The Boomerang PWN G106.6+2.9 and the Magnetic Field Structure in Pulsar Wind Nebulae

Abstract

We present new observations at high radio frequencies made with the Effelsberg 100 m radio telescope of the "Boomerang" pulsar wind nebula (PWN) G106.65+2.96, which is part of the supernova remnant (SNR) G106.3+2.7. Including low-frequency data from the Canadian Galactic Plane Survey allows us to develop a complete picture of the PWN and its evolution in a very complex environment. The radio continuum spectrum reveals a break between 4 and 5 GHz that is the result of synchrotron cooling. We find evidence that the reverse shock of the initial supernova shock wave has driven away the original PWN, which explains why the current nebula around the pulsar has such a low radio luminosity. An analysis of the energy budget of the pulsar, the magnetic field in the nebula, and the lifetime of relativistic electrons gives a magnetic field of 2.6 mG in the PWN with an age of 3900 yr. This is not the age of the SNR but the time that has passed since the reverse shock crushed the original PWN. A comparison of our polarization measurements with those of other PWNe leads to a simple model of the magnetic field structure within such an object. Using this model, we derive an angle of about 30° between the Galactic north and the spin axis of the pulsar, projected onto the plane of the sky. The spin axis is pointing away from us, toward the northwest. This agrees nicely with the results of Ng and Romani, who in a 2004 paper calculated an angle of 20° from their fits of a model torus to a Chandra measurement of the Boomerang. The nebula contains a toroidal magnetic field structure, yet the rotation measure distribution indicates a radial magnetic field as its cause. Apparently, the observed radio emission originates on the far side of the object and passes through the inner region, which has a radial magnetic field.