Observational Evidence for Magnetic Field Amplification in SN 1006

Abstract

We report the first observational evidence for magnetic field amplification in the north-east/south-west (NE/SW) shells of supernova remnant SN 1006, one of the most promising sites of cosmic ray (CR) acceleration. In previous studies, the strength of magnetic fields in these shells was estimated to be $B_{\rm SED}$ $\simeq$ 25$μ$G from the spectral energy distribution, where the synchrotron emission from relativistic electrons accounted for radio to X-rays, along with the inverse Compton emission extending from the GeV to TeV energy bands. However, the analysis of broadband radio data, ranging from 1.37~GHz to 100~GHz, indicated that the radio spectrum steepened from $α_1 = 0.52 \pm 0.02$ to $α_2 = 1.34 \pm 0.21$ by $Δα$ = 0.85 $\pm$ 0.21. This is naturally interpreted as a cooling break under strong magnetic field of $B_{\rm brk}$ $\ge$ 2~mG. Moreover, the high-resolution MeerKAT image indicated that the width of the radio NE/SW shells was broader than that of the X-ray shