The principal goal of this whitepaper is not so much to demonstrate that
gravitational wave detectors like LIGO and LISA will help answer many central
questions in astronomy and astrophysics, but to make the case that they can
help answer a far greater range of questions if we prepare to make the
(sometimes substantial) effort to identify electromagnetic counterparts to the
gravitational wave sources.Comment: Science White Paper submitted to the Astro2010 Decadal Surve

Phinney, E. S.

English

arXiv

The principal goal of this whitepaper is not so much to demonstrate that gravitational wave detectors like LIGO and LISA will help answer many central questions in astronomy and astrophysics, but to make the case that they can help answer a far greater range of questions if we prepare to make the (sometimes substantial) effort to identify electromagnetic counterparts to the gravitational wave sources

Phinney, E. Sterl

Caltech Authors - Main

ar
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v1
  [
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09
Finding and Using Electromagnetic
Counterparts of Gravitational Wave Sources
A white paper for the Astro2010 Decadal Review
E. Sterl Phinney
Theoretical Astrophysics
130-33 California Institute of Technology
Pasadena, CA 91125
esp@tapir.caltech.edu
August 27, 2018
Abstract
The principal goal of this whitepaper is not so much to demonstrate that gravita-
tional wave detectors like LIGO and LISA will help answer many central questions in
astronomy and astrophysics, but to make the case that they can help answer a far
greater range of questions if we prepare to make the (sometimes substantial) effort
to identify electromagnetic counterparts to the gravitational wave sources.
1
1 Extreme Sports in the Universe: past decades
The most dramatic events in the universe —the deaths of stars, the collisions of stellar
remnants and giant black holes, the feeding of the monsters in galactic nuclei —are believed to
produce the most electromagnetically luminous objects in the universe: supernovae, gamma-
ray bursts, quasars.
The objects involved in these events are worthy of study for three reasons: 1) their intrin-
sic interest and the great distances to which they can be seen, 2) the impact of the energy
they release on the rest of the astronomical universe, and 3) (perhaps most significantly)
because they test our theories of matter and energy in ways we cannot hope to do on earth.
They involve the most extreme physical conditions in the present universe: the highest den-
sities of both matter and radiation, the highest magnetic fields, the deepest gravitational
fields, the most relativistic bulk motions.
In previous decades, we have studied these events using just the light emitted from them
(plus a couple of dozen marvelous neutrinos from supernova 1987A). This light comes from
atoms and electrons in tenuous gas, generally far removed from the main action. Because
this action is buried deep behind layers of obscuring gas and in deep gravitational potential
wells, the light we can see has given us many circumstantial clues, but the deepest questions
remain unanswered.
• How and why do some stars explode as supernovae?
• Are the things we call black holes actually precisely the vacuum space-time solutions
of the equations of general relativity?
• How and when do black holes form in stars and in the centers of galaxies?
• Do black holes form in other places?
• How do gas, stars and black holes interact in the nuclei of galaxies and compact clus-
ters?
• What are the internal ingredients of neutron stars?
• What happens when two white dwarfs merge?
2 The next decade: adding information from gravita-
tional waves
Strong gravitational waves are produced by the rapid motion of massive compact bodies:
exactly those extreme objects discussed above. The waves encode the history of those mo-
tions: exactly the information that has proven so difficult to obtain electromagnetically.
Gravitational waves will provide us with a detailed look deep into the interiors of the most
exotic objects in our Universe. Ground-based detectors such as enhanced-LIGO (2009) and
2
advanced LIGO (2014) will detect high-frequency (HF) gravitational waves (∼ 10−1000Hz)
[1]. They can detect the merging of binary black holes, and the tidal disruption and merger of
neutron stars in black hole and neutron star binaries at ∼50Mpc and ∼200Mpc respectively.
They may also be able to detect accretion-induced collapse events, and some types of super-
novae and pulsars [2]. The ESA-NASA space-based mission LISA will detect low-frequency
(LF) gravitational waves (0.1-10mHz) [3, 4]. It can detect merging binary supermassive
black holes (to z ∼ 30), their captures of intermediate mass black holes (to z ∼ 3), and their
captures of the compact objects (stellar mass black holes to z ∼ 1, neutron stars and white
dwarfs to z ∼ 0.1 ) in galactic nuclei. It will measure the masses, spins and distances of all
these objects to precisions unprecedented in astrophysics. It will also measure the orbital
parameters of thousands of ultracompact binary stars in the Milky Way and its satellites.
