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Stellar Tidal Disruption Flares: an EM Signature of Black Hole Merger and Recoil

Stellar Tidal Disruption Flares: an EM Signature of Black Hole Merger and Recoil. Nicholas Stone in collaboration with Avi Loeb GWPAW – Milwaukee – 1/ 28 /11. Motivation. EM counterpart necessary to study host galaxy properties, SMBH population statistics

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Stellar Tidal Disruption Flares: an EM Signature of Black Hole Merger and Recoil

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  1. Stellar Tidal Disruption Flares: an EM Signature of Black Hole Merger and Recoil Nicholas Stone in collaboration with Avi Loeb GWPAW – Milwaukee – 1/28/11

  2. Motivation • EM counterpart necessary to study host galaxy properties, SMBH population statistics • If EM counterpart exists, BH mergers could be used as standard sirens • Precision cosmology independent of the standard cosmological distance ladder (Holz & Hughes 2005) • Previous proposed EM counterparts require uncertain premerger accretion flows • We propose flares from tidally disrupted stars as prompt and perhaps repeating EM signatures for a wide class of SMBH mergers • Key numerical relativity prediction: high-velocity (>100 km/s) recoils as generic feature of black hole mergers

  3. Supermassive Black Hole Mergers • SMBH binaries regularly form as consequence of hierarchical galaxy evolution • Final parsec problem: • Dynamical friction can reduce abin to ~pc scales • But GW emission only merges in less than a Hubble time on ≤mpc scales • Possible solutions (Milosavljevic & Merritt 2003): • Collisional relaxation (effective only for MBH<107M) • Significant nuclear triaxiality • Presence of accreting gas (also suppresses vk)

  4. Black Hole Recoil • Numerical relativity simulations increasingly convergent between groups (Lousto et al. 2010) • Gas accretion can align spins, suppress large vk (Bogdanovic et al 2007) • Post-Newtonian resonances could also align spins (Kesden et al 2010) Lousto 2010 vk distribution: -Unaligned spins -30° alignment -10° alignment

  5. Tidal Disruption Events (TDEs) • Tidal disruption radius • Above ~108 M, rt≤rs • Exception: Kerr BHs, up to ~5x108 M(Beloborodov et al. 1992) • At least half the stellar mass unbound with large spread in energy • Mass fallback rate • Supernova-like UV/X-ray emission, some optical • Observed rate ~10-5 /galaxy/yr • Donley et al. 2002 Evans & Kochanek 1989 Strubbe & Quataert 2009

  6. Tidal Disruption Rates • For a stationary SMBH, governed by relaxation into 6D loss cone (LC) • Theoretical estimates 10-4 – 10-6 stars/yr • Rates highest in small, cuspy galaxies • SMBH recoil instantaneously shifts phase space and refills loss cone • Loss cone drains on a dynamical time (<< relaxational time) • TDE rate up to 104-5x stationary SMBH rate Merritt & Milosavljevic 2003

  7. Our Model • Phase space shift could identify recoil in two ways • TDE signal after LISA signal • Repeating TDEs within one galaxy • Use pre-coalescence distribution functions of stars, f(J, E) • Then shift coordinates in velocity space, and integrate over new loss cone to get total number of draining stars • Cuts in energy limit us to short period (<100 yr) stars • Two models for f(J, E) • Wet merger • Dry merger

  8. Dry Mergers • Final parsec problem solved by • Collisional relaxation (if MBH<107M) • Triaxiality • These lead respectively to the following density profiles ρ=kr-γ: • Joint core-cusp profiles (transition at 0.2rinfl) • Cores • Therefore we consider both core galaxies (γ=1) and the joint (γ=1, 1.75) result of Merritt et al 2007 • Salpeter mass function

  9. Dry Mergers: Pre-Merger Loss Cone • SMBHs decouple from stellar population when , at separation aE • Remove all stars with a<aE • But relaxation in J is faster than in E • To fill a gap in J-space takes • So there is a second decoupling (aJ) when Tgap>TGW • Remove all stars with pericentersrp<aJ

  10. Dry Mergers: Results • N<(t) is the number of stars disrupted < t years after SMBH merger • As mass increases: • More stars in post-kick LC • Orbital periods in post-kick LC increase • As velocity increases: • Overlap between post- and pre-kick LCs shrink • Fewer stars remain on bound orbits

  11. Dry Mergers: Results • The first post-merger TDEs occurs sooner for: • Higher kicks (up to a point) • Lighter SMBHs • The opposite characteristics lead to more total post-merger TDEs • Pure core models produce negligible TDEs

  12. Wet Mergers • Large accretion flows can solve final parsec problem • Will dynamically produce low-density stellar core => no post-kick TDEs? • But – two factors could dramatically increase N<(t) • Star formation • Disk migration • We model f(J,E) with a simple power-law cusp • We set the inner boundary for pericenters to where TGW=Tvisc • Note that large vk will be suppressed

