Signatures of Supermassive Black Hole Coalescence

1. Signatures of Supermassive Black Hole Coalescence 14 Nov 2011 STScI Colloquium • Tamara Bogdanović • Einstein Postdoctoral Fellow • University of Maryland

2. Antennae galaxy Credit: NASA/CXC/MPE/S. Komossa et al. NGC 6240 B. Whitmore (STSci), F. Schweizer (DTM), NASA Credit: NASA/JPL/MPI/SSC Mice galaxy Stephan’s Quintet Credit: NASA/ESA-ACS Science Team SBHBs: Formation and Evolution • Galactic merger • Interaction with stars & gas • GW phase • Coalescence • GW recoil

3. GW astrophysics of SBHBs Credit: www.srl.caltech.edu/lisa/graphics/master.html • SBHBs are one of the prime GW sources for LISA/NGO/eLISA/LISAlight. • GWs imprinted with detailed information about the system • New “window” into the universe

4. Synergy of EM and GW signatures • Observational best case scenario: coincident detections of EM and GW signatures. • May be a while before GW interferometer is launched − EM a critical tool for ≳10 yr • Likelihood of observable EM counterpart determined by the physical properties of binary environment. • Knowledge about these systems so far drawn from theoretical considerations.

5. Non-relativistic simulations of mergers • Large dynamic range: from galactic mergers (~10s of kpc) to coalescence (~μpc). Simulations spanning the entire dynamical range still prohibitively computationally expensive. 10s kpc sub-pc 10s pc sub-pc Mayer+ 07 Escala+ 05 Hayasaki+ 07 Cuadra+ 09

6. Recently: non-relativistic MHD simulation Shi, Krolik, Lubow, & Hawley 11 • First MHD study of disk structures and angular momentum transport density with density weighted velocity field

7. Relativistic simulations of mergers • Surrounded by EM fields • Surrounded by test particles (van Meter+ 09) (Palenzuela+ 09, 10; Mösta+ 10, 11, Neilsen+ 2010) • Surrounded by matter (Bode+ 10, 11; Farris+ 10, 11)

8. Environment in vicinity of a coalescing binary • How EM counterpart depends on the mass ratio, spins and environment of the binary? • Circumbinary disk model: Cooling is efficient, the gas settles into a rotationally supported, geometrically thin accretion disk. • Radiatively inefficient hot gas flow: Cooling is inefficient, the BBH is immersed in a pressure supported, geometrically thick torus.

9. Merger of SBHs in a hot accretion flow • Fully relativistic hydro study (with Maya code) • Late inspiral and merger • Equal and unequal mass, spinning BHs • Not modeled: radiative cooling, magnetic fields, viscosity. a=8M (GM/c2) ≡ M (when G = c = 1) “1 M”≈ 1.5x1012cm (M/107M⊙) ≈ 50s (M/107M⊙) (Bode, Haas, TB, Laguna, Shoemaker 10 TB, Bode, Haas, Laguna, Shoemaker 10 Bode, TB, Haas, Healy, Laguna, Shoemaker 11)

10. q=1, s1= s2= 0.6, Merger in a hot accretion flow − gas density 10M

11. Shocks triggered by the orbiting BHs Mach number > 1 q=1, s1= s2= 0.6,

12. q=1, q=1/2, sudden dropoff q=1/2, rise q=1/2, L quasi-periodic variability -400 -300 -200 -100 0 100 t(M) 1M7 ≈ 50s coalescence Merger in a hot accretion flow − EM signatures • Rapid rise + drop-off are robust features of all light curves regardless of parameters or initial conditions (Bode+ 10, Farris+ 10)

13. q=1/2, L GW t(M) Correlated EM & GW emission • Frequency of EM quasi-periodic variability correlates with GWs and q and spin imprinted in the shape of oscillations... • ..., but their amplitude is relatively low

14. q=1, q=1/2, q=1/2, q=1/2, 1 Lnorm 0.96 -400 -300 -200 -100 0 100 t(M) Correlated EM & GW emission • Frequency of EM quasi-periodic variability correlates with GWs and q and spin imprinted in the shape of oscillations... • ..., but their amplitude is relatively low ☹

15. 1M7 ≈ 50s q=1, q=1/2, sudden dropoff q=1/2, rise q=1/2, L quasi-periodic variability -400 -300 -200 -100 0 100 t(M) coalescence Hot accretion flow - luminosities

16. (Nemmen+ 10) Comparison to LLAGNs ADAFs have been used to model SEDs of some LLAGN and Sgr A* (e.g., Yuan 07, Narayan & McClintock 08) Some dominated by emission from ADAF (synchrotron, inverse Compton, and bremsstrahlung) , and some by jets (synchrotron).