From the gravitational waveforms we will be able to decode precise information about
the masses, spins, distances, interior properties and space-time dynamics of the detected
sources. This information will be of an accuracy and robustness far exceeding what can
be obtained by electromagnetic measurements. Alone, these will enable precision tests of
general relativity in the strong-field limit[5, 6] and the structure and dynamics of compact
objects, and tell us about the merger rates as a function of cosmic time for supermassive
black holes[7], intermediate mass black holes, and stellar mass black holes, neutron stars and
white dwarf binaries of all types[8, 9].
3 Why electromagnetic counterparts to gravitational
wave sources are so important
However this new information from gravitational waves can have a far broader impact if it
can be put into the context of our existing electromagnetic view of the universe.
Imagine the frustration of measuring gravitational waves from what appears to be the
formation of a rapidly spinning neutron star, but missing the electromagnetic counterpart
that would determine that it was from a massive type Ic supernova in a starburst galaxy,
not an accretion-induced collapse of an accreting white dwarf in the outskirts of an elliptical
galaxy.
And what a lost opportunity if gravitational waves measured the properties of a merging
pair of massive black holes, but we missed the electromagnetic fireworks that would have
enabled us to identify the host high-redshift galaxy and its clustering environment, and to
diagnose the properties of the black holes’ circumbinary gas disk.
Gravitational waves alone will generally determine sky positions only to ∼ degrees 1. In
a few cases, bright electromagnetic events such as nearby gamma-ray bursts or supernovae
may trigger LIGO searches[10]. But most commonly, gravitational wave detections by LIGO
and LISA will have to trigger electromagnetic searches, because many of the predicted coun-
1Tens of degrees for short-lived LF (LISA) sources and high-frequency sources (LIGO), improving for
sources with > year lifetimes to ∼ 0.1 degree for strong LISA sources and arcseconds for strong LIGO
sources.
3
terparts are faint (V magnitudes 18-27) and can be short-lived and inconspicuous. Much
effort on the part of electromagnetic observations will be required to identify them in the
error boxes that may be as large as tens of square degrees. Wide field synoptic survey instru-
ments with a fast cadence are ideal (e.g. Palomar Transient Factory, Pan-Starrs, SkyMapper,
LSST in the optical, plus wide-field or all-sky radio, X-ray and gamma-ray instruments[11]).
Finding electromagnetic counterparts will require rapid, but feasible data analysis and dis-
semination from the gravitational wave detectors (within minutes to hours for short-lived
HF sources, and days to weeks for merging LF sources).
Fortunately, finding counterparts of both HF and LF gravitational wave sources requires
quite similar electromagnetic instrumentation, of a type envisaged for the coming decade.
Flexibility in the cadences and modes of operation will, however, be required.
In the subsections below, we give just a few selected examples of science questions that
could be answered by combining electromagnetic and gravitational wave data on the sources
listed. The tables briefly summarise additional topics for which space precludes discussion.
2.
3.1 What causes some pairs of dead stars to merge, and others
not to?
The fates of interacting binary stars remain poorly understood, despite their importance
for the nature, formation and evolution of Type I (a,b,c) supernovae, binary pulsars, X-ray
binaries and cosmic nucleosynthesis. A priori calculations involve accretion disks, magnetic
fields, and the 3-D radiation hydrodynamics of common-envelope evolution. Parametrised
population synthesis has many uncertainties and difficulties explaining existing population
data, which is however subject to severe selection effects.
LISA will provide a complete Galactic sample orders of magnitude larger than the handful
of ultracompact binaries now known, measuring the individual parameters of about 20,000
ultracompact white dwarf binaries (orbital periods less than around 30 minutes) in the Milky
Way and its satellites [12], along with a less well-determined number of neutron star and
black hole binaries. The frequency f of the gravitational wave signal is twice the orbital
frequency. The polarisation of the gravitational wave signal determines the orbit inclination
and orientation on the sky. The amplitude of the gravitational signal h ∝ f 2/3M5/3/D
depends on the chirp mass M = (M1M2)
3/5(M1 +M2)
−1/5 and distance D to the source. If
the components are not interacting by mass transfer or tides, the orbital frequency evolution
ḟ is determined just by gravitational radiation (and the braking index f̈ f/ḟ 2 = 11/3), and
D can be determined accurately for the ∼ 3000 ultracompact binaries for which ḟ will be
measurable. But for many of the white dwarf binaries, tides and mass transfer will affect
ḟ , even in sign. For these, M can still be determined from h if D is known. GAIA will
measure parallaxes for at least ∼ 400 (more if tidal heating is important) eclipsing binary
2In keeping with US legal tradition “innocent until proven guilty”, we use the term “dead stars” instead
of the more common terms “compact objects” or “white dwarfs, neutron stars, and black holes”, which
presuppose proof beyond a reasonable doubt.
