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Gas Driven Supermassive Black Hole Binaries: periodic quasar
variability and thegravitational wave backgroundBence Kocsis
(CFA)Einstein Symposium, 10/26/2009
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Galaxies merge ignite quasars black holes merge AGN variability
surveys and Pulsar Timing Arrays detects them
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Evolution of binariesCollisionless damping (~kpc; dynamical
friction, Landau damping)3-body encounters with stars (~ 1 pc)Gas
driven migration (~0.1 pc, Type II migration)Gravitational waves
(~0.01 pc)Note: sub-parsec SMBH binaries ~ weeks months orbital
periods~ 103 104 km/s velocity
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Evolution of binariesCollisionless damping (~kpc; dynamical
friction, Landau damping)3-body encounters with stars (~ 1 pc)Gas
driven migration (~0.1 pc, Type II migration)Gravitational waves
(~0.01 pc)Note: sub-parsec SMBH binaries ~ weeks months orbital
periods~ 103 104 km/s velocityNumber of binaries reduced at
corresponding separation due to gas!
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Within the last pcThin gaseous diskDisk aligns with binary plane
(Bardeen & Peterson 1975, Ivanov et al. 1999)Binary evacuates
cavity (Artymowicz & Lubov 1994)Viscous decay (Type II
migration)Secondary dominatedDisk dominatedGravitational Wave
driven evolutionCuadra et al. 2009; see also Ivanov et al. 1999;
Armitage & Natarayan 2002, 2005; MacFadyen & Milosavljevic
2008;
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Within the last pcThin gaseous diskDisk aligns with binary plane
(Bardeen & Peterson 1975, Ivanov et al. 1999)Binary evacuates
cavity (Artymowicz & Lubov 1994)Viscous decay (Type II
migration)Secondary dominatedDisk dominatedGravitational Wave
driven evolutionCuadra et al. 2009; see also Ivanov et al. 1999;
Armitage & Natarayan 2002, 2005; MacFadyen & Milosavljevic
2008;Accretion Rate
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Within the last pcThin gaseous diskDisk aligns with binary plane
(Bardeen & Peterson 1975, Ivanov et al. 1999)Binary evacuates
cavity (Artymowicz & Lubov 1994)Viscous decay (Type II
migration)Secondary dominatedDisk dominatedGravitational Wave
driven evolutionHaiman, Kocsis, Menou, 2009, ApJ, 700,
1952Residence Time
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Within the last pcThin gaseous diskDisk aligns with binary plane
(Bardeen & Peterson 1975, Ivanov et al. 1999)Binary evacuates
cavity (Artymowicz & Lubov 1994)Viscous decay (Type II
migration)Secondary dominatedDisk dominatedGravitational Wave
driven evolutionHaiman, Kocsis, Menou, 2009, ApJ, 700,
1952Residence Time
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Detecting Decaying binariesOptimistic Assumptions:binary is
producing bright emission (~30% Ledd)non-negligible fraction (~10%)
of this emission is variableclearly identifiable period tvar~
torbitin-spiraling binary = periodically variable quasarIdentifying
such binaries statistically?fraction of quasars with period tvar =
(1+z) torbfvar = tres / tQ
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Requirements for an (optical) surveyfor finding periodic
variable sourcesRequire: 100 sources @ tvar 1 yr 5 sources @ tvar
20 wk
Assume: fEdd= 0.3 fvar = 0.1 tQ = 107 yr Hopkins et al. QSOLF @
z=2Conclude: wide survey best to probe GW-decay disk physics at
i~26.5Haiman, Kocsis, Menou, 2009, ApJ, 700, 1952
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Pulsar Timing Arrays
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PPTA (Parkes pulsar timing array) LEAP (large European array for
pulsars) NanoGrav (north American nHz observatory for gravitational
waves)
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GW background for PTAsCharacteristic gravitational wave (GW)
signal
Merger history Millennium Run (Springel et al. 2005; Sesana et
al. 2009)
Residence time at sub-pc scales From our previous plot
Millennium Run
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Gravitational Waves for PTAsGas OFFGas ONContribution of
individual sourcesUnresolved backgroundTotal signalSpectrum
averaged over 1000 Monte Carlo realizationsKocsis & Sesana
(2009)
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SummarySMBH binaries, gas/GW driven dynamics
AGN surveysLook for week-month year periodic variabilityLook for
spectral features ~ several x 1,000 km/s
Pulsar Timing ArraysGas suppresses the stochastic background
Individually resolvable sources remain
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Higher signal variance: impossible to characterize the slope of
the background a prioriStatistics of resolvable sources basically
unaffected
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GW background for PTAsCharacteristic gravitational wave (GW)
signalThis depends onMerger history Millennium Run (Springel et al.
2005; Sesana et al. 2009)Residence time at subparsec scales From
our previous plotMillennium Run
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Millennium simulation (Springel et al. 2005)N-body numerical
simulation of halo hierarchySemi-analytical models for galaxy
formation and evolutionWe extract catalogs of merging galaxies and
populate them with sensible MBH prescriptionsSMBH Merger
history
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Cartoon Model of Binary + Gas evolutionGas cools and settles
into a thin circumbinary diskDisk aligned with binary orbital plane
(Bardeen & Peterson 1975, Ivanov et al. 1999)Torques from
binary evacuate central cavity r ~ 2a(Artymowicz & Lubov
1994)Orbit decays due to torques and viscosity, gas
followsAnalogous to Type II planetary migrationWhen local disk mass
< binary mass migration slows downtGW becomes shorter than tvis
when r ~ 100 RS
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Punctured disk
