Vincent L. Fish MIT Haystack Observatory and the Event ...member.ipmu.jp/bh2011/pdf/5Fish.pdf · and the Event Horizon Telescope collaboration Model courtesy C. Gammie Wednesday,

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


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)


Evidence that Sgr A* hosts a black hole• Stellar orbits


Evidence that Sgr A* hosts a black hole• Stellar orbits• Density

Maoz 1998


Evidence that Sgr A* hosts a black hole• Stellar orbits• Density• Lack of motion

Reid & Brunthaler 2004


Evidence that Sgr A* hosts a black hole• Stellar orbits• Density• Lack of motion• X-ray flares

Baganoff et al. 2001


Evidence that Sgr A* hosts a black hole• Stellar orbits• Density• Lack of motion• X-ray flares• NIR flares?

Genzel et al. 2003


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


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


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


GR MHD modelNatural variability

Features similar to RIAF model: photon orbit Doppler boosting etc.


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


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



• Atmospheric opacity

courtesy CSO Atmospheric Transmission Interactive Plotter




• 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


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


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)


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*


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


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


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)


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)


Spin

Incl

inat

ion

Position angle


Constraining model parametersIn 2009, we detected Sgr A* on Hawaii-California baseline also

Even better constraints


Spin

Incl

inat

ion


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


M87

Walker et al. 2008, 43 GHz

M87 hosts a supermassive black hole with a jet

Angular scale (rSch) similar to Sgr A*


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


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


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%


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


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


Event Horizon TelescopeRed: Phase IYellow: Phases II and III? some telescopes exist (SPT, SEST, Haystack) new sites (ATF dishes?)

as seen from Sgr A*


The Event Horizon TelescopePhase I telescope locations• Hawaii (phased JCMT + CSO + SMA)• Arizona (ARO/SMT)• California (phased CARMA)


The Event Horizon TelescopePhase I telescope locations• Hawaii (phased JCMT + CSO + SMA)• Arizona (ARO/SMT)• California (phased CARMA)• Chile (ASTE, APEX, and/or ALMA)


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)


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)


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


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 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


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


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


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


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


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


Testing general relativity: The no-hair theorem

a = 0

a = 0.4

ε = 0 ε = 0.5Figures courtesy T. Johannsen & D. Psaltis


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


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


The Event Horizon Telescope collaborationMIT HaystackNRAOHarvard-Smithsonian CfA / SMAU. Arizona / AROCARMAJCMTCaltech / CSOASIAANAOJMPIfR-BonnIRAM / Pico Veleta + Plateau de BureUC BerkeleyUMass Amherst / LMT& others


Documents

Vincent L. Fish MIT Haystack Observatory and the Event ...member.ipmu.jp/bh2011/pdf/5Fish.pdf · and the Event Horizon Telescope collaboration Model courtesy C. Gammie Wednesday,