4
white dwarfs, with substantial overlap with the LISA sample, so for these sources, the
combination of EM and GW information will determine the component radii, masses and
orbital dynamics[13]. Identifying these eclipsing white dwarf pairs in the ∼ 1◦ error boxes
from LISA will require fast (minute cadence) wide field synoptic surveys to V = 19, and
followup with high-speed spectroscopy of the identified sources.
Source GW data GW+EM data
ultracompact
binaries (LISA)
complete census
in Milky Way and
satellites, binary
periods, compo-
nent masses, orbit
inclination and
orientation, in- or
out-spiral rates,
formation rate,
merger rate
parallaxes (GAIA) needed for some GW
masses, eclipses: white dwarf radii, white dwarf
temperatures: tidal heating?, tidal synchroni-
sation?, accretion rates and types (disk, direct
impact, magnetic guiding), winds, SN Ia, AIC?
3.2 What happens during the mergers of dead stars, and what
determines the nature of their remnants?
Merging neutron star binaries (and/or neutron stars tidally disrupted in merging neutron
star-black hole binaries) have been proposed as the engines of short-hard gamma-ray bursts,
r-process nucleosynthesis, and ultra-high energy neutrinos. Yet these identifications remain
hypothetical for lack of evidence. Gravitational wave detections will settle the matter. But
because of relativistic beaming, the majority of mergers may not be detectable as gamma-
ray bursts, and such optical and radio afterglows (likely less beamed) as have been seen
are faint, and will require concerted effort from wide-field synoptic survey telescope and
much followup to distinguish them from other transient sources. Yet the payoff will be
tremendous —for example, the gravitational waveform will determine the orbit inclination
as well as component masses and radii and the time of merger. This will provide vastly
improved constraints on models of the electromagnetic emission as a function of angle from
the angular momentum axis.
Similarly, a combination of gravitational wave and electromagnetic detections could im-
mediately clarify our uncertainty about what happens when two white dwarfs or a white
dwarf and a neutron star accrete and merge: outcomes range from neutron stars or black
holes (accretion-induced collapse, AIC) to various explosive ejections up to type Ia super-
novae.
5
Source GW data GW+EM data
NS-BH (LIGO) masses, spins, NS
radius/structural
parameters, orbit
inclination and
orientation
Gamma-rays (GRB?), jet and outflow (after-
glow) properties as function of GW-determined
inclination, neutrinos, nucleosynthesis.
NS-NS (LIGO) masses, neu-
tron star
radii/structural
parameters
Gamma-rays (GRB?), jet and outflow (after-
glow) properties as function of GW-determined
inclination, neutrinos, nucleosynthesis.
WD-WD, WD-NS
(LIGO, LISA)
final NS or BH
mass, radius, spin
nucleosynthesis, outflow energetics, explosion
vs remnant?, magnetic field of AIC remnant
3.3 What are the fates of stars and stellar remnants near the giant
black holes in galactic nuclei?
Observations of the center of our Milky Way and other nearby galaxies have revealed that
short-lived massive stars form surprisingly close to the supermassive black holes that lurk
there. Mass segregation implies that black hole remnants of such stars should be the dom-
inant population close to the black hole, and they would provide valuable clues to the star
formation history and stellar dynamics in galactic nuclei. Detection of these is only possible
through gravitational radiation. White dwarfs may be tidally disrupted late in their inspiral,
providing both electromagnetic and gravitational wave signatures and the combination of
gravitational wave determined distance (see section 3.1) and electromagnetic redshift would
enable precision cosmography [14, 15]. These sources also will provide maps of the spacetime
geometry of unprecedented precision (black hole multipole moments with precision as high
as ∼ 10−4). While these enable tests of strong-field general relativity ([5]), comparison of the
unambiguously known masses and spins for a population of quiescent and active black holes
will at last provide a firm basis for electromagnetic models of accretion and the evolution of
black holes in galactic nuclei ([8, 7]).
Source GW data GW+EM data
extreme mass
ratio inspiral onto
massive black
holes in galactic
nuclei (LISA)
mass, spin of mas-
sive black hole,
mass of compact
object, distance,
high-precision
spacetime, merger
rate and dead star
mass function in
local universe,
orbit eccentric-
ity, inclination,
orientation
redshift, host galaxy ID, properties and dy-
namics of host nucleus, GW advance notice for
EM study of tidal disruption of white dwarfs,
brown dwarfs of known M,R.
