1. C. Darren Dowell Jet Propulsion Laboratory 2006 Oct 24
Observing the Environment of Sagittarius A*, the Nearest Supermassive Black Hole, with CCAT
C. Darren Dowell
Jet Propulsion Laboratory
2006 Oct 24

2. 200 mm is the perfect l for observation, and only CCAT can do it.
Executive Summary: CCAT will probe (for the first time at the spectral peak) the magnetized gas very near the event horizon of a supermassive black hole.
Sgr A*
environment of M = 4×106 Msolar black hole in the center of the Milky Way
The majority of photons in the accretion zone are submillimeter wavelength, produced by synchrotron; some may be upscattered to X-rays during flare events.
200 mm is the perfect l for observation, and only CCAT can do it.
What CCAT will see: accretion/jet events, magnetic field order changing on timescales of minutes

3. The spectrum of Sgr A* turns over somewhere in the far-infrared.
Genzel et al. (2003)
CCAT coverage (l = mm) shown in yellow.
Note ability of l = 200 mm to distinguish models.

4. Sgr A* is bright in the submm/far-IR, but surrounded by even brighter dust emission.
Only CCAT has the combination of angular resolution and sensitivity to study Sgr A* at 200 mm.

5. polarization fraction
Detecting submillimeter variability and polarization of Sgr A* is now routine from Mauna Kea.
polarization fraction
polarization angle
Marrone et al. (2006) SMA, l = 850 mm

6. However…
Very recent data (time delay from X-ray to submillimeter flare) suggests emission is optically thick at l = 850 mm  To see all the way in to source, need to go to shorter wavelengths, where existing telescopes lose detection speed.
Existing telescopes apparently do not have enough sensitivity to detect orbital signatures:
2 hour (25 RSchwarzschild): obvious in radio, maybe seen in submm?
20 minute (2 RSchwarzschild): seen in near-IR, not reported in submm
Chandra (F. Baganoff)
CSO (D. Dowell)

7. Investigations with CCAT
Concentrate on highest frequencies (l = 200 and 350 mm), where the emission is likely to be optically thin.
Signal-to-noise of (respectively) in one second of integration, sufficient to see any tiny orbital modulations.
Use surrounding emission to monitor calibration stability with <<10% uncertainty.
Polarimetry should be easy, with significant detection likely after ~1 minute integrations. Faraday rotation can be exploited:
Small rotation at shortest wavelengths, yielding absolute position angle of net magnetic field.
Field strength estimate using longer wavelengths.
Admittedly, CCAT comes nowhere close to angularly resolving the emission from Schwarzschild radii, which is in principle a mix of Bondi-Hoyle and disk accretion and a jet, both relativistic. However, until far-IR/submm VLBI, CCAT will provide the strongest constraints on the accretion rate and magnetic field near the event horizon.