Imaging the Event Horizon: Past, Present

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., 105 range for M_dot

4
Sgr A Basic Properties

• Supermassive Black Hole
• 3 x 106 M_sun
• Extremely underluminous
• L L_sun 10-10 L_edd
• Inverted Spectrum
• a gt 0.1 0.7
• Compact, nonthermal
• Size lt 1 AU _at_ 3mm
• Tb gt 109 K

5
Models of Sgr AWhy is L ltlt L_edd?

• Under-fed systems
• Jet
• CDAF
• Bondi-Hoyle
• Under-luminous systems
• ADAF

6
Models of Sgr AWhy is L ltlt L_edd?

• Under-fed systems
• Jet
• CDAF
• Bondi-Hoyle
• Under-luminous systems
• ADAF

1mm Polarization Indicates dM/dt lt 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
VmnVpq
Cmnpq -----------------
VmqVnp
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 N7
• 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 ResultsConsistent with Scattering
9 Q, 4 K, 1 U experiments
3 7 13 20 mm
3 7 13 20 mm
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 mas ? 60 /- 30 R_g
• Delta Minor axis 40 mas ? 90 /- 90 R_g
• No outflow? Slow outflow? Along line of sight?

22
Whats 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 lt 1 at 1.3 mm
• Sgr A is the only realistic candidate

25
Whats 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
• gt 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!