Theoretical Motivation for Submm-VLBI of Sgr A*

1
Theoretical Motivation for Submm-VLBI of Sgr A

• Heino Falcke
• ASTRON, Dwingeloo
• University of Nijmegen

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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)
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Black Hole plus Dark MatterDark 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)
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Radio Emission from Neutralino Annihilation near
Sgr A
?no spike or no neutralino
Gondolo (2000)
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Correlation between Size and Spectrum of Sgr A
submm-bump
cut-off
The spectrum cuts off at the size scale of the
event horizon!
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Optical Depth

• The submm bump has an optical depth t1, because
• High-frequency spectrum turns over
• is highly variable
• Suggested by SSC models for the X-ray emission
  (implying equipartition B-fields)

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Predictions for submm-interferometryThe Shadow
of a Black Hole
?0.6mm VLBI
?1.3mm VLBI
GR Model
a0.998 Ir-2
a0 Iconst
(Falcke, Melia, Agol 2000)
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Varying the Models
Jeta0.998i90ºIhollow
Infalla0.998i90ºIr-2
Whatever the model looks likethe shadow is
always visible! If there is a black hole, we
aregoing to see it.
Infalla0i90ºIr-2
Jeta0i45ºIhollow
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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)
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Issues

• All models must go GR at 1.3 mm.
• Optimal range for shadow detection is 0.8-0.6 mm
  VLBI, need 1001 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.