Vincent L. Fish MIT Haystack Observatory and the Event ... and the Event Horizon Telescope collaboration

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  • Resolving Black Holes with Millimeter VLBI Vincent L. Fish

    MIT Haystack Observatory and the Event Horizon Telescope collaboration

    Model courtesy C. Gammie

    Wednesday, March 9, 2011

  • Bringing resolution to black holes There 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 model Natural variability

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

    Wednesday, March 9, 2011

  • The apparent sizes of black holes Sgr A*:

    Mass ~ 4.3 x 106 Msun (Gillessen et al. 2009) Distance ~ 8.0 to 8.4 kpc RSch ~ 0.08 AU = 10 μas

    Sgr A* has the largest apparent event horizon from Earth RSch = 2GM/c2 scales linearly with mass Stellar-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. 2008 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

    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 Interferometry Visibility 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 observations Observations in 2007, 2009, and 2010

    4630 km

    403 0km

    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 size The 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

  • Density Putting 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 1998 Wednesday, March 9, 2011

  • Existence of an event horizon In 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 parameters If 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 parameters If 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

    In cl

    in at

    io n

    Position angle

    Wednesday, March 9, 2011

  • Constraining model parameters In 2009, we detected Sgr A* on Hawaii-California baseline also

    Even better constraints

    Broderick et al. 2011

    Spin

    In cl

    in at

    io n

    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 results Millimeter 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 Coherence SNR limited by at