1. mm/submm VLBI of Sagittarius A*: An Event Horizon Telescope
MBH – paradigm, useful but dangerous, need to really push techniques to make MBH case solid.
Shep Doeleman
MIT Haystack Observatory

2. Centaurus A: Optical
D=3Mpc, Elliptical eating a smaller spiral galaxy, star formation along dust lanes, dust lane disk offset from radio jet. Nearest extragalactic AGN

3. Centaurus A: Radio

4. Chandra, VLA, VLT

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

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

7. 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.

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

9. High Frequency VLBI Resolution: /D (cm) ~ 0.5 mas
/D (1.3mm) ~ 30 as
/D (0.8mm) ~ 20 as
ISM Scattering:
scat ~ 
Self Absorption:
Intrinsic SgrA* size
decreases with 

10. VLBI Basics Earth Rotation Baseline Coverage Interferometer F T
Fourier Components (visibilities) Map

11. 3mm and 7mm VLBI
7mm (Bower et al 2004): VLBA
Scattering size 670as, Observed size 712as
Intrinsic size = 24 Rsch
3mm (Shen et al 2005): VLBA
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.
High Frequency Challenges: Atmosphere, LO’s, weather, frequency reference stability.
Increased sensitivity is critical.

12. Resolving Rsch-scale structures
Falcke
Melia
Agol
Spinning (a=1)
Non-spinning (a=0)
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)

13. New 4Gb/s VLBI System Total cost $40-50K per station.
Digital Backend (DBE)
Digital Recorder (Mark5)
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.

14. 1.3mm Observations of SgrA*
908km
4030km
4630km
Carried out 230GHz VLBI. 2. High resolution See through scattering 4. Design of experiment simple (3 baselines) and aimed at determining if there was structure on scales implicated by all light curve, vlbi, proper motion evidence already shown. These high freq obs make use of non-vlbi-facility instruments: have to create arrays using separate mm/submm facilities, but it can be done.
DAYS OBSERVING - April 2007

15. 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

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

17. 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.
m87 is largest MBH candidate, but enclosed mass poorly constrained.
Maoz 1998

18. A Surface or Event Horizon
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.
Broderick & Narayan 2006

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

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

21. But…April 2009 SgrA* Detections

22. 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.
RIAFs : Radiatively Inefficient Accretion Flows

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

24. Constraining RIAF Models with VLBI
Broderick, Fish, Doeleman & Loeb (2009)

25. Adding Telescopes
Hawaii, CARMA, SMT
Adding LMT
Adding ALMA

26. 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.

27. Hot Spot Model for SgrA* Flares

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

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

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

31. Detecting Rsch Polarization Structures

32. 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 leverage investments from ALMA and other development projects.
Collaboration: necessarily a international project but with well defined boundaries.
Strong element of training at intersection of submm science and interferometry.

33. Event Horizon Telescope
Phase 1: 7 Telescopes Phase 2: 10 Telescopes
Phase 3: 13 Telescopes

34. Progression to an Image: 345GHz
GR Model
7 Stations
13 Stations

35. Technical Elements of EHT
Adding Telescopes: uv coverage, flare coverage, closure quantities for real-time modeling.
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

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

37. EHT Phases: Phase I: 7 station 8Gb/s array
ALMA phasing, preliminary Rx/LO work, new frequency standards, new site studies, operations.
Phase II: 10 station 32Gb/s dual-pol array
Activate SEST, equip S.Pole, new frequency standards, install new 0.8/1.3mm dual-pol Rx, increase bandwidth of VLBI backends/recorders, relocate ATF dishes, operations.
Phase III: 12 station array up to 64Gb/s
install ATF dishes, array operations for 5 years.

38. VLBA Movie of M87 @ 43 GHz (7 mm) Walker et al. 2008
More luminous class of AGN with more massive central BH
Eg M87, half the apparent size of SgrA* (1000 x more massive)
Beam: 0.43x0.21 mas mas = 0.016pc = 60Rs mas/yr = 0.25c

39. April 2009 M87 1.3mm VLBI Detections
CARMA1-CARMA2
CARMA-ARO/SMT
JCMT-CARMA
JCMT-ARO/SMT
Resolved component
is > 300as
Compact component
is 35as FWHM, or
~ 4.5 Rsch.

40. A Closeup of M87 Mass of M87 Black Hole = 6.4x109 Msol
The angular size of the Schwarzschild radius is similar to SgrA*’s: 8as.
Apparent size of Black Hole ‘Shadow’ is 40as (5.2 Rsch for non spinning BH).
Apparent Size of the Innermost Stable Circular Orbit (ISCO) is 60as.
These are the relevant size scales for models of TeV photon generation and jet formation.
1.3mm VLBI is now detecting compact structure on these size scales.
With higher sensitivity, more VLBI sites, and the phased array at SMA, this technique can image a radio loud AGN on Rsch scales.

41. Summary Non-Imaging VLBI can extract BH parameters.
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.

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

43. 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