Constraining the accretion rate onto Sagittarius A* using linear polarization

Two possible explanations for the low luminosity of the supermassive black hole at the center of our Galaxy are (1) an accretion rate of the order of the canonical Bondi value (similar to 10(-5) M. yr(-1)) but a very low radi ative efficiency for the accreting gas or (2) an accretion rate much less t han the Bondi rate. Both models can explain the broadband spectrum of the G alactic center. We show that they can be distinguished using the linear pol arization of synchrotron radiation. Accretion at the Bondi rate predicts no linear polarization at any frequency because of Faraday depolarization. Lo w accretion rate models, on the other hand, have much lower gas densities a nd magnetic field strengths close to the black hole; polarization may there fore be observable at high frequencies. If confirmed, a recent detection of linear polarization from Sgr A* at greater than or similar to 150 GHz argu es for an accretion rate of similar to 10(-8) M. yr(-1), much less than the Bondi rate. This test can be applied to other low-luminosity galactic nucl ei.