A Radio Polarimetric Study of the Galactic Center Threads

Abstract

The Very Large Array has been used to carry out a multifrequency, polarimetric study of the nonthermal filaments (NTFs), G0.08+0.15, and G359.96+0.09, also known as the Northern and Southern Threads (Morris & Yusef-Zadeh). These linear structures have been observed at λ = 20, 6, 3.6, and 2 cm, with high enough spatial resolution to be resolved for the first time at 6 and 3.6 cm. The 20 cm image reveals a wealth of new detail in the radio sources lying within the inner 60 pc of the Galaxy. The Southern Thread has a prominent split along its length, similar to splitting at the ends of previously studied NTFs. In addition to the prominent Northern and Southern Threads, there are several elongated features that resemble NTFs but that extend for only 5-7 pc. With resolutions as fine as 2'', the 3.6 and 6 cm images reveal a high degree of continuity and little substructure internal to the filament. However, the width of the Northern Thread varies along its length between ~4'' and ~12'' (0.15-0.5 pc) and becomes markedly diffuse at its northwest extremity. The spectral index of the Northern Thread has been determined over a broad range of frequencies. Its flux density falls with frequency, (α = -0.5) between 90 and 6 cm (where Sν ∝ να) and becomes much steeper (α = -2.0) between 6 and 2 cm. The spectral index does not vary significantly along the length of the Northern Thread, which implies either that the diffusion timescale for the emitting electrons is less than their synchrotron lifetime or that the emitting electrons are reaccelerated continuously at multiple positions along the filament. Because of the lack of spectral index variation, we have not located the source of relativistic electrons. Polarization observations at 6 and 3.6 cm confirm the nonthermal nature of the emission from the Northern Thread. The fractional polarization in the Northern Thread reaches 70% in some regions, although the polarized emission is patchy. Large rotation measures (RM ≳ 2000 rad m-2) have been observed with irregular variations across the filament; typical values of RM are 100-2000 rad m-2. The lack of any apparent pattern in the distribution of RM suggests that the Faraday rotating medium is not physically associated with the Northern Thread since the filament itself is so highly ordered. The intrinsic magnetic field in the Northern Thread is predominantly aligned along its long axis. The data on the Southern Thread were less conclusive: the Southern Thread was not detected in total intensity at 6 or 3.6 cm, while the polarized emission at 3.6 cm arising from the Southern Thread is evident, with a bifurcated structure similar to that detected in total 20 cm intensity.