Detection of the Intrinsic Size of Sagittarius A* with Closure Amplitude Imaging

Geoffrey C. Bower

UC Berkeley

Team Closure

Backer, D.C. (UCB) Falcke, H. (MPIfR) Goss, W.M. (NRAO) Herrnstein, R. (CfA/Columbia) Zhao, J.-H. (CfA)

Science, accepted

What We Want to See What We See

7mm VLBA

Scattering Inhibits Imaging &Points to Higher Frequencies

Lo et al. 1998

Difficulty of mm Imaging SgrA* Axisymmetric Structure

Purely an amplitude measurement Low Declination & High Frequency

Poor and variable antenna gain Variable opacity High Tsys Short and variable coherence time Lack of North-South resolution

Solution Closure Amplitudes

Closure Amplitude Properties

Independent of station-based gain errors Still dependent on baseline-based errors

Decorrelation Reduced sensitivity

2/3 for N=7 Non-Gaussian errors

Doeleman et al. 2000 --- 3mm imaging

Cmnpq = -----------------VmnVpq

VmqVnp

Data & Techniques

New and archival VLBA & VLA data 10Q, 4K, 1U, 2X, 2C, 3L

Fringe-fit, form c.a. Non-linear model-fitting

of c.a. Grid of models around

best-fit solution to determine errors

Results: 43 GHz

Equal Scales

New Results:Deviation from Scattering

0.3 cm 20 cm

10 Q, 4 K, 1 U, 2X, 2C, 3L experiments

0.3 cm 20 cm

Alternatives

Errors in modeling Monte Carlo simulations show we get what we put in

Phase decorrelation No change in intrinsic size with 15-120 sec averaging time

Multiple components Reduced chi^2 not improved

Refractive effects Observations over a short span give larger 7mm size

Slope = 1.96 +/- 0.01 Strongly inconsistent with scattering theory! Weakly inconsistent with data

Intrinsic Size

New scattering law derived Major Axis

24 +/- 2 Rs at 0.69 cm Alpha_size = 1.6 +/- 0.2 Flare in 1/9 Q band epochs: 60 +25 -15%

Minor Axis 26 +/- 6 Rs at 0.69 cm < Lo et al size Alpha_size= 2.2 +/- 0.5

Intrinsic Size

Emission/Accretion Implications Axisymmetry? Tb > 1010 λ-1.2 K Requires inhomogeneous models

Inconsistent with ADAF, single zone models Jet RIAF

Source comparable to Rs at 1.3mm Physics in strong gravity

BH mass density 4 x 104 Msun AU-3

What’s Next?

Add GBT at 7mm Links SC/HN to rest of

array Increased SNR for

closure amplitude 3mm Model-dependent

deconvolution

The Future

Meeting Friday after dinner