Searching for supermassive black hole binaries with CRTS
Matthew J. GrahamCenter for Data-Driven Discovery, Caltech
and NOAO
May 12, 2015
Hotwiring The Transient Universe IV
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The physics of a SMBH binary merger
Stage I (> 1pc) SMBHs dissipate angular momentum through dynamical
friction with surrounding starsStage II (0.01 – 1pc) Stalled phase due to stellar depletion (~106 – 107 yrs)Stage III ( < 0.01pc) Orbital angular momentum lost by gravitational radiationStage IV Coalescence and recoil----
The “final parsec” problem Subparsec systems are not resolvable
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Structure of a close SMBH binary
Circumbinary disk
Primary black hole
Secondary black hole
Minidisk
BLR
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Spectroscopic searches A fraction of all quasar spectra exhibit double-peaked broad
emission lines (attributable to a number of causes) If two BLRs bound to two SMBHs coorbiting, we should see
characteristic Doppler-shifts reflecting orbital motion Monitor samples of broad-line double peakers or systems with
single, kinematically-shifted broad lines Limited to > 0.1 pc separation systems
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Catalina Real-time Transient Survey (2005-) Collaborative survey with Catalina Sky Survey (LPL, UA) Unfiltered observations 21 nights/lunation covering up to 2000 deg2/night Covers 33000 sq. deg. (0 < RA < 360, -75 < Dec < 70). Calibrated photometry for 500 million objects (> 100 billion data points) Depth V = 19 to 21.5 100 – 600 observations in most regions (median ~ 250) More published SNe and CVs than any other survey (public instantly) Open data policy (http://catalinadata.org) ~3% LSST
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Quasar variability
First quasar identified 3C 48 – most striking feature was that the optical radiation varied
Physical origin of photometric variability in optical/UV is unclear
Many studies based on small sample size or (very)sparse time sampling
The current best statistical description is damped random walk (CAR(1)):
characterized by σ2 and τ 335000 spectroscopically-confirmed quasars in CRTS archive:
250000 have sufficient coverage (npts > 50)
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The best binary candidate to date
OJ 287 shows a pair of outburst peaks every 12.2 years for at least the last century
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Periodicity search criteriaWavelets Peak value Period Slepian wavelet characteristic timescaleAutocorrelation function Period Amplitude of exponentially damped cosine Decay constant of exponentially damped cosineShape and coverage Scatter around best-fit Fourier series At least 1.5 cycles
Train SVM to better describe discriminating hyperplane
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How many SMBH binaries detected? From merger tree assembly models and hydro simulations with
period from 20 – 300 weeks sky coverage of 2π ster. V < 20 0.0 < z < 4.5
=> 450 binaries are predicted 111 candidates identified out of 250000 quasars Simulated data set of objects following
a CAR(1) model gives no candidates
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PG 1302-102
z = 0.28, core-dominated flat-spectrum radio source Luminous elliptical host with nearby companions Coincident radio and optical structural features
(Graham et al. 2015,Nature, 518, 74)
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Quasi-periodicity in CARMA models
5 candidates show a periodicity consistent with best-fit CARMA but bandwidth of these covers full temporal baseline
100 mock CARMA light curves for each candidate give statistically different selection parameters
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Possible interpretations
Superposition of thermal emission from accretion disk and non-thermal contribution from a precessing jet driven by SMBH binary
Warped accretion disk caused by SMBH binary Periodic accretion rates from a SMBH binary can lead to an
overdense hump in the inner circumbinary accretion disk Hydrodynamical simulations suggest
that strongest periodicity associatedwith cavity in circumbinary disk => true binary period 3-8 times shorter
Relativistic boosting for line-of-sightmotion of minidisk around secondaryorbiting around system barycenter
Scaled version of QPOs seen in stellar blackhole binaries (D’Orazio et al. 2015)
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What about other SMBH binary candidates?
No spectroscopic binary candidates in the literature show any sign of significant periodicity in their CRTS photometry
Some suggestion of differences in light curve morphology for specific spectroscopic samples
The quiescent milliparsec binary J120136.02+300305.5 shows no optical variation in CRTS data
Blazars with reported (quasi-)periodicity do not show consistent periodicity in CRTS data
FBQS J221648.7+012427 does not pass our selection criteria
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Testing the binary hypothesis
Further monitoring looking for amplitude/phase trends with wavelength
Spectroscopic monitoring Multiple periods with cavity and broad line widths X-ray imaging to detect relativistic effects in Fe Kα SED effects Reverberation mapping to distinguish between different
scenarios
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The GW-driven population
Haiman, Kocsis & Menou (2009) predict:
The power law at longer timescales depends on accretion physics Circumbinary gas is present at small orbital radii and is being
perturbed by the black holes
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Detecting gravitational waves
Are any of the candidates resolvable by PTAs as continuous GW sources and not just part of the stochastic background?
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Summary Supermassive black holes are an expected consequence of
galaxy mergers To date, they have been very difficult to detect CRTS presents an unprecedented data set for exploring all
types of variable astronomical phenomena Using a robust selection technique, we have identified 111
periodic candidates out of 250000 with CRTS coverage The best is PG 1302-105 showing strong periodicity over 20 yrs Several physical mechanisms can explain the variability but all
involve a supermassive black hole binary Various followup tests can distinguish between different
mechanisms Potentially resolvable by PTAs in the next decade or so
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