High Resolution VLA Radio Observations of the Boomerang Pulsar Wind Nebula

Abstract

We present a radio polarimetric study of the Boomerang pulsar wind nebula G106.65+2.96 with VLA observations at the 6 GHz band. Our high-resolution image discovers new small-scale features in the nebula, including an elliptical core of $40''\times20''$ surrounding the central pulsar and a $2'$-long arc wrapping around the core in the north. The latter shows a clear gap from the core, and it consists of a bright lobe in the northwest and a tongue-like structure in the northeast. These could be resulting from the pulsar wind interaction with the environment. Our polarization measurement reveals a highly ordered magnetic field with toroidal geometry. The small scale features are all highly linearly polarized. In particular, the lobe has a polarization fraction of $\sim$60%, close to the synchrotron limit. This is also much higher than the value measured at a lower frequency, implying significant depolarization. We show that this can be explained by Faraday rotation in the nebula, and we constructed a simple 3D model accordingly to infer a magnetic field strength of $\sim$50-105$μ$G.

Figures (7)

• Figure 1: (a) VLA total intensity and (b) linear polarized intensity maps of the Boomerang PWN at 6 GHz. The white crosses mark the position of PSR J2229+6114. The radio images have a beam size of $2\farcs3\times1\farcs8$ FWHM and rms noise of 5 mJy beam$^{-1}$. (c) same as (a) with different PWN features labeled. The arrow pointing west shows the pulsar spin axis direction inferred from the X-ray PWN ng04. The arrow pointing southeast indicates the direction to the relic PWN kothes+01. (d) Exposure corrected Chandra X-ray image of the same field, showing the central pulsar J2229+6119 and the surrounding torus.
• Figure 2: RM map of the Boomerang PWN overlaid with the 6 GHz total intensity contours. The RM uncertainties are $<$45 rad m$^{-2}$.
• Figure 3: Total intensity map same as Figure \ref{['fig:boomerang']}a, overlaid with unit vectors that show the intrinsic $B$-field orientation after corrected for Faraday rotation. The vectors are only plotted for regions with Stokes I signal-to-noise ratio $>3$ and PA-fit uncertainty $<15\arcdeg$. The white cross marks the position of PSR J2229+6114.
• Figure 4: Selected spectral tomography images for the Boomerang between the 4.5 GHz and 7.5 GHz subband total intensity images. The top left panel shows the full band intensity image as Fig. \ref{['fig:boomerang']}a to illustrate the PWN features. For other panels, the trial spectral index $\alpha_t$ is labeled in top right. The cross marks the position of PSR J2229+6114.
• Figure 5: PF of the Boomerang PWN at different frequencies, from our new data and previously reported values kothes06. The last column lists the PF values after correction for the beam depolarization effect.