Interferometric Measurements of Variable 340 GHz Linear Polarization in Sagittarius A*

Daniel P Marrone,James M Moran,Ramprasad Rao,Jun‐Hui Zhao

doi:10.1086/500106

Abstract

Using the Submillimeter Array, we have made the first high angular resolution measurements of the linear polarization of Sagittarius A* at submillimeter wavelengths and the first detection of intraday variability in its linear polarization. We detected linear polarization at 340 GHz (880 μm) at several epochs. At the typical resolution of 14 × 22, the expected contamination from the surrounding (partially polarized) dust emission is negligible. We found that both the polarization fraction and the position angle are variable, with the polarization fraction dropping from 8.5% to 2.3% over 3 days. This is the first significant measurement of variability in the linear polarization fraction in this source. We also found variability in the polarization and total intensity within single nights, although the relationship between the two is not clear from these data. The simultaneous 332 and 342 GHz position angles are the same, setting a 1 σ rotation measure (RM) upper limit of 7 × 105 rad m-2. From position angle variations and comparison of quiescent position angles observed here and at 230 GHz, we infer that the RM is a few times 105 rad m-2, a factor of a few below our direct detection limit. A generalized model of the RM produced in the accretion flow suggests that the accretion rate at small radii must be low, below 10-6-10-7 M☉ yr-1 depending on the radial density and temperature profiles, but in all cases below the gas capture rate inferred from X-ray observations.

Highlights

The radio source Sagittarius A* (Sgr A*) has been conclusively identified in the radio and infrared with a black hole of mass ∼ 3.5 × 106M⊙ at the center of our galaxy (Reid & Brunthaler 2004; Schodel et al 2003; Ghez et al 2005; Eisenhauer et al 2005)
We examined the polarization by fitting point sources to the central parts of the images; the point source flux densities matched well with those obtained from the central pixel when the signal was well above the noise, but the point source positions and peak flux densities became erratic for low signal-to-noise images
The rotation measure associated with a plasma screen located between the source and observer can be inferred from the measurement of χ at two frequencies, since it introduces a frequency dependent change in the position angle given by χ(ν) where the RM is given by (e.g., Gardner & Whiteoak 1966)

Summary

Introduction

The radio source Sagittarius A* (Sgr A*) has been conclusively identified in the radio and infrared with a black hole of mass ∼ 3.5 × 106M⊙ at the center of our galaxy (Reid & Brunthaler 2004; Schodel et al 2003; Ghez et al 2005; Eisenhauer et al 2005). Sgr A* is the nearest super-massive black hole, 100 times closer than its nearest neighbor, M31*, and should provide a unique opportunity to understand the physics and life cycle of these objects. For a black hole of its size, Sgr A* is extremely under-luminous, only a few hundred solar luminosities and 10−8LEdd. For a black hole of its size, Sgr A* is extremely under-luminous, only a few hundred solar luminosities and 10−8LEdd This surprisingly low luminosity has motivated many theoretical and observational efforts to understand the processes at work very near to Sgr A*. The physics of this source are not well constrained by these observations alone

Results

Discussion

Conclusion