imaging the event horizon: past, present & future vlbi of sgr a* geoffrey c. bower uc berkeley
TRANSCRIPT
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!