17. Merger of SBHs in a circumbinary disk • Late inspiral and merger (BH separation 8M) • Equal and unequal mass, spinning BHs • Initially, SBHB orbital plane in the plane of the disk • Gas pressure supported disk, h/r = 0.2 (co-R and counter-R) and 0.4 (co-R) • Not modeled: radiative cooling, magnetic fields, viscosity. Rin=16M (Bode, TB, Haas, Healy, Laguna, Shoemaker 11)

18. Circumbinary disk: snapshots h/r=0.2, BH separation 2M q=1, q=1/2, q=1, retrograde disk 60M

19. q=1/2, , h/r=0.2 Merger in circumbinary disk - gas density

20. q=1/2, , h/r=0.2 Merger of SBHs in a circumbinary disk Gas density, vertical slice • No shocks arise from SBHB in the disk body • Variable emission confined to the diffuse gas within the disk “hole” 60M

21. q=1, q=1, q=1/2, , h/r=0.2 , h/r=0.4 disk , h/r=0.4 disk , h/r=0.2 retro disk , h/r=0.2 retro disk q=1/2, q=1, L t(M) 1M7 ≈ 50s coalescence SBHBs in circumbinary disks - light curves Emission from diffuse gas within the disk “hole”

22. SBHBs in circumbinary disks - luminosities emission from diffuse gas within the disk “hole” assuming magnetic field of nearly equipartition strength: • Inferred luminosities of the VARIABLE signal in circumbinary disk models are modest in observational terms. • Presence of MHD stresses can help introduce more gas into the hole (Shi+ 11)

23. narrow Hβ [OIII] doublet Search for SBH binaries in circumbinary disks Collaborators: Mike Eracleous, Todd Boronson, Jules Halpern, Steinn Sigurdsson, Hélène Flohic • Targeting SBHBs with orbital separations ~0.1 pc and P~10-100 yr • 3 epochs of data: archival (SDSS DR7) and new (HST, MDM, Kitt Peak, Palomar, HET) • Spectra of ~16k SDSS QSOs triaged to 88 “unusual” QSOs for monitoring • Selection criterion: shifted broad Hβ emission line profiles • Out of 68 for which followup data is delivered 14 exhibit profile shifts consistent with SBHB model. Rest Wavelength (Å)

24. CAUTION! Displaced peaks do NOT always mean binaries... Arp 102B PKS 0235+023 3C390.3 Nor do displaced peaks that move. (Eracleous+ 97)

25. broad Hβ (Eracleous+ 11) Understanding the population of binary candidates • Cautiously optimistic: theoretical predictions of population size of SBHBs (Volonteri+ 09) in broad agreement with observed numbers • Population modeling: distribution of observed velocity shifts as a function of binary properties and evolution histories • Multi-wavelength followup: optical imaging of the host, VLBA imaging of nuclei, X-ray properties of AGN(s)

26. Conclusions • Coincidental observations of EM and GW signatures from coalescences are a key to understanding SBHBs, but we may have to wait ≳10 yr for eLISA. • Detection of SBHBs from EM searches is an ongoing effort. Theoretical models can offer some guidance. • Numerical relativity has stepped into the astrophysical regime. More development will follow to include MHD and RMHD. • Merging SBH binaries in hot flows more likely to be sufficiently luminous to be observed. Serendipitous discovery of ≳108 M⊙ SBHB coalescence cannot be excluded. • More systematic search will require deep monitoring of the transient sky with multi-wavelength synoptic sky surveys.

27. Image credit: Galaxy Zoo

28. Koss+ 10 (SDSS/Kitt Peak)

29. J1048+0055 Zhou+ 04 VLBA, z= 0.65 z=0.055 VLBA 0402+379 Rodriguez+ 06 Maness+ 04 Morganti+ 09 Proj. separation ∼20 pc Projected separation ∼7 pc Observational evidence: gravitationally bound SBHBs? • 1pc @ 100 Mpc ≈ 2 mas

30. Dense, radiatively efficient ✔ • Gas expelled by binary torques ✗ • kT ∼10−100 eV (UV) ✗ Circumbinary disk model: Radiatively inefficient hot gas flow: • Tenuous, radiatively inefficient ✗ • Binary immersed in hot gas until coalescence ✔ • kT ∼ 0.1−1 MeV (hard X-ray, γ-ray) ✔

31. (Baumgarte & Shapiro 11; Physics Today)