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

Imaging the Event Horizon: Past, Present & Future VLBI of Sgr A*

Geoffrey C. Bower

UC Berkeley

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)

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

Sgr A*: Basic Properties

Supermassive Black Hole 3 x 106 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

Models of Sgr A*:Why is L << L_edd?

Under-fed systems Jet CDAF Bondi-Hoyle

Under-luminous systems ADAF

Models of Sgr A*:Why is L << L_edd?

Under-fed systems Jet CDAF Bondi-Hoyle

Under-luminous systems ADAF

1mm Polarization IndicatesdM/dt < 10-7 M_sun y-1

What We Want to See

Structure Ejection of components Correlated changes

with X-ray variability Astrometric

measurements (Reid talk)

…?

What We See

Elliptical Gaussian 2 x 1 ratio East-West major axis

No detection of … Extended structure Separate components

Scattering Inhibits Imaging &Points to Higher Frequencies

Lo et al. 1998

Is there Structure?

Lo et al. 1998

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

Closure Amplitude

Cmnpq = -----------------

VmnVpq

VmqVnp

Independent of station-based gain errors!

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

Sample Closure Amplitudes

Error Surfaces

Slices through the Error Surface

Herrnstein et al. 2003, Zhao et al. 2003

Results: 22 GHz

Equal scales

Results: 43 GHz

Equal Scales

New Results:Consistent with Scattering

3 7 13 20 mm 3 7 13 20 mm

9 Q, 4 K, 1 U experiments

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?

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

The Future

Falcke, Melia & Agol 2000Bardeen 1973

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 Hole Horizon Size

Sgr A* 6.5 μ arcsec

M87 3.7

NGC 4649 2.4

Cen A 1.0

What’s Necessary for the Future?

3- or 4-station “Image”

1.5 Jy

Shadow

Best-fitGaussian

Technical Requirements

High frequency receivers & antenna performance 230/350 GHz

Phase stability Water vapor radiometers

Time standards Array Phasing

Correlator options > Gigabit recording

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!

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!