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Bence Kocsis (CFA)

Gas Driven Supermassive Black Hole Binaries: periodic quasar variability and the gravitational wave background. Bence Kocsis (CFA). Einstein Symposium, 10/26/2009. Galaxies merge  ignite quasars  black holes merge AGN variability surveys and Pulsar Timing Arrays detects them.

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Bence Kocsis (CFA)

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  1. Gas Driven Supermassive Black Hole Binaries: periodic quasar variability and thegravitational wave background Bence Kocsis (CFA) Einstein Symposium, 10/26/2009

  2. Galaxies merge  ignite quasars  black holes merge AGN variability surveys and Pulsar Timing Arrays detects them

  3. Evolution of binaries • Collisionless 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

  4. Evolution of binaries • Collisionless 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 Number of binariesreduced at corresponding separation due to gas!

  5. Within the last pc • Thin gaseous disk • Disk aligns with binary plane (Bardeen & Peterson 1975, Ivanov et al. 1999) • Binary evacuates cavity(Artymowicz & Lubov 1994) • Viscous decay (“Type II migration”) • Secondary dominated • Disk dominated • Gravitational Wave driven evolution Cuadra et al. 2009; see also Ivanov et al. 1999; Armitage & Natarayan 2002, 2005; MacFadyen & Milosavljevic 2008;

  6. Within the last pc • Accretion Rate • Thin gaseous disk • Disk aligns with binary plane (Bardeen & Peterson 1975, Ivanov et al. 1999) • Binary evacuates cavity(Artymowicz & Lubov 1994) • Viscous decay (“Type II migration”) • Secondary dominated • Disk dominated • Gravitational Wave driven evolution Cuadra et al. 2009; see also Ivanov et al. 1999; Armitage & Natarayan 2002, 2005; MacFadyen & Milosavljevic 2008;

  7. Within the last pc • Residence Time • Thin gaseous disk • Disk aligns with binary plane (Bardeen & Peterson 1975, Ivanov et al. 1999) • Binary evacuates cavity(Artymowicz & Lubov 1994) • Viscous decay (“Type II migration”) • Secondary dominated • Disk dominated • Gravitational Wave driven evolution Haiman, Kocsis, Menou, 2009, ApJ, 700, 1952

  8. Within the last pc • Residence Time • Thin gaseous disk • Disk aligns with binary plane (Bardeen & Peterson 1975, Ivanov et al. 1999) • Binary evacuates cavity(Artymowicz & Lubov 1994) • Viscous decay (“Type II migration”) • Secondary dominated • Disk dominated • Gravitational Wave driven evolution Haiman, Kocsis, Menou, 2009, ApJ, 700, 1952

  9. Detecting Decaying binaries • Optimistic Assumptions: • binary is producing bright emission (~30% Ledd) • non-negligible fraction (~10%) of this emission is variable • clearly identifiable periodtvar~ torbit • in-spiraling binary = periodically variable quasar • Identifying such binaries statistically? • fraction of quasars with period tvar = (1+z) torb fvar = tres / tQ

  10. Requirements for an (optical) surveyfor finding periodic variable sources • Require: ≥ 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=2 • Conclude: • wide survey best to probe GW-decay • disk physics at i~26.5 Haiman, Kocsis, Menou, 2009, ApJ, 700, 1952

  11. Pulsar Timing Arrays Pulsar Intensity Earth Time

  12. PPTA (Parkes pulsar timing array)‏ NanoGrav (north American nHz observatory for gravitational waves)‏ LEAP (large European array for pulsars)‏

  13. Millennium Run GW background for PTAs • Characteristic 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

  14. Gravitational Waves for PTAs Gas OFF Gas ON Kocsis & Sesana (2009) Unresolved background Contribution of individual sources Spectrum averaged over 1000 Monte Carlo realizations Total signal

  15. Summary • SMBH binaries, gas/GW driven dynamics • AGN surveys • Look for week-month year periodic variability • Look for spectral features~ several x 1,000 km/s • Pulsar Timing Arrays • Gas suppresses the stochastic background • Individually resolvable sources remain

  16. Statistics of resolvable sources basically unaffected Higher signal variance: impossible to characterize the slope of the background a priori

  17. Millennium Run GW background for PTAs • Characteristic gravitational wave (GW) signal • This depends on • Merger history  Millennium Run(Springel et al. 2005; Sesana et al. 2009) • “Residence time” at subparsec scales  From our previous plot

  18. SMBH Merger history • Millennium simulation (Springel et al. 2005) • N-body numerical simulation of halo hierarchy • Semi-analytical models for galaxy formation and evolution • We extract catalogs of merging galaxies and populate them with sensible MBH prescriptions

  19. Cartoon Model of Binary + Gas evolution • Gas cools and settles into a thin circumbinary disk • Disk 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 follows • Analogous to Type – II planetary migration • When local disk mass < binary mass  migration slows down • tGW becomes shorter than tvis when r ~ 100 RS

  20. Punctured disk

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