  13. Wet Mergers: Results • Much higher values of N<(100) • Sequential TDEs detectable on timescale of years • Significantly more uncertainties in this model • Star formation • Resonances with disk • Wide range of disk parameters • Note that we assume (M, R) for all stars

  14. Other Factors • Cosmological enhancement • Higher rate, longer delay until first event? • Unequal mass SMBH binaries • Resonance in dry mergers • Resonant capture can in principal migrate stars inward as binary hardens • Demonstrated for the 1:1 Trojan resonance by Seto & Muto 2010 • Could be relevant for higher-order mean-motion resonances also – we are currently investigating this

  15. Conclusions • The phase space shift caused by BH recoil will: • Produce TDEs at a time t~10s of years after GW signal for dry mergers • Perhaps produce repeating TDEs for wet mergers at t~few years after GW signal • The dry merger rates could be dramatically enhanced if MMRs can migrate 10s-100s of stars • Time domain surveys in LISA era can use this effect for localization of SMBH merger • Confirm strong GR predictions • Precision cosmology (standard sirens) • Independent confirmation of recoil possible if repeating TDEs observed • Calibration of LISA event rate

  16. Questions?

  17. Observational Constraints • Time-domain surveys expected to observe ~10s-1000s of TDEs/yr (Gezari et al. 2009, Strubbe & Quataert 2009) • LSST particularly promising • Spatial offsets: we assume LSST resolution ~0.8” • With photometric subtraction of bulge astrometric precision is FWHM/SNR • We assume SNR~10 in our calculations, so detectable offsets of ~0.08” • Kinematic offsets: • UV spectral followup ideal, but uncertain in LSST era • Next best is X-ray, we consider SXS (ASTRO-H) as example • 7eV resolution at 10 keV => ~200 km/s offsets detectable if wind velocity is small or can be firmly modeled

  18. Tidal Disruption Flares • Recent work (Strubbe & Quataert 2009, 2010) models lightcurves/spectra in more detail • Accretion torus radiates in the UV/soft X-ray for ~months to ~years • Becomes bluer with time • Optical and line emission from unbound gas • Possible super-Eddington outflow lasting ~weeks • Dynamics not settled, but super-Eddington outflows potentially highly luminous in optical (~1043-44 erg/s) Strubbe & Quataert 2009

  19. A Kinematic Recoil Candidate • Interpretation of this spectra, by Komossa et al. 2008, has since been disputed • Other possibilities: • SMBH binary • Chance quasar superposition

  20. Absorption in Super-Eddington Outflows • Predicted by Strubbe & Quataert 2010 (SQ) and Loeb & Ulmer 1997 (LU) for very different super-Eddington models • LU scenario: radiation pressure isotropizes returning debris • Radiation pressure supports quasi-spherical envelope with smaller accretion disk in center • X-ray/UV absorption lines on surface of envelope, thermally broadened ~10s km/s • SQ scenario: super-Eddington fallback launches polar wind • Wind speed highly uncertain, but features X-ray/UV absorption lines • Spectral detection not feasible if vwind>>vkick

  21. LISA Localization Capabilities • LISA taskforce estimates: • 8.2 events/yr localized to within 10 deg2 • 2.2 events/yr localized to within 1 deg2 • Holz & Hughes 2005 provide galaxy column density

  22. Eliminating Sources of Confusion • Triple SMBH systems with gravitational slingshot • Presence of 1 or more SMBH in galactic center (Civano et al. 2010) • Host galaxies have very large mass deficits, velocity anisotropy (Iwasawa et al 2008) • No GW signal • SMBH binaries • Very hard (<pc) scale binaries will display interrupted tidal flares • Wider binaries potentially resolvable (spatially or spectrally) • No TDE kinematic offset for Kozai scenario • No GW signal

  23. Observability • Time-domain sky surveys expected to observe ~10s-1000s of TDEs/yr (Gezari et al. 2009, Strubbe & Quataert 2009) • LSST particularly promising • Higher numbers (1000s/yr) if super-Eddington outflows behave as in Strubbe & Quataert 2009 • Two ways to verify a recoil-associated TDE • Spatial offsets • Spectral offset between host galaxy and absorption lines in super-Eddington outflow (less certain)

  24. Observational Constraints • Peak optical luminosity ~1040-42 erg/s for disk, ~1043-44 for super-Eddington outflows • Spatial offsets: we assume LSST resolution ~0.8” • With photometric subtraction of bulge astrometric precision is FWHM/SNR • We assume SNR~10 in our calculations, so detectable offsets of ~0.08” • Kinematic offsets: • UV spectral followup ideal, but uncertain in LSST era • Next best is X-ray, we consider SXS (ASTRO-H) as example • 7eV resolution at 10 keV => ~200 km/s offsets detectable if wind velocity is small or can be firmly modeled

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