Polarization Studies with the Submillimeter Array
 

Polarization of the Galactic Center Sgr A*

Sagittarius A* is the radio/IR/X-ray source associated with the supermassive black hole at the center of the Milky Way. It is remarkable both for its proximity, which allows us to study it in great detail, and its very low luminosity, 10-8 Eddington luminosities, the lowest of any known AGN. Despite the considerable observational resources devoted to this object, particularly in the last several years, much mystery remains about the origin of its low luminosity. Submillimeter polarimetry provides a unique tool for studying this source because 1) submillimeter emission arises within a few Schwarzschild radii of the event horizon, 2) polarization signals are modified by propagation through the accretion flow in ways that can be used to probe the structure of that flow, and 3) linear polarization is not observed at frequencies below ~100 GHz.

The first detection of millimeter linear polarization (Aitken et al. 2000) was used to place upper limits on the accretion rate of Sgr A*, and these observations provided a significant part of the motivation for the construction of the SMA polarimeter. This instrument has contributed significantly to our understanding of this source and revealed new mysteries, and the observational requirements of the Sgr A* observations have driven advancements in the capabilities of the instrument and the entire SMA. The first results from the SMA polarimeter confirmed the submillimeter (880 µm) polarization of Sgr A*, isolating the signal from the surrounding polarized emission and revealed intraday variability in the linear polarization (Marrone et al. 2006). Following an upgrade that allowed 230 GHz polarimetry, Marrone et al. (2007) made the first measurement of Faraday rotation in this source, which immediately restricted the accretion rate to the range 2 x 10-7 to 2 x 10-9 solar masses per year, implying very significant mass loss between the scales on which gas is captured and the inner accretion flow.

Interestingly, the rotation measure was not observed to vary, and has remained stable for several years, contrary to the expectation for a turbulent accretion flow. This stability may indicate a magnetically dominated accretion/outflow structure, or a special sight line to the submillimeter emission. The sensitivity of the SMA has allowed high-time resolution observations of the polarization, revealing structure on timescales corresponding to orbits in the inner accretion flow (Figure 1, Marrone et al. 2006b). The periodic structure of these polarization changes are suggestive of an orbiting plasma hot spot (e.g. Broderick & Loeb 2006), although such an origin cannot yet be confirmed. These polarization changes were not previously known but could be used to constrain the black hole spin if they can be uniquely identified with these hot spots. Finally, the SMA has found circular polarization in Sgr A*, the first in an AGN at submillimeter wavelengths (Muñoz et al., in prep). This is completely unexpected and adds to the mystery of this source. In other objects CP is associated with radio jets, although this interpretation is not required. As with the Faraday rotation, the circular polarization is found to be stable over years.



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Figure 1: The variability of Sgr A* in total intensity (top), polarization fraction (middle) and position angle (bottom) on 2007 March 31. The time resolution of this light curve is 4 minutes, to allow the identification of the orbital timescales expected very near to the black hole. The dramatic polarization modulation is not obviously connected to the strong flare in the top panel around 16UT (Marrone et al. 2006b).



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