Signatures of Supermassive Black Hole Coalescence

Tamara Bogdanović Einstein Postdoctoral Fellow

University of Maryland

14 Nov 2011 STScI Colloquium

SBHBs: Formation and Evolution

•Galactic merger

•Interaction with stars & gas

•GW phase

•Coalescence

•GW recoil

Antennae galaxy

NGC 6240

Credit: NASA/CXC/MPE/S. Komossa et al.

Credit: NASA/ESA-ACS Science Team

Mice galaxy

Stephan’s Quintet

Credit: NASA/JPL/MPI/SSC

B. Whitmore (STSci), F. Schweizer (DTM), NASA

GW astrophysics of SBHBs

• 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

Credit: www.srl.caltech.edu/lisa/graphics/master.html

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.

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.

Hayasaki+ 07Escala+ 05Mayer+ 07 Cuadra+ 09

10s kpc 10s pc sub-pc sub-pc

Recently: non-relativistic MHD simulation

•First MHD study of disk structures and angular momentum transport

Shi, Krolik, Lubow, & Hawley 11

density with density weighted velocity field

Relativistic simulations of mergers

•Surrounded by EM fields

•Surrounded by matter

(Palenzuela+ 09, 10; Mösta+ 10, 11, Neilsen+ 2010)

(Bode+ 10, 11; Farris+ 10, 11)

(van Meter+ 09)

•Surrounded by test particles

Environment in vicinity of a coalescing 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.

•How EM counterpart depends on the mass ratio, spins and environment of the binary?

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.

(Bode, Haas, TB, Laguna, Shoemaker 10TB, Bode, Haas, Laguna, Shoemaker 10

Bode, TB, Haas, Healy, Laguna, Shoemaker 11)

q spins

1 0

1

1

1/2

1/2

a=8M

(GM/c2) ≡ M (when G = c = 1)“1 M” ≈ 1.5x1012cm (M/107M⊙) ≈ 50s

(M/107M⊙)

QuickTime™ and a decompressor

are needed to see this picture.

Merger in a hot accretion flow − gas density

q=1, s1= s2= 0.6,

10M

Shocks triggered by the orbiting BHs

q=1, s1= s2= 0.6, Mach number > 1

coalescence

-400 -300 -200 -100 0 100t(M)

1M7 ≈ 50s

L

rise

sudden dropoff

quasi-periodic variability

q=1, q=1/2, q=1/2, q=1/2,

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)

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

q=1/2,

L

t(M)

GW

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

q=1, q=1/2, q=1/2, q=1/2,

Lnorm

-400 -300 -200 -100 0 100

t(M)

1

0.96

☹

Hot accretion flow - luminosities

coalescence -400 -300 -200 -100 0 100

t(M)

1M7 ≈ 50s

L rise sudden

dropoffquasi-periodic

variability

q=1, q=1/2, q=1/2, q=1/2,

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).

(Nemmen+ 10)

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.

q spins

1

1/2

1/2

Rin=16M

(Bode, TB, Haas, Healy, Laguna, Shoemaker 11)

Circumbinary disk: snapshotsq=1, q=1/2, q=1,

retrograde disk

h/r=0.2, BH separation 2M

60M

QuickTime™ and a decompressor

are needed to see this picture.

Merger in circumbinary disk - gas density

q=1/2, , h/r=0.2

Merger of SBHs in a circumbinary disk

QuickTime™ and a decompressor

are needed to see this picture.

• No shocks arise from SBHB in the disk body

• Variable emission confined to the diffuse gas within the disk “hole”

Gas density, vertical slice q=1/2, , h/r=0.2

60M

SBHBs in circumbinary disks - light curves

L

t(M)

coalescence 1M7 ≈ 50s

q=1, , h/r=0.4 disk

q=1, , h/r=0.2 retro disk

q=1, , h/r=0.2

q=1/2, , h/r=0.4 diskq=1/2, , h/r=0.2 retro disk

Emission from diffuse gas within the disk “hole”

SBHBs in circumbinary disks - luminositiesemission 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)

Search for SBH binaries in circumbinary disks

•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.

Collaborators: Mike Eracleous, Todd Boronson, Jules Halpern, Steinn Sigurdsson, Hélène Flohic

narrow Hβ

[OIII] doublet

Rest Wavelength (Å)

CAUTION! Displaced peaks do NOT always mean binaries...

Nor do displaced peaks that move.

3C390.3Arp

102B PKS 0235+023

(Eracleous+ 97)

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)

broad Hβ (Eracleous+ 11)

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.

Image credit: Galaxy Zoo

Koss+ 10 (SDSS/Kitt Peak)

Rodriguez+ 06 Maness+ 04

Morganti+ 09

Projected separation ∼7 pc

0402+379z=0.055 VLBA

Observational evidence: gravitationally bound SBHBs?

•1pc @ 100 Mpc ≈ 2 mas

Proj. separation ∼20 pc

VLBA, z= 0.65 J1048+0055 Zhou+ 04

•Dense, radiatively efficient ✔

•Gas expelled by binary torques ✗

•kT ∼10−100 eV (UV) ✗

•Tenuous, radiatively inefficient ✗

• Binary immersed in hot gas until coalescence ✔

•kT ∼ 0.1−1 MeV (hard X-ray, γ-ray) ✔

Circumbinary disk model: Radiatively inefficient hot gas flow:

(Baumgarte & Shapiro 11; Physics Today)