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 intra-day variability in its linear polarization .
We detected linear polarization at 340 GHz ( 880 \mu m ) at several epochs .
At the typical resolution of 1 \farcs 4 \times 2 \farcs 2 , the expected contamination from the surrounding ( partially polarized ) dust emission is negligible .
We found that both the polarization fraction and position angle are variable , with the polarization fraction dropping from 8.5 % to 2.3 % over three 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 one-sigma rotation measure ( RM ) upper limit of 7 \times 10 ^ { 5 } 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 10 ^ { 5 } 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 _ { \sun } yr ^ { -1 } depending on the radial density and temperature profiles , but in all cases below the gas capture rate inferred from X-ray observations .