6
3.4 How and when do black holes form in the centers of galaxies?
The field of electromagnetic counterparts of merging supermassive black holes has burgeoned
in the past few years [16, 15]. Signatures range from prompt optical, UV and X-ray flares
to year-timescale events in infrared and other bands. Identification of these transients will
enable us to identify the host galaxy, testing models of galaxy mergers, and also, because
of the known time and amplitude of disk perturbation around a black hole of known mass,
potentially revolutionise the study of accretion disks, as well as testing modified gravity
models by comparing the electromagnetic and gravitational z −DL(z) relations [15].
Source GW data GW+EM data
merging massive
or intermediate
mass black holes
(LISA)
masses, spins
(initial and final),
distances, merger
rates, spacetime
dynamics
host galaxy morphology, luminosity, dynam-
ics, redshift; circumbinary gas disk properties
and response to mass loss, kicks, gravitational
waves; magnetodynamics
3.5 How and why do some stars explode as supernovae?
Most supernovae probably are not strong sources of gravitational waves, but the most inter-
esting ones (e.g. rapidly rotating hypernovae implicated in long-duration gamma-ray bursts,
magnetar progenitors, etc) may be [17]. These gravitational waveforms will likely only be
identified in coincidence with electromagnetic observations, but would provide a view of the
interior dynamics of supernova unobtainable in any other way.
Source GW data GW+EM data
Supernovae
(LIGO)
extreme core
(magneto) hy-
drodynamics
and instabilities,
rotation, bars
relation of core dynamics to explosion type,
progenitors, nucleosynthesis, black hole vs neu-
tron star remnant
3.6 What are the internal ingredients of neutron stars?
Rapidly rotating, magnetised and accreting neutron stars may not be axisymmetric. Grav-
itational waves can provide one of our few ways to determine the interior structure and
motions[2].
Source GW data GW+EM data
radio and X-ray
pulsars, low-mass
X-ray binaries
(LIGO)
spin, interior
structures, neu-
tron star moun-
tains and mass
multipole mo-
ments induced by
interior magnetic
fields or accretion
accretion rate, spin up/down, exterior mag-
netic field, detection of core-crust differential
rotation, thermal properties
7
References
[1] “Advanced LIGO: Reference Design and Astrophysical Targets”
http://www.ligo.caltech.edu/advLIGO
[2] M. Vigelius, D. Payne, A. Melatos “Gravitational Radiation from Accreting Millisecond
Pulsars” (2008) http://arxiv.org/abs/0811.2031
[3] T. Prince et al, “The Promise of Low-Frequency Gravitational Wave Astronomy”, As-
tro2010 WP (2009)
[4] LISA Mission and Science Documents, including the science case document are at
http://www.lisa-science.org/resources/mission/mission_body.
[5] B. Schutz et al “Will Einstein Have the Last Word on Gravity?”, Astro2010 WP (2009)
[6] S.E. Whitcomb et al “Probing Gravity and Cosmology with Gravitational Waves”,
Astro2010 WP (2009).
[7] P. Madau et al, “Massive Black Holes Across Cosmic Time”, Astro2010 WP (2009)
[8] M.C. Miller et al “Probing Stellar Dynamics in Galactic Nuclei”, Astro2010 WP (2009)
[9] G. Nelemans et al, “The Astrophysics of Ultra-Compact Binaries”, Astro2010 WP
(2009)
[10] B. Abbott et al “Astrophysically triggeredsearches for gravitational waves: status and
prospects” (2008)Class Quant Grav 25, 114051 http://arxiv.org/abs/0802.4320
[11] J. Bloom et al, “Coordinated Science in the Gravitational and Electromagnetic Skies”,
Astro2010 WP (2009) http://arxiv.org/abs/0902.1527
[12] M. Vallisneri “A LISA Data-Analysis Primer”, http://arXiv.org/abs/0812.0751
(2009).
[13] A. Stroeer, A. Vecchio & G. Nelemans “LISA astronomy of double white dwarf binary
systems” ApJ 633, L33 (2005) http://arxiv.org/abs/astro-ph/0509632
[14] C. Hogan et al, “Precision Cosmology with Gravitational Waves”, Astro2010 WP
(2009a)
[15] Z. Haiman et al (2008) “Identifying Decaying Supermassive Black Hole Binaries from
their Variable Electromagnetic Emission” http://arxiv.org/abs/0811.1920
[16] M. Milosavljevic, E.S. Phinney “The Afterglow of Massive Black Hole Coalescence” ApJ
622, L93 (2005) http://arxiv.org/abs/astro-ph/0410343
[17] C. D. Ott “The Gravitational Wave Signature of Core-Collapse Supernovae” (2009)
http://arxiv.org/abs/0809.0695
8


Finding and Using Electromagnetic Counterparts of Gravitational Wave

  Sources

ar
X
iv
:0
90
3.
