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Theoretical Motivation for Submm-VLBI of Sgr A*

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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Theoretical Motivation for Submm-VLBI of Sgr A*

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  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 Rs • Requires high-resolution observations to rule out Torres et al. (2000)

  3. Black Hole plus Dark Matter:Dark Matter Spike at the GC Immediate vicinity of the black hole. • 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)

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

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

  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 0.6mm VLBI 1.3mm VLBI GR Model a=0.998 I=r-2 a=0 I=const (Falcke, Melia, Agol 2000)

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

  9. 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)

  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.

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