1. Theoretical Motivation for Submm-VLBI of Sgr A* Heino Falcke ASTRON, Dwingeloo University of Nijmegen

2. Why bother? “Boson Star” Instead of Black Hole? 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 R s  Requires high-resolution observations to rule out Torres et al. (2000)

3. 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 Gondolo & Silk (1999) Immediate vicinity of the black hole.

4. Radio Emission from Neutralino Annihilation near Sgr A* Gondolo (2000)  no spike or no neutralino …

5. Correlation between Size and Spectrum of Sgr A* event horizon shadow of event horizon “submm-bump” cut-off The spectrum cuts off at the size scale of the event horizon! size 10 g 100 R g 1000 R g 1 R g Size of Sgr A*

6. 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 B- fields)

7. 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)

8. Varying the Models Infall: a=0.998 i=90º I=r -2 Infall: a=0 i=90º I=r -2 Jet: a=0 i=45º I=hollow Jet: a=0.998 i=90º I=hollow Whatever the model looks like the shadow is always visible! If there is a black hole, we are going to see it. Whatever the model looks like the shadow is always visible! If there is a black hole, we are going to see it.

9. Simulate mm-VLBI imaging of Sgr A* 3D General Relativistic Ray-Tracing of a 2.6 ·10 6 M  black hole at the Galactic Center. Include interstellar scattering and instrumental resolution. (Falcke, Melia, Agol 2000) The shadow of the event horizon is 35  arcsec — resolvable by mm-VLBI! decreasing wavelength (mm)

10. 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.