00
98
v1
  [
as
tro
-p
h.C
O]
  2
8 F
eb
 20
09
Finding and Using Electromagnetic
Counterparts of Gravitational Wave Sources
A white paper for the Astro2010 Decadal Review
E. Sterl Phinney
Theoretical Astrophysics
130-33 California Institute of Technology
Pasadena, CA 91125
esp@tapir.caltech.edu
August 27, 2018
Abstract
The principal goal of this whitepaper is not so much to demonstrate that gravita-
tional wave detectors like LIGO and LISA will help answer many central questions in
astronomy and astrophysics, but to make the case that they can help answer a far
greater range of questions if we prepare to make the (sometimes substantial) effort
to identify electromagnetic counterparts to the gravitational wave sources.
1
1 Extreme Sports in the Universe: past decades
The most dramatic events in the universe —the deaths of stars, the collisions of stellar
remnants and giant black holes, the feeding of the monsters in galactic nuclei —are believed to
produce the most electromagnetically luminous objects in the universe: supernovae, gamma-
ray bursts, quasars.
The objects involved in these events are worthy of study for three reasons: 1) their intrin-
sic interest and the great distances to which they can be seen, 2) the impact of the energy
they release on the rest of the astronomical universe, and 3) (perhaps most significantly)
because they test our theories of matter and energy in ways we cannot hope to do on earth.
They involve the most extreme physical conditions in the present universe: the highest den-
sities of both matter and radiation, the highest magnetic fields, the deepest gravitational
fields, the most relativistic bulk motions.
In previous decades, we have studied these events using just the light emitted from them
(plus a couple of dozen marvelous neutrinos from supernova 1987A). This light comes from
atoms and electrons in tenuous gas, generally far removed from the main action. Because
this action is buried deep behind layers of obscuring gas and in deep gravitational potential
wells, the light we can see has given us many circumstantial clues, but the deepest questions
remain unanswered.
• How and why do some stars explode as supernovae?
• Are the things we call black holes actually precisely the vacuum space-time solutions
of the equations of general relativity?
• How and when do black holes form in stars and in the centers of galaxies?
• Do black holes form in other places?
• How do gas, stars and black holes interact in the nuclei of galaxies and compact clus-
ters?
• What are the internal ingredients of neutron stars?
• What happens when two white dwarfs merge?
2 The next decade: adding information from gravita-
tional waves
Strong gravitational waves are produced by the rapid motion of massive compact bodies:
exactly those extreme objects discussed above. The waves encode the history of those mo-
tions: exactly the information that has proven so difficult to obtain electromagnetically.
Gravitational waves will provide us with a detailed look deep into the interiors of the most
exotic objects in our Universe. Ground-based detectors such as enhanced-LIGO (2009) and
2
advanced LIGO (2014) will detect high-frequency (HF) gravitational waves (∼ 10−1000Hz)
[1]. They can detect the merging of binary black holes, and the tidal disruption and merger of
neutron stars in black hole and neutron star binaries at ∼50Mpc and ∼200Mpc respectively.
They may also be able to detect accretion-induced collapse events, and some types of super-
novae and pulsars [2]. The ESA-NASA space-based mission LISA will detect low-frequency
(LF) gravitational waves (0.1-10mHz) [3, 4]. It can detect merging binary supermassive
black holes (to z ∼ 30), their captures of intermediate mass black holes (to z ∼ 3), and their
captures of the compact objects (stellar mass black holes to z ∼ 1, neutron stars and white
dwarfs to z ∼ 0.1 ) in galactic nuclei. It will measure the masses, spins and distances of all
these objects to precisions unprecedented in astrophysics. It will also measure the orbital
parameters of thousands of ultracompact binary stars in the Milky Way and its satellites.
From the gravitational waveforms we will be able to decode precise information about
the masses, spins, distances, interior properties and space-time dynamics of the detected
sources. This information will be of an accuracy and robustness far exceeding what can
be obtained by electromagnetic measurements. Alone, these will enable precision tests of
general relativity in the strong-field limit[5, 6] and the structure and dynamics of compact
objects, and tell us about the merger rates as a function of cosmic time for supermassive
black holes[7], intermediate mass black holes, and stellar mass black holes, neutron stars and
white dwarf binaries of all types[8, 9].
