDONA: 10-2 M⊙

23. The EM Counterpart Challenge
Sky Localization: LIGO + VIRGO
LIGO+
VIRGO
+ Southern
Hemisphere
Telescope
Aylott+ 2011
area (deg2)
NS-NS/BH Mergers:
- Large FOV ~ many deg2
- rapid cadence ≲ day
- sensitivity to sources
~ 30 x fainter than SNe
reasonably matched to
optical imaging surveys:
PTF, LSST, ...

24. NS-NS Merger
EM Counterpart
0
10
1
10
2
Luminous Supernovae
Core−Collapse
Supernovae
10
−24
−22
−20
−18
−16
−14
−12
−10
−8
−6
Characteristic Timescale [day]
]
V
Peak Luminosity [M
Thermonuclear
Supernovae
Classical Novae
Luminous
Red
Novae
.Ia Explosions
Ca−rich
Transients
45
10
44
10
43
10
42
10
41
10
40
10
39
10
38
10
Peak Luminosity [erg s−1]
Optical Transients circa 2004
Kasliwal (2011) PTF

25. NS-NS Merger
EM Counterpart
‘Blind’ Detection
PTF: ~1 yr-1
LSST: ~103 yr-1
0
10
PTF11bij
PTF09dav
1
10
Luminous Supernovae
PTF09cnd
Core−Collapse
Supernovae
M85 OT
2
10
−24
−22
−20
−18
−16
−14
−12
−10
−8
−6
SN2005ap SN2008es
SN2005E
PTF10acbp
Characteristic Timescale [day]
]
V
Peak Luminosity [M
V838 Mon
M31 RV
SCP06F6
SN2006gy
SN2007bi
SN2008S
NGC300OT
SN2008ha
SN2002bj
PTF10iuv
PTF10bhp
PTF10fqs
PTF09atu
PTF10cwr PTF09cwl
Thermonuclear
Supernovae
Classical Novae
Luminous
Red
Novae
.Ia Explosions
Ca−rich
Transients
P60−M81OT−071213
P60−M82OT−081119
45
10
44
10
43
10
42
10
41
10
40
10
39
10
38
10
Peak Luminosity [erg s−1]
Optical Transients circa 2011
Kasliwal (2011) PTF

26. EM Counterparts to kHz GW Sources
Observational Requirements:
- Large FOV ~ many deg2
- rapid cadence ≲ day
- sensitivity to sources
~ 30 x fainter than SNe
feasible w/ optical imaging
surveys: PTF, LSST, ...
Prediction: NS-NS/BH Mergers
few day “kilonova”:
L ~ 3 1041 ergs s-1
(MV ~ -15)
spectroscopic: all n-rich elements
(no Ni, Fe, C, O, He, Si, H, Ca, ...)
“There are more things in Heaven and Earth, Horatio,
than are dreamt of in your philosophy” (Hamlet)

27. The Biermann Lectures:
Adventures in Theoretical Astrophysics
• work on a range of problems: ‘model building’ & studying key processes
• Compact Object Astrophysics
• gamma-ray bursts, transients, accretion theory, the Galactic Center
• Galaxy Formation
• massive black hole growth, galactic winds, ‘feedback’, star formation in galaxies
• Plasma Astrophysics
• plasma instabilities (disks, galaxy clusters, ...), plasma turbulence (incl solar wind)
• Stellar Astrophysics
Thanks for your hospitality!
• stellar seismology, tides