Imaging the Event Horizon: Past, Present & Future VLBI of Sgr A*

Why Study Sgr A*? • “Unique Laboratory for Astrophysics” • 1 mas ~ 0.1 milli-parsec ~ 150 R_g • Unprecedented multi-wavelength information • Degeneracy in Measurements and Models • Role of inflow, outflow, jets not settled • i.e., 10^5 range for M_dot

Sgr A*: Basic Properties • Supermassive Black Hole • 3 x 106 M_sun • Extremely underluminous • L ~ L_sun ~ 10-10 L_edd • Inverted Spectrum • α > 0.1 – 0.7 • Compact, nonthermal • Size < 1 AU @ 3mm • Tb > 10^9 K

Models of Sgr A*:Why is L << L_edd? • Under-fed systems • Jet • CDAF • Bondi-Hoyle • Under-luminous systems • ADAF

Models of Sgr A*:Why is L << L_edd? • Under-fed systems • Jet • CDAF • Bondi-Hoyle • Under-luminous systems • ADAF 1mm Polarization Indicates dM/dt < 10-7 M_sun y-1

What We Want to See • Structure • Ejection of components • Correlated changes with X-ray variability • Astrometric measurements (Reid talk) • …?

What We See • Elliptical Gaussian • 2 x 1 ratio • East-West major axis • No detection of … • Extended structure • Separate components

Scattering Inhibits Imaging &Points to Higher Frequencies Lo et al. 1998

Is there Structure? Lo et al. 1998

Difficulty of mm Imaging SgrA* • Axisymmetric Structure • Purely an amplitude measurement • Low Declination & High Frequency • Poor and variable antenna gain • High Tsys • Variable opacity • Short and variable coherence time • Lack of North-South resolution

Closure Amplitude VmnVpq Cmnpq = ----------------- VmqVnp Independent of station-based gain errors!

Closure Amplitude Properties • Independent of station-based gain errors • Still dependent of baseline-based errors • Decorrelation, for example • Reduced sensitivity • 2/3 for N=7 • Non-Gaussian errors • Doeleman et al. 2000 --- 3mm imaging

Sample Closure Amplitudes

Error Surfaces

Slices through the Error Surface

Herrnstein et al. 2003, Zhao et al. 2003

Results: 22 GHz Equal scales

Results: 43 GHz Equal Scales

New Results:Consistent with Scattering 9 Q, 4 K, 1 U experiments 3 7 13 20 mm 3 7 13 20 mm

Past and Present Conclusions • Mean properties consistent with scattering • Axisymmetric structure only • Based on closure phases • Max variability between high and low flux states: no N-S extension • Delta Major axis: ~30 mas  60 +/- 30 R_g • Delta Minor axis: ~40 mas  90 +/- 90 R_g • No outflow? Slow outflow? Along line of sight?

What’s Next for the VLBA? • Add GBT at 7mm • Links SC/HN to rest of array • Increased SNR for closure amplitude • 3mm • Doeleman et al (2000) • VLBA + ad hoc • Resolution over the scattering

The Future Falcke, Melia & Agol 2000 Bardeen 1973

Event Horizon Shadow • Shadow with radius 5 R_g must exist • Optically thin emission required • Polarization suggests tau < 1 at 1.3 mm • Sgr A* is the only realistic candidate

What’s Necessary for the Future?

3- or 4-station “Image” 1.5 Jy Shadow Best-fit Gaussian

Technical Requirements • High frequency receivers & antenna performance • 230/350 GHz • Phase stability • Water vapor radiometers • Time standards • Array Phasing • Correlator options • > Gigabit recording

How Will We Do It? • NSF-STC Gravity proposal • UC Berkeley, Stanford, U Washington • CMB, Quantum Gravity, Small-scale r-2 tests & • 3 station, full-polarization image by 2010 • Provide support for technical development, instrumentation and observations • Collaboration!

Summary • Gold standard of imaging • Closure amplitude • Closure phase • VLBA Future Observations • Deviations in size of 10s of micro-arcseconds • Detecting the event horizon • Technical innovation • Collaboration • Proof of existence of black holes!

Imaging the Event Horizon: Past, Present & Future VLBI of Sgr A*