3 Why electromagnetic counterparts to gravitational
wave sources are so important
However this new information from gravitational waves can have a far broader impact if it
can be put into the context of our existing electromagnetic view of the universe.
Imagine the frustration of measuring gravitational waves from what appears to be the
formation of a rapidly spinning neutron star, but missing the electromagnetic counterpart
that would determine that it was from a massive type Ic supernova in a starburst galaxy,
not an accretion-induced collapse of an accreting white dwarf in the outskirts of an elliptical
galaxy.
And what a lost opportunity if gravitational waves measured the properties of a merging
pair of massive black holes, but we missed the electromagnetic fireworks that would have
enabled us to identify the host high-redshift galaxy and its clustering environment, and to
diagnose the properties of the black holes’ circumbinary gas disk.
Gravitational waves alone will generally determine sky positions only to ∼ degrees 1. In
a few cases, bright electromagnetic events such as nearby gamma-ray bursts or supernovae
may trigger LIGO searches[10]. But most commonly, gravitational wave detections by LIGO
and LISA will have to trigger electromagnetic searches, because many of the predicted coun-
1Tens of degrees for short-lived LF (LISA) sources and high-frequency sources (LIGO), improving for
sources with > year lifetimes to ∼ 0.1 degree for strong LISA sources and arcseconds for strong LIGO
sources.
3
terparts are faint (V magnitudes 18-27) and can be short-lived and inconspicuous. Much
effort on the part of electromagnetic observations will be required to identify them in the
error boxes that may be as large as tens of square degrees. Wide field synoptic survey instru-
ments with a fast cadence are ideal (e.g. Palomar Transient Factory, Pan-Starrs, SkyMapper,
LSST in the optical, plus wide-field or all-sky radio, X-ray and gamma-ray instruments[11]).
Finding electromagnetic counterparts will require rapid, but feasible data analysis and dis-
semination from the gravitational wave detectors (within minutes to hours for short-lived
HF sources, and days to weeks for merging LF sources).
Fortunately, finding counterparts of both HF and LF gravitational wave sources requires
quite similar electromagnetic instrumentation, of a type envisaged for the coming decade.
Flexibility in the cadences and modes of operation will, however, be required.
In the subsections below, we give just a few selected examples of science questions that
could be answered by combining electromagnetic and gravitational wave data on the sources
listed. The tables briefly summarise additional topics for which space precludes discussion.
2.
3.1 What causes some pairs of dead stars to merge, and others
not to?
The fates of interacting binary stars remain poorly understood, despite their importance
for the nature, formation and evolution of Type I (a,b,c) supernovae, binary pulsars, X-ray
binaries and cosmic nucleosynthesis. A priori calculations involve accretion disks, magnetic
fields, and the 3-D radiation hydrodynamics of common-envelope evolution. Parametrised
population synthesis has many uncertainties and difficulties explaining existing population
data, which is however subject to severe selection effects.
LISA will provide a complete Galactic sample orders of magnitude larger than the handful
of ultracompact binaries now known, measuring the individual parameters of about 20,000
ultracompact white dwarf binaries (orbital periods less than around 30 minutes) in the Milky
Way and its satellites [12], along with a less well-determined number of neutron star and
black hole binaries. The frequency f of the gravitational wave signal is twice the orbital
frequency. The polarisation of the gravitational wave signal determines the orbit inclination
and orientation on the sky. The amplitude of the gravitational signal h ∝ f 2/3M5/3/D
depends on the chirp massM = (M1M2)
3/5(M1 +M2)
−1/5 and distance D to the source. If
the components are not interacting by mass transfer or tides, the orbital frequency evolution
f˙ is determined just by gravitational radiation (and the braking index f¨ f/f˙ 2 = 11/3), and
D can be determined accurately for the ∼ 3000 ultracompact binaries for which f˙ will be
measurable. But for many of the white dwarf binaries, tides and mass transfer will affect
f˙ , even in sign. For these, M can still be determined from h if D is known. GAIA will
measure parallaxes for at least ∼ 400 (more if tidal heating is important) eclipsing binary
2In keeping with US legal tradition “innocent until proven guilty”, we use the term “dead stars” instead
of the more common terms “compact objects” or “white dwarfs, neutron stars, and black holes”, which
presuppose proof beyond a reasonable doubt.
