Observing an Event Horizon: submm VLBI of Sagittarius A*

1. Observing an Event Horizon:submm VLBI of Sagittarius A* Shep Doeleman MIT Haystack Observatory

2. Haystack Observatory

3. 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 JHU - Applied Physics Labs: Greg Weaver Honeywell: Irv Diegel

4. Centaurus A: Optical

5. Centaurus A: Radio

6. Chandra, VLA, VLT

7. Accretion • Radiation • Outflow • GR in strong field regime • Accretion/jet models • Black hole parameters A. Fabian Chandra Web site

8. Vstars ~104 km/s Central Mass M ~ 4x106 M Within 45 AU. Rsch = 10as

9. Proper Motion Studies • Is SgrA* the central mass? • Luminosity consistent with stellar source. • Proper motion of SgrA* measured with VLA and VLBA : +/- 1km/s upper limits (Reid & Brunthaler 2004, Backer et al 1999) • If SgrA* in dynamical equilibrium with stars, then M>4x105 Msol. • So SgrA* is not Stellar and is coincident with dynamical center of Galaxy.

10. 10000 20000 30000 Time offset (s) X-ray/NIR Flares: An Indirect Size VLT: Genzel et al 2003 ~17 min periodicity? Orbiting hot-spot at R=4 Rsch Baganoff et al 2001 Rise time < 300s Light crossing < 12 Rsch

11. What we really want: the ‘Shadow’ Falcke et al free fall rotating orbiting non- rotating 0.6mm VLBI GR Code 1.3mm VLBI SgrA* has the largest apparent Schwarzschild radius of any BH candidate. BUT… SgrA* scattered ~ 

12. The VLBI Technique /D (cm) ~ 0.5 mas /D (1.3mm) ~ 30 as

13. VLBI Basics Earth Rotation Baseline Coverage Interferometer F T • Map must be real valued • Usually most of map is blank • Visibilities Map • Sparsely Sampled

14. Radio Scattering of OH/IR Stars Due to turbulence in ionized screen near the GC. Anisotropic scattering results from B field alignment. SgrA* is scattered into an ellipse. Frail et al 1994

15. Scattering towards SgrA* • Scattering size ~ l2 • Intrinsic Structure masked by scattering • Use high frequency VLBI : resolution increases and scattering descreases.

16. 3mm and 7mm VLBI • 7mm (Bower et al 2004) • Scattering size 670as, Observed size 712as • Intrinsic size = 24 Rsch • 3mm (Shen et al 2005) • Scattering size 170as, Observed size 210as • Intrinsic size = 12.6 Rsch • Both in scattering dominated regime and insensitive to ISCO scale (60as for a=0). • Can’t use ‘facility’ VLBI instruments VLBA: ad hoc arrays using (smaller) mm/submm apertures.

17. New 4Gb/s VLBI System Digital Recorder (Mark5) Digital Backend (DBE) • Total cost $40-50K per station. • x16 in BW over current VLBA sustainable rates. • Equivalent to replacing VLBA with 50m antennas. • Planned VLBA/HSA 4Gb/s upgrade by early 2009: x4 in sensitivity over current VLBA sustainable rate.

18. 1.3mm Observations of SgrA* 908km 4030km 4630km days out of 5 possible (weather), 4Gb/s at each site, new Hydrogen maser at Mauna Kea and CARMA.

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

20. Seeing Through the Scattering OBS deviates from scattering for cm INT  SCAT for mm INT 

21. Alternatives to a MBH Most condensations of smaller mass objects evaporate on short timescales. Remaining possibility: Boson Star R=Rsch + epsilon Depends on Boson mass M87? Mass BH = 3.2x109Msol Distance = 16Mpc Rsch=4 micro arc sec. Maoz 1998

22. Does SgrA* have a surface? • A quiescent surface would radiate. • The radius R and dM/dt set the expected NIR flux. • NIR limits set dM/dt limits. • These are too low to power SgrA*. • If no surface then Event Horizon. A Surface or Event Horizon Broderick & Narayan 2006

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

24. SMT-CARMA SMT-JCMT Caveat: Very Interesting Structures 14 Rsch (140as) Gammie et al

25. SgrA*: Luminosity • Leddington ~ 3.3x1044 erg/s • Accretion estimates : 1.6x10-5 Msol/yr to 3x10-6 Msol/yr, stellar loss ~3x10-3 Msol/yr • If 10% efficiency then Lacc ~ 1041 erg/s • Lx (Chandra) = 2x1033 erg/s (2-10 keV) • Lradio < 1036 erg/s • SgrA* is 10-8 Ledd and over 10-5 less than expected from accretion arguments.

26. Radiatively Inefficient Accretion Flow • Much (over 90%) of captured material is convected away. • Flares in IR and Xray caused by SSC process. • RIAF can be combined with jet (mini-AGN) • Can such models be constrained with 1.3mm VLBI data? Yuan, Quataert, Narayan 2003

27. Constraining RIAF Models andSimulating VLBI Observations of Flares in the Galactic Center V. Fish, A.E.E. Rogers (MIT Haystack) Avery Broderick (CfA/CITA) Avi Loeb (Harvard CfA)

28. Model Correlated Flux Density Quiescent disk models Courtesy Gammie et al and Broderick&Loeb

29. Constraining RIAF Models with VLBI Broderick, Fish, Doeleman & Loeb (2008)

30. Constraining the RIAF Model with VLBI Fish et al (2008)

31. 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.

32. Hot Spot Model for SgrA* Flares

33. ISCO and Black Hole Spin Orbital Period gives spin. Other (indirect) methods for detection of spin: -- QPO’s -- Modeling thin disks -- GR distortions of Fe lines -- IXO: spectral obs of individual hot-spots. -- GRAVITY: NIR obs of SgrA* centroid motion. C. Reynolds

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

35. Hot Spot Model (a=0, i=30) SMTO-Hawaii-CARMA, 8Gb/s, 230GHz, 10sec points

36. Closure Phases: Hawaii-CARMA-Chile Spin = 0.9 Hot-spot at ~ 6Rg Period = 27 min.

37. Phased array processors: creating large effective VLBI apertures (with SAO). Technical Advances • Increasing bandwidth of VLBI systems: Burst Mode recorders using COTS. • Designing new VLBI freq. standards (UWA). • Adding new VLBI sites (Chile, Mexico). • VLBI Polarimetry - building new low noise dual polarization receivers.

38. A Unique Opportunity“Right object, right technique, right time” • SgrA* presents the largest Event Horizon of any BH candidate - Orbital timescales are minutes. • VLBI at 1.3mm and 0.8mm yields ~2 Rsch resolution for SgrA*. • Technical advances and new facilities now make this possible.

39. Event Horizon Telescope

40. 1.3mm VLBI confirms ~4Rsch diameter for SgrA* • Non-Imaging VLBI can extract BH parameters. • submm VLBI is able 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. Summary