1. Imaging the Event Horizon: Past, Present & Future VLBI of Sgr A* Geoffrey C. Bower UC Berkeley

2. Principal Collaborators Backer, D.C. (UCB) Doeleman, S. (MIT) Falcke, H. (MPIfR) Goss, W.M. (NRAO) Herrnstein, R. (CfA/Columbia) Quataert, E. (UCB) Wright, M.C.H. (UCB) Zhao, J.-H. (CfA)

3. 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

4. Sgr A*: Basic Properties Supermassive Black Hole 3 x 10 6 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

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

6. 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

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

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

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

10. Is there Structure? Lo et al. 1998

11. 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

12. Closure Amplitude C mnpq = ----------------- V mn V pq V mq V np Independent of station-based gain errors!

13. 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

14. Sample Closure Amplitudes

15. Error Surfaces

16. Slices through the Error Surface

17. Herrnstein et al. 2003, Zhao et al. 2003

18. Results: 22 GHz Equal scales

19. Results: 43 GHz Equal Scales

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

21. 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  as  60 +/- 30 R_g Delta Minor axis: ~40  as  90 +/- 90 R_g No outflow? Slow outflow? Along line of sight?

22. 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

23. The Future Falcke, Melia & Agol 2000 Bardeen 1973

24. 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 Black HoleHorizon Size Sgr A*6.5 μ arcsec M873.7 NGC 46492.4 Cen A1.0

25. What’s Necessary for the Future?

26. 3- or 4-station “Image” 1.5 Jy Shadow Best-fit Gaussian

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

28. 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!

29. 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!