4
white dwarfs, with substantial overlap with the LISA sample, so for these sources, the
combination of EM and GW information will determine the component radii, masses and
orbital dynamics[13]. Identifying these eclipsing white dwarf pairs in the ∼ 1◦ error boxes
from LISA will require fast (minute cadence) wide field synoptic surveys to V = 19, and
followup with high-speed spectroscopy of the identified sources.
Source GW data GW+EM data
ultracompact
binaries (LISA)
complete census
in Milky Way and
satellites, binary
periods, compo-
nent masses, orbit
inclination and
orientation, in- or
out-spiral rates,
formation rate,
merger rate
parallaxes (GAIA) needed for some GW
masses, eclipses: white dwarf radii, white dwarf
temperatures: tidal heating?, tidal synchroni-
sation?, accretion rates and types (disk, direct
impact, magnetic guiding), winds, SN Ia, AIC?
3.2 What happens during the mergers of dead stars, and what
determines the nature of their remnants?
Merging neutron star binaries (and/or neutron stars tidally disrupted in merging neutron
star-black hole binaries) have been proposed as the engines of short-hard gamma-ray bursts,
r-process nucleosynthesis, and ultra-high energy neutrinos. Yet these identifications remain
hypothetical for lack of evidence. Gravitational wave detections will settle the matter. But
because of relativistic beaming, the majority of mergers may not be detectable as gamma-
ray bursts, and such optical and radio afterglows (likely less beamed) as have been seen
are faint, and will require concerted effort from wide-field synoptic survey telescope and
much followup to distinguish them from other transient sources. Yet the payoff will be
tremendous —for example, the gravitational waveform will determine the orbit inclination
as well as component masses and radii and the time of merger. This will provide vastly
improved constraints on models of the electromagnetic emission as a function of angle from
the angular momentum axis.
Similarly, a combination of gravitational wave and electromagnetic detections could im-
mediately clarify our uncertainty about what happens when two white dwarfs or a white
dwarf and a neutron star accrete and merge: outcomes range from neutron stars or black
holes (accretion-induced collapse, AIC) to various explosive ejections up to type Ia super-
novae.
5
Source GW data GW+EM data
NS-BH (LIGO) masses, spins, NS
radius/structural
parameters, orbit
inclination and
orientation
Gamma-rays (GRB?), jet and outflow (after-
glow) properties as function of GW-determined
inclination, neutrinos, nucleosynthesis.
NS-NS (LIGO) masses, neu-
tron star
radii/structural
parameters
Gamma-rays (GRB?), jet and outflow (after-
glow) properties as function of GW-determined
inclination, neutrinos, nucleosynthesis.
WD-WD, WD-NS
(LIGO, LISA)
final NS or BH
mass, radius, spin
nucleosynthesis, outflow energetics, explosion
vs remnant?, magnetic field of AIC remnant
3.3 What are the fates of stars and stellar remnants near the giant
black holes in galactic nuclei?
Observations of the center of our Milky Way and other nearby galaxies have revealed that
short-lived massive stars form surprisingly close to the supermassive black holes that lurk
there. Mass segregation implies that black hole remnants of such stars should be the dom-
inant population close to the black hole, and they would provide valuable clues to the star
formation history and stellar dynamics in galactic nuclei. Detection of these is only possible
through gravitational radiation. White dwarfs may be tidally disrupted late in their inspiral,
providing both electromagnetic and gravitational wave signatures and the combination of
gravitational wave determined distance (see section 3.1) and electromagnetic redshift would
enable precision cosmography [14, 15]. These sources also will provide maps of the spacetime
geometry of unprecedented precision (black hole multipole moments with precision as high
as ∼ 10−4). While these enable tests of strong-field general relativity ([5]), comparison of the
unambiguously known masses and spins for a population of quiescent and active black holes
will at last provide a firm basis for electromagnetic models of accretion and the evolution of
black holes in galactic nuclei ([8, 7]).
Source GW data GW+EM data
extreme mass
ratio inspiral onto
massive black
holes in galactic
nuclei (LISA)
mass, spin of mas-
sive black hole,
mass of compact
object, distance,
high-precision
spacetime, merger
rate and dead star
mass function in
local universe,
orbit eccentric-
ity, inclination,
orientation
redshift, host galaxy ID, properties and dy-
namics of host nucleus, GW advance notice for
EM study of tidal disruption of white dwarfs,
brown dwarfs of known M,R.
6
3.4 How and when do black holes form in the centers of galaxies?
The field of electromagnetic counterparts of merging supermassive black holes has burgeoned
in the past few years [16, 15]. Signatures range from prompt optical, UV and X-ray flares
to year-timescale events in infrared and other bands. Identification of these transients will
enable us to identify the host galaxy, testing models of galaxy mergers, and also, because
of the known time and amplitude of disk perturbation around a black hole of known mass,
potentially revolutionise the study of accretion disks, as well as testing modified gravity
models by comparing the electromagnetic and gravitational z −DL(z) relations [15].
Source GW data GW+EM data
merging massive
or intermediate
mass black holes
(LISA)
masses, spins
(initial and final),
distances, merger
rates, spacetime
dynamics
host galaxy morphology, luminosity, dynam-
ics, redshift; circumbinary gas disk properties
and response to mass loss, kicks, gravitational
waves; magnetodynamics
3.5 How and why do some stars explode as supernovae?
Most supernovae probably are not strong sources of gravitational waves, but the most inter-
esting ones (e.g. rapidly rotating hypernovae implicated in long-duration gamma-ray bursts,
magnetar progenitors, etc) may be [17]. These gravitational waveforms will likely only be
identified in coincidence with electromagnetic observations, but would provide a view of the
interior dynamics of supernova unobtainable in any other way.
Source GW data GW+EM data
Supernovae
(LIGO)
extreme core
(magneto) hy-
drodynamics
and instabilities,
rotation, bars
relation of core dynamics to explosion type,
progenitors, nucleosynthesis, black hole vs neu-
tron star remnant
3.6 What are the internal ingredients of neutron stars?
Rapidly rotating, magnetised and accreting neutron stars may not be axisymmetric. Grav-
itational waves can provide one of our few ways to determine the interior structure and
motions[2].
Source GW data GW+EM data
radio and X-ray
pulsars, low-mass
X-ray binaries
(LIGO)
spin, interior
structures, neu-
tron star moun-
tains and mass
multipole mo-
ments induced by
interior magnetic
fields or accretion
accretion rate, spin up/down, exterior mag-
netic field, detection of core-crust differential
rotation, thermal properties
7
References
[1] “Advanced LIGO: Reference Design and Astrophysical Targets”
http://www.ligo.caltech.edu/advLIGO
[2] M. Vigelius, D. Payne, A. Melatos “Gravitational Radiation from Accreting Millisecond
Pulsars” (2008) http://arxiv.org/abs/0811.2031
[3] T. Prince et al, “The Promise of Low-Frequency Gravitational Wave Astronomy”, As-
tro2010 WP (2009)
[4] LISA Mission and Science Documents, including the science case document are at
http://www.lisa-science.org/resources/mission/mission_body.
[5] B. Schutz et al “Will Einstein Have the Last Word on Gravity?”, Astro2010 WP (2009)
[6] S.E. Whitcomb et al “Probing Gravity and Cosmology with Gravitational Waves”,
Astro2010 WP (2009).
[7] P. Madau et al, “Massive Black Holes Across Cosmic Time”, Astro2010 WP (2009)
[8] M.C. Miller et al “Probing Stellar Dynamics in Galactic Nuclei”, Astro2010 WP (2009)
[9] G. Nelemans et al, “The Astrophysics of Ultra-Compact Binaries”, Astro2010 WP
(2009)
[10] B. Abbott et al “Astrophysically triggeredsearches for gravitational waves: status and
prospects” (2008)Class Quant Grav 25, 114051 http://arxiv.org/abs/0802.4320
[11] J. Bloom et al, “Coordinated Science in the Gravitational and Electromagnetic Skies”,
Astro2010 WP (2009) http://arxiv.org/abs/0902.1527
[12] M. Vallisneri “A LISA Data-Analysis Primer”, http://arXiv.org/abs/0812.0751
(2009).
[13] A. Stroeer, A. Vecchio & G. Nelemans “LISA astronomy of double white dwarf binary
systems” ApJ 633, L33 (2005) http://arxiv.org/abs/astro-ph/0509632
[14] C. Hogan et al, “Precision Cosmology with Gravitational Waves”, Astro2010 WP
(2009a)
[15] Z. Haiman et al (2008) “Identifying Decaying Supermassive Black Hole Binaries from
their Variable Electromagnetic Emission” http://arxiv.org/abs/0811.1920
[16] M. Milosavljevic, E.S. Phinney “The Afterglow of Massive Black Hole Coalescence” ApJ
622, L93 (2005) http://arxiv.org/abs/astro-ph/0410343
[17] C. D. Ott “The Gravitational Wave Signature of Core-Collapse Supernovae” (2009)
http://arxiv.org/abs/0809.0695
8


Finding and Using Electromagnetic Counterparts of Gravitational Wave Sources

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