Abstract
We report the first detections of circularly polarized emission at submillimeter wavelengths from the compact radio source and supermassive black hole candidate Sgr A* at a level of 1.2\pm0.3% at 1.3 mm wavelength (230 GHz) and 1.6\pm0.3% at 860 microns (345 GHz) with the same handedness as observed at lower frequencies (1.4-15 GHz). The observations, taken with the Submillimeter Array in multiple epochs, also show simultaneous linear polarization (LP) at both wavelengths of about 6%. These properties differ sharply from those at wavelengths longer than 1 cm (frequencies below 30 GHz), where weak circular polarization (CP) (~ 0.5%) dominates over LP, which is not detected at similar fractional limits. We describe an extensive set of tests to ensure the accuracy of our measurements. We find no CP in any other source, including the bright quasar 1924-292, which traces the same path on the sky as Sgr A* and therefore should be subject to identical systematic errors originating in the instrument frame. Since a relativistic synchrotron plasma is expected to produce little CP, the observed CP is probably generated close to the event horizon by the Faraday conversion process. We use a simple model to show that the phase shift associated with Faraday conversion can be nearly independent of frequency, a sufficient condition to make the handedness of CP independent of frequency. Because the size of the tau=1-surface changes by more than an order of magnitude between 1.4 and 345 GHz, the magnetic field must be coherent over such scales to consistently produce left CP.
Highlights
The Galactic Center source Sagittarius A* (Sgr A*), the nearest supermassive black hole (SMBH), is extremely underluminous for its mass (∼ 4 × 106 M, Ghez et al 2008; Gillessen et al 2009), radiating at only 10−9 LEdd
In addition to the stochasticity of the magnetic field—which appears to play a crucial role in building up circular polarization (CP) by propagation effects—the helical geometry of jets might be important in high levels of linear polarization (LP), above 100 GHz
The orientation of Bsky is free to change in time without affecting the value of V. This is a consequence of the sequence Q → U → V given by eq 5, where the first step is performed by Faraday rotation depending only on Blos and the fact that Stokes V does not change with coordinate transformations in the plane of the sky
Summary
The Galactic Center source Sagittarius A* (Sgr A*), the nearest supermassive black hole (SMBH), is extremely underluminous for its mass (∼ 4 × 106 M , Ghez et al 2008; Gillessen et al 2009), radiating at only 10−9 LEdd. Some models seeking to explain the millimeter and submillimeter LP have predicted CP at these high frequencies due to the conversion of LP to CP in a turbulent jet (e.g., Beckert & Falcke 2002) In these models, in addition to the stochasticity of the magnetic field—which appears to play a crucial role in building up CP by propagation effects—the helical geometry of jets might be important in high levels of LP, above 100 GHz. Coupled mechanisms to produce both LP and CP in relativistic outflows have been studied in detail by many authors (e.g., Ruszkowski & Begelman 2002; Beckert & Falcke 2002; Beckert 2003; Huang et al 2008; Homan et al 2009; Shcherbakov et al 2010). Mechanisms for generating CP, and their applicability to Sgr A*
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