Resolving Black Holes with Millimeter VLBIVincent L. Fish
MIT Haystack Observatoryand the Event Horizon Telescope collaboration
Model courtesy C. Gammie
Wednesday, March 9, 2011
Bringing resolution to black holesThere is lots of interesting physics to be done with black holes:• Accretion physics• Outflow/jet collimation• Tests of general relativity
Sgr A* is a great laboratory for testing GR, but we need to understand the astrophysics of the material we see
Our understanding of black holes is limited by the lack of resolution
The goal of the Event Horizon Telescope collaboration is to study the black holes with the largest apparent angular sizes (especially Sgr A*, M87) at high angular resolution
Wednesday, March 9, 2011
Outline• Background on the Galactic Center BH Sgr A*
• Current millimeter VLBI observations of Sgr A* and M87
• Scientific implications of long-baseline detections
• Overview of present capabilities of the mm VLBI array
• Progression of the Event Horizon Telescope
• Future scientific possibilities (testing GR)
Wednesday, March 9, 2011
Evidence that Sgr A* hosts a black hole• Stellar orbits
Wednesday, March 9, 2011
Evidence that Sgr A* hosts a black hole• Stellar orbits• Density
Maoz 1998
Wednesday, March 9, 2011
Evidence that Sgr A* hosts a black hole• Stellar orbits• Density• Lack of motion
Reid & Brunthaler 2004
Wednesday, March 9, 2011
Evidence that Sgr A* hosts a black hole• Stellar orbits• Density• Lack of motion• X-ray flares
Baganoff et al. 2001
Wednesday, March 9, 2011
Evidence that Sgr A* hosts a black hole• Stellar orbits• Density• Lack of motion• X-ray flares• NIR flares?
Genzel et al. 2003
Wednesday, March 9, 2011
Evidence that Sgr A* hosts a black hole• Stellar orbits• Density• Lack of motion• X-ray flares• NIR flares?
Conclusion: There is a very dense, very massive object in the center of the Galaxy
Wednesday, March 9, 2011
What might Sgr A* look like?Emission likely comes from a disk and/or a jet
Emission is variable on short timescales, suggesting source structure variation on spatial scales of a few RSch
We need to be able to resolve Sgr A*
cartoon courtesy Chandracourtesy C. Gammie
courtesy A. Broderick
Wednesday, March 9, 2011
Features of optically thin disk emission in GR• Doppler boosting/deboosting
Low-inclination disk models look “big” (would be resolved out on long baselines) High-inclination disk models produce a bright crescent of emission that is much more compact• Photon orbit• Gravitational redshift• Secondary/tertiary images• Back side of disk lensed into field of view• Innermost stable circular orbit
6 GM/c2 for a=0 (no spin) 1 GM/c2 for a=1 (maximal spin) but lensed to appear bigger (RSch = 2 GM/c2)
1〫
20〫
40〫
60〫
80〫
Models courtesy A. Broderick
Approaching Receding
Wednesday, March 9, 2011
GR MHD modelNatural variability
Features similar to RIAF model: photon orbit Doppler boosting etc.
Wednesday, March 9, 2011
The apparent sizes of black holesSgr A*:
Mass ~ 4.3 x 106 Msun (Gillessen et al. 2009)Distance ~ 8.0 to 8.4 kpcRSch ~ 0.08 AU = 10 μas
Sgr A* has the largest apparent event horizon from EarthRSch = 2GM/c2 scales linearly with massStellar-mass black holes appear much smaller (factor of 106 less massive, but nowhere near factor of 106 closer)
Next largest: M87
These angular scales are very small, but it is possible to achieve them now from the surface of the Earth
Wednesday, March 9, 2011
The need for millimeter VLBI• Resolution: at 230 GHz,
Hawaii-Arizona ~ 60 μas, Hawaii-Chile ~ 30 μas• Interstellar scattering: goes as λ2
Doeleman et al. 2008Wednesday, March 9, 2011
The need for millimeter VLBI• Resolution: at 230 GHz,
Hawaii-Arizona ~ 60 μas, Hawaii-Chile ~ 30 μas• Interstellar scattering: goes as λ2
• Atmospheric opacity
courtesy CSO Atmospheric Transmission Interactive Plotter
Wednesday, March 9, 2011
The need for millimeter VLBI• Resolution: at 230 GHz,
Hawaii-Arizona ~ 60 μas, Hawaii-Chile ~ 30 μas• Interstellar scattering: goes as λ2
• Atmospheric opacity
Sweet spot is 230 or 345 GHz window
Note that resolution is tens of microarcseconds, corresponding to a few Schwarzschild radii (unlensed)
At present, no other technique can achieve this level of angular resolution
Wednesday, March 9, 2011
Very Long Baseline InterferometryVisibility is a Fourier component of sky image (amplitude and phase)
Longer baseline = finer angular resolution
We cannot yet image Sgr A* (not enough baselines)
Nevertheless, observables (e.g., visibility amplitude, closure phase) give information about small structure
Wednesday, March 9, 2011
The current state of mm VLBI observationsObservations in 2007, 2009, and 2010
4630km
4030km
908km
JCMT(+ SMA + CSO)
CARMA
SMT
Used with permission from University of Arizona, T. W. Folkers, photographer
In 2010 also observed with ASTE in Chile (but no detections yet)
Wednesday, March 9, 2011
2007 observations: Detected SMT-CARMA and SMT-JCMT
2009 observations (3 epochs): Detected JCMT-CARMA also
Assuming circular Gaussian:
Measured size 43 μas (+14, -8)
Deconvolved size 37 μas (+16, -10) ~ 3.7 RSch
Sizes are consistent across all 4 epochs
Size of emitting region in Sgr A*
Wednesday, March 9, 2011
Minimum apparent sizeThe emission from Sgr A* cannot be centered on the black hole
Close to the black hole, strong lensing affects apparent size from the viewpoint of a distant observer
Implication: the emission at 230 GHz must be (partially) optically thin and offset from the center of the black hole
Newtonian
General relativity
Wednesday, March 9, 2011
DensityPutting 4 x 106 Msun into 37 μas at 8 kpc yields a lower limit on density of several x 1023 Msun pc-3
Any collection of ordinary matter would collapse or disperse on a very short time scale
Alternate theories exist, but a black hole is the most mundane possibility for the mass of Sgr A*
Maoz 1998Wednesday, March 9, 2011
Existence of an event horizonIn the absence of an event horizon, near infrared fluxes and VLBI sizes place a lower limit on the efficiency of conversion of gravitational binding energy: > 99.6%
There must be an event horizon
Broderick et al. 2009
Wednesday, March 9, 2011
Constraining model parametersIf the emitting region in Sgr A* can be described by a disk, current detections place limits on model parameters (e.g., spin of black hole, inclination of axis, orientation on sky)
Wednesday, March 9, 2011
Constraining model parametersIf the emitting region in Sgr A* can be described by a disk, current detections place limits on model parameters (e.g., spin of black hole, inclination of axis, orientation on sky)
RIAF simulations strongly disfavor low-inclination models (i.e., Sgr A* is not face-on)
GR MHD simulations give same result (Mościbrodzka et al. 2009)
Broderick et al. 2009
Spin
Incl
inat
ion
Position angle
Wednesday, March 9, 2011
Constraining model parametersIn 2009, we detected Sgr A* on Hawaii-California baseline also
Even better constraints
Broderick et al. 2011
Spin
Incl
inat
ion
Wednesday, March 9, 2011
Constraining model parameters
Spin Inclination Position Angle
In 2009, we detected Sgr A* on Hawaii-California baseline also
Even better constraints on black hole and disk parameters
Wednesday, March 9, 2011
M87
Walker et al. 2008, 43 GHz
M87 hosts a supermassive black hole with a jet
Angular scale (rSch) similar to Sgr A*
Wednesday, March 9, 2011
Brief summary of current resultsMillimeter VLBI is already producing interesting science
• Sgr A* contains a black hole- Density/coalescence arguments- Existence of an event horizon
• There is very compact emission- This emission must be offset from center of black hole
•We can place constraints on emission morphology- Very likely not a face-on disk
• M87 has very compact emission as well
Increasing sensitivity and array coverage will lead to even more interesting science
Wednesday, March 9, 2011
Sensitivity and Atmospheric CoherenceSNR limited by atmospheric coherence
Timescale for coherent integration is 1 - 30 sec (typically <10)
Can segment data (coherently) and incoherently average segments, but...
SNR for incoherent averaging asymptotes to t¼ (not t½)
It is important to get signal as quickly as possible
Wednesday, March 9, 2011
Atmospheric coherenceAtmospheric turbulence smears out delay rate spectrum
Good coherence: SNR 90 Bad coherence: SNR 25
Nearly identical scans on the same calibrator on two different days
Necessary to segment data and average incoherentlyDifferent source
Initial probability of false detection: 40%
Wednesday, March 9, 2011
TechnologyWideband digital backends and Mark 5B+ recorders
Current capability: 4 Gbit s-1 sustained rateNear future: 16 Gbit s-1
Goal: 32 Gbit s-1 sustained, full polarization, phased arrays
Wednesday, March 9, 2011
Phased arraysCfA/SMA group working to produce phased-array processor for Mauna Kea (JCMT+CSO+SMA)
Processor can easily be adapted to CARMA, Plateau de Bure
International team (including Mareki Honma) has proposed to phase ALMA
Wednesday, March 9, 2011
Event Horizon TelescopeRed: Phase IYellow: Phases II and III? some telescopes exist (SPT, SEST, Haystack) new sites (ATF dishes?)
as seen from Sgr A*
Wednesday, March 9, 2011
The Event Horizon TelescopePhase I telescope locations• Hawaii (phased JCMT + CSO + SMA)• Arizona (ARO/SMT)• California (phased CARMA)
Wednesday, March 9, 2011
The Event Horizon TelescopePhase I telescope locations• Hawaii (phased JCMT + CSO + SMA)• Arizona (ARO/SMT)• California (phased CARMA)• Chile (ASTE, APEX, and/or ALMA)
Wednesday, March 9, 2011
The Event Horizon TelescopePhase I telescope locations• Hawaii (phased JCMT + CSO + SMA)• Arizona (ARO/SMT)• California (phased CARMA)• Chile (ASTE, APEX, and/or ALMA)• Pico Veleta (IRAM 30 m)• Plateau de Bure (phased)
Wednesday, March 9, 2011
The Event Horizon TelescopePhase I telescope locations• Hawaii (phased JCMT + CSO + SMA)• Arizona (ARO/SMT)• California (phased CARMA)• Chile (ASTE, APEX, and/or ALMA)• Pico Veleta (IRAM 30 m)• Plateau de Bure (phased)• Mexico (LMT)
Wednesday, March 9, 2011
The Event Horizon TelescopePhase I telescope locations• Hawaii (phased JCMT + CSO + SMA)• Arizona (ARO/SMT)• California (phased CARMA)• Chile (ASTE, APEX, and/or ALMA)• Pico Veleta (IRAM 30 m)• Plateau de Bure (phased)• Mexico (LMT)
(u,v) coverage is important because we don’t know a priori what the interesting spatial scales and orientations are
Wednesday, March 9, 2011
Tracking rapid source structure changesSgr A* is known to be variable on timescale of a few minutesInnermost stable circular orbital period ranges from ~4 min (maximally rotating) to ~30 min (Schwarzschild)
Wednesday, March 9, 2011
Closure quantitiesTracking structural changes on rapid timescales at millimeter wavelengths (where the coherence time is short) will require robust, non-imaging observables
Closure phase and amplitude are independent of most complex gain, clock, and tropospheric delay errors
Wednesday, March 9, 2011
Tracking rapid source structure changesSgr A* is known to be variable on timescale of a few minutesInnermost stable circular orbital period ranges from ~4 min (maximally rotating) to ~30 min (Schwarzschild)Closure phases will be able to detect source structure changesPeriodicity could give clue to spin
Wednesday, March 9, 2011
Closure phases including ChileIn some models (especially high spin and/or inclination), long baselines are needed to see large closure phases
Sensitivity is very important; a single dish may not be enough
Phased ALMA
a = 0.9, hot spot at 6 RG
Wednesday, March 9, 2011
ImagingEventual goal will be to produce an image of Sgr A* (and M87)
This will require both sensitivity and good (u,v) coverage
Model 7 stations 13 stations345 GHz, interstellar scattering included
Wednesday, March 9, 2011
Testing general relativity: Shadow sizeBlack hole “shadow” or “silhouette” predicted from any optically-thin emission
The size of the shadow is only very weakly dependent on the spin of the black hole
dshadow ~ 10.4 GM/c2 (Schwarzschild black hole) 9.0 GM/c2 (maximally spinning black hole)
To within a few percent, we know:• the predicted shadow diameter in GM/c2
• the mass of Sgr A*• the distance to Sgr A*
Testable prediction of GR: size of shadow to within less than 10 μas Falcke et al. (2001)
GM/c2 in μas
Wednesday, March 9, 2011
Testing general relativity: The no-hair theoremBlack holes have three hairs: mass, spin, and charge
Improbable that black holes have (much) charge, so only 2 hairs
Mass (M): monopole momentSpin (a): dipole moment
In the Kerr metric, all higher order terms are set by M and a
Glampedakis & Babak (2006) and Johannsen & Psaltis (2010) investigate divergence from Kerr in the quadrupole term Q = -M(a2 + εM2)
Non-Kerr solutions would show deviations in shape of photon orbit
Wednesday, March 9, 2011
Testing general relativity: The no-hair theorem
a = 0
a = 0.4
ε = 0 ε = 0.5Figures courtesy T. Johannsen & D. Psaltis
Wednesday, March 9, 2011
Testing general relativity: The no-hair theorem
Phased ALMA will be especially sensitive to the spatial scales on which we might see deviations from GR
Wednesday, March 9, 2011
ConclusionsMillimeter VLBI observations have constrained the sizes of emitting regions in Sgr A* and M87 to be in the GR regime
These observations are already producing interesting science (existence of event horizon, emission offset from black hole, constraints on model parameters...)
Sensitivity upgrades and the inclusion of additional telescopes in future observations will be able to detect signatures of changing source structures in Sgr A*
Future expansion of the Event Horizon Telescope collaboration may allow for very high resolution images of Sgr A* and M87, enabling tests of general relativity in the vicinity of a black hole
Wednesday, March 9, 2011
The Event Horizon Telescope collaborationMIT HaystackNRAOHarvard-Smithsonian CfA / SMAU. Arizona / AROCARMAJCMTCaltech / CSOASIAANAOJMPIfR-BonnIRAM / Pico Veleta + Plateau de BureUC BerkeleyUMass Amherst / LMT& others
Wednesday, March 9, 2011