# Presentation: 208634

## Detecting the Shadow of the Galactic Center Black Hole

- James M. Bardeen
- University of Washington
- Supermassive Black Holes, Santa Fe
- July 14, 2006

## References and Acknowledgements

- J. M. Bardeen, in Black Holes, Les Houches 1972
- Falke, Melia, Agol, astro-ph/9912263
- Melia and Falcke, Ann. Rev. Astron. Astrophys. 2001
- Bower, et al, astro-ph/0404001
- Shen, et al, astro-ph/0512515
- Bower, et al, submitted to Ap J Letters
- Special thanks to Heino Falcke for lending slides from his 2005 talk at Garching.

## Photon trajectories in the Kerr metric

## Finding the shadow edge

## Shape a= 0.5 I = 90

- <date/time>

## Shape a = 0.9 I = 90

- <date/time>

## Shape a = 0.99 I = 90

- <date/time>

## Shape a = 0.99 I = 70

- <date/time>

## Shape a = 0.99 I = 45

- <date/time>

## Emission not subject to shadowing

## Headlight Pos

- <date/time>

## Headlight Blueshift

- <date/time>

## The Shadow of a Black HoleIt’s getting closer!

- GR Model

- 0.6mm VLBI

- 1.3mm VLBI

- a=0.998
- I=r-2

- a=0
- I=const

- (Falcke, Melia, Agol 2000, ApJL)

- 10 Rg=44 μas!

## Varying the Models

- _Infall:_a=0.998i=90oI=r-2

- _Infall:_a=0i=90oI=r-2

- _Jet:_a=0i=45oI=hollow

- _Jet:_a=0.998i=90oI=hollow

- Agol, Falcke, Melia, et al. (2001), conf. proc.

## High-Precision VLBI Measure-ments of the Size of Sgr A*

- High-Precision VLBI Measure-ments of the Size of Sgr A*

**Size/********2**

**Size/********2**

******2****-law**

******2****-law**

- intrinsic size = 2 AUbut frequency dependent (>7σ effect)

- pure scatter broadening

- ±2% size

**                    ****43 GHz        22 GHz  15 GHz         8 GHz        5 GHz                             1.4 GHz**

- Bower, Falcke, Herrnstein, et al. (2004), Science

## Correlation between Size and Spectrum of Sgr A*

- “submm-bump”

- cut-off

- The spectrum cuts off at the size scale of the event horizon!

- BR-1nR-2

- Size of    collimation region   is <10 Rg!>21023 M/pc3
- From Tb(ν)-2 & ν-1 Tb~const, cs ~ 0.1 cSgr A* unbound   (at least ν<15 GHz)

## Submm Telescopes

- SMA

- Hawaii

- California

- Arizona

- Chile

- Mexico

## High Frequency VLBI Challenges

- Sensitivity limited by
- atmospheric coherence,
- telescope apertures,
- bandwidth,
- weather (opacity and coherence).
- Baseline coverage: small number of high demand sub/mm telescopes.
- Receiver Electronics noisier.

## High Frequency VLBI Challenges

- Scheduling mm/submm telescopes is difficult.
- Array phasing: difficult to phase submm arrays into single aperture.
- Hydrogen masers: very stable but at high frequencies they introduce signal loss.
- Amplitude calibration is difficult: array phasing, pointing, atmosphere, gain curves.
- But: Major improvements can be expected in some of these areas.

## Future Plans

- Technical improvements in VLBI developing rapidly: BW up x10, cost down x 20.
- Initial experiments show 230GHz VLBI on SgrA* possible.
- Timeline:
- Formation of international consortium
- Jan 2006: 230GHz VLBI on SgrA*: JCMT-HHT.
- Jan 2007: 230GHz VLBI: JCMT-HHT-CARMA/ ALMApt
- APEX-SMA. Test 345GHz systems.
- Jan 2008: 230/345 GHz VLBI: add LMT
- ‘Shadow’ imaging within 5-7 years still possible

## Conclusions

- A sustained effort should make it possible to resolve the Sgr A* black hole within 5-10 years.
- The shadow itself is not a good diagnostic of the properties of the black hole, but it would be really neat to see it.
- The real payoff will be understanding much more about how accretion works near the event horizon.  Is there a disk?  Is there a jet?  How important is the innermost stable circular orbit?