Title  The general relativistic shadow of Sgr A*
Pi     C. Carilli
Time   16 hrs

5.1.1: Name -- The general relativistic shadow of Sgr A*
    Authors: C. Carilli

2. Science goal: We propose VLBI imaging at 220 GHz of Sgr A* using
the 'Pacific array' (see note below). These observations will allow
for reasonable imaging at 20 uas resolution, well matched to the scale
of the expected general relativistic shadow of the SMBH in Sgr A*
(Falcke et al.2000 528, L13). These observations will provide the
final evidence for the existence of a SMBH at the Galactic center,
provide a fundamental test of strong field GR, and are the most direct
method for separating a Kerr (ie. spinning) from a Schwarzschild
black hole.  At a minimum, the sensitivity per baseline is adequate to
perform model fitting on relatively short timescales (minutes), while
the array itself has enough antennas to provide both closure amplitude
and phases, and hence should be adequate for hybrid imaging of the GR
shadow of Sgr A*. The existence of reasonable mm-VLBI calibrators (eg
NRAO 530) in the vicinity of Sgr A* will allow for phase-referenced
fringe fitting, although the source itself is strong enough, and the
UV coverage dense enough, to allow for hybrid mapping as well.  The
source Sgr A* has been detected at 220 GHz on the PdBI -- Pico Veleta
baseline (resolution = 300 uas) with a flux density of 2.0 Jy, and an
upper limit to the size of order 100 uas (Krichbaum et al. 1998 335,
L106). The proposed observations will have more than an order of
magnitude better resolution, more than two orders of magnitude better
sensitivity, and, again, enough antennas to perform proper imaging of
Sgr A*.

3. Number of sources: 1

4. Coordinates:

  4.1. 1742-2859

  4.2. Moving target: no

  4.3. Time critical: no

5.  Spatial scales:

  5.1. Angular resolution: 20uas

  5.2. Range of spatial scales/FOV: 20 uas -- 100 uas

  5.3. Single dish: no

  5.4. ACA: no

  5.5. Subarrays: no

6. Frequencies:

  6.1. Receiver band: Band 6 -- 220 GHz  in Configuration D or E,
       phased  array

  6.2. Lines and Frequencies

  6.3. Spectral Resolution (km/s)

  6.4. Bandwidth or spectral coverage: 1 GHz x 2pol (set by VLBI
recorder)

7. Continuum flux density:

  7.1. Typical value:

  7.2. Continuum peak value: <= 2 Jy

  7.3. Required continuum rms:
        ALMA - HHT baseline, 10 min rms = 0.7 mJy
        ALMA - LMT baseline, 10 min rms = 0.2 mJy

  7.4. Dynamic range in image: 1e3

8. Line intensity:

  8.1. Typical value:

  8.2. Required rms per channel:

  8.3. Spectral dynamic range:

9. Polarization: yes

10. Integration time per setting:
  4 tracks of 4hrs each

11. Total integration time for program: 16 hr

Notes: The pacific array at 220 GHz consists of: ALMA, HHT, LMT,
CARMA, and any of CSO/JCMT/SMA. We estimate mutual visibility of Sgr
A* of about 4hrs. Note that an Atlantic array might also be
considered, including PdBI, IRAM 30m.

****************************************************************************

Review Jean Turner: Fascinating project to detect the "shadow" of Sgr
A*, which is predicted to be at 10 Rsch. If the hole is spinning
(Kerr) the shadow will be flattened and offset. Unclear from the
proposal what will be seen, or what is "shadowed" in the radio but
whatever is there in SgrA* is well worth the effort. This is a modest
amount of time for a great project.

I have to take Chris's numbers on faith since they involve
other telescopes and VLBI. They seem very reasonable given the ALMA
sensitivities.


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Review v2.0:
 
1.6.3 The general releativisitic shadow of Sgr A* (Carilli)

Not revised since DRSP 1.1. Nothing needs to be added to the review
back then, no severe impact through reduction to 50 antennas, no need
for ACA.