1. An Event Horizon Telescope: (sub)mm VLBI of Sgr A* Shep Doeleman MIT Haystack Observatory

2. SgrA*: Best Case for a SMBH Stellar orbits within 45 AU: M~4x10 6 Msol Proper motions 10^5 Msol (Backer & Sramek, Reid & Brunthaler) Submm/NIR/X-ray flares variability. VLT: Genzel et al 2003 Baganoff et al 2001 Ghez et al 2005

3. Resolving Rsch-scale structures Spinning (a=1) Non-spinning (a=0) Falcke Melia Agol SgrA* has the largest apparent Schwarzschild radius of any BH candidate. Rsch = 10  as Shadow = 5.2 Rsch (non-spinning) = 4.5 Rsch (maximally spinning)

4. High Frequency VLBI Resolution: /D (cm) ~ 0.5 mas /D (1.3mm) ~ 30  as /D (0.8mm) ~ 20  as ISM Scattering:  scat ~  Can’t use VLBA Short Challenges: Weather, freq. stnds, electronics noise Increased Sensitivity

5. Wideband VLBI Systems Total cost $40-50K per station. x16 in BW over current VLBA sustainable rates. Equivalent to replacing VLBA with 50m antennas. Used for VLBA upgrade: x4 in sensitivity over current VLBA sustainable rate. Digital Backend (DBE) Digital Recorder (Mark5)

6. mm/submm VLBI Collaboration MIT Haystack: Alan Rogers, Alan Whitney, Mike Titus, Dan Smythe, Brian Corey, Roger Cappallo, Vincent Fish U. Arizona Steward Obs: Lucy Ziurys, Robert Freund CARMA: Dick Plambeck, Douglas Bock, Geoff Bower Harvard Smithsonian CfA: Jonathan Weintroub, Jim Moran, Ken Young, Dan Marrone, David Phillips, Ed Mattison, Bob Vessot, Irwin Shapiro, Mark Gurwell, Ray Blundell, Bob Wilson James Clerk Maxwell Telescope: Remo Tilanus, Per Friberg UC Berkeley SSL: Dan Werthimer Caltech Submillimeter Observatory: Richard Chamberlain MPIfR: Thomas Krichbaum ASIAA: Makoto Inoue, Paul Ho

7. 1.3mm Observations of SgrA* 4630km  DAYS OBSERVING - April 2007 4030km 908km

8. Determining the size of SgrA* SMT-CARMA SMT-JCMT  OBS = 43  as (+14, -8)  INT = 37  as (+16, -10) Doeleman et al 2008 1 Rsch = 10  as JCMT-CARMA

9. The minimum apparent size. Broderick & Loeb Noble & Gammie Event Horizon

10. Caveat: Very Interesting Structures Gammie et al 14 Rsch (  as)  SMT-CARMA SMT-JCMT JCMT-CARMA

11. But…April 2009 SgrA* Detections

12. Adding ALMA Adding Telescopes: RIAF example Adding LMT Hawaii, CARMA, SMT

13. Time Variable Structures Variabilty in NIR, x-ray, submm, radio. Probe of metrics near BH, and of BH spin. Violates Earth Rotation aperture synthesis. Use ‘good’ closure observables to probe structure as function of time.

14. Hot Spot Model for SgrA* Flares

15. Hot Spot Models (P=27min) Spin=0, orbit = ISCO Spin=0.9, orbit = 2.5xISCO Models: Broderick & Loeb 230 GHz, ISM scattered

16. Closure Phases: Hawaii-CARMA-Chile Spin = 0.9 Hot-spot at ~ 6R g Period = 27 min.

17. Detecting Rsch Polarization Structures

18. The Event Horizon Telescope An international collaborative project assemble a global submm-VLBI array for observing and resolving Event Horizons. Key observations have removed scientific uncertainty (i.e. will we see anything?). Technical Elements: low risk extensions of ongoing efforts and leveraging investments from ALMA and other development projects. Collaboration: necessarily a international project using many facilities. Strong element of training at intersection of submm science and interferometry.

19. Adding Telescopes Phase 1: 7 TelescopesPhase 2: 10 Telescopes Phase 3: 13 Telescopes

20. Technical Elements of EHT ALMA quality, dual pol rx and LO for all sites. Testing hardware for in-situ verification. Central correlation facility. VLBI backends/recorders at rates up to 64Gb/s Phased Array processors (ALMA, PdeBure, CARMA, Hawaii) Low noise freq. references. Logistics/Observations/Project Management

21. Hawaii Phased Array Success (CSO+JCMT+SMA)-CARMA

22. Progression to an Image: 345GHz GR Model7 Stations 13 Stations

23. Summary 1.3mm VLBI confirms ~4Rsch diameter for SgrA* Non-Imaging VLBI can extract BH parameters. Imaging soon possible with EHT developments. Increased sensitivity enables submm VLBI to directly probe Event Horizon scales and trace time variable structure (complements GRAVITY, IXO). Over the next decade: A VLBI Event Horizon Telescope will address fundamental questions of BH physics and space-time.

24. A Unique Opportunity “Right object, right technique, right time”