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Theoretical Motivation for
Submm-VLBI of Sgr A*
Heino Falcke
ASTRON, Dwingeloo
University of Nijmegen
Why bother?
“Boson Star” Instead of Black Hole?
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Dark matter particles: weakly
interacting bosons and scalar fields may
contribute to the astrophysical mass
budget: Higgs scalar, Axions, etc. …
Can one form a central dark mass
concentration out of bosons?
prevented from collapse (pressure) by
uncertainty principle
particles are mildly relativistic
no solid surface (“boson sponge”) and
no horizon
wide mass range of particles can be
accommodated
Mimics black hole outside some 10 Rs
Requires high-resolution observations to
rule out
Torres et al. (2000)
Black Hole plus Dark Matter:
Dark Matter Spike at the GC
• If dark matter is weakly
interacting, there will be slow
accretion towards the center.
• This process can grow black
holes (see also Ostriker 2000
or Munyaneza & Biermann
2003)
• A spike in the dark matter
distribution is expected.
• If the spike is steep any
products from dark matter
interactions will be dominated
by the GC.
 Radio and gamma-rays
Immediate vicinity
of the black hole.
Gondolo & Silk (1999)
Radio Emission from Neutralino
Annihilation near Sgr A*
Gondolo (2000)
no spike or no neutralino …
Correlation between
Size and Spectrum of Sgr A*
“submm-bump”
1000 Rg
cut-off
10 g
shadow of event horizon
event horizon
1 Rg
The spectrum cuts off at the size scale of the event horizon!
Size of Sgr A*
100 Rg
Optical Depth
• The submm bump has an
optical depth τ≤1,
because:
– High-frequency spectrum
turns over
– is highly variable
– Suggested by SSC models
for the X-ray emission
(implying equipartition Bfields)
Predictions for submm-interferometry:
The Shadow of a Black Hole
GR Model
0.6mm VLBI
1.3mm VLBI
a=0.998
I=r-2
a=0
I=const
(Falcke, Melia, Agol 2000)
Varying the Models
Infall:
a=0.998
i=90º
I=r-2
Infall:
a=0
i=90º
I=r-2
Whatever the model looks like
the shadow is always visible!
If there is a black hole, we are
going to see it.
Jet:
a=0.998
i=90º
I=hollow
Jet:
a=0
i=45º
I=hollow
Simulate mm-VLBI
imaging of Sgr A*
decreasing wavelength (mm)
• 3D General Relativistic
Ray-Tracing of a 2.6 ·106
M black hole at the
Galactic Center.
• Include interstellar
scattering and
instrumental resolution.
• The shadow of the event
horizon is 35 arcsec —
resolvable by mm-VLBI!
(Falcke, Melia, Agol 2000)
Issues
• All models must go GR at 1.3 mm.
• Optimal range for shadow detection is 0.8-0.6 mm VLBI, need
100:1 dynamic range.
• Explore closure quantities – what can we identify?
• Polarization can probably not be ignored!
• Minute time scale variability can shift the source but also reveal
physical properties!
• Relative location and size of shadow can give spin.
• Dual-frequency experiments to separate (achromatic) GR effects
from (wavelength-dependent) optical depth effects?
• The program should be set up and funded like a dedicated physics
experiment: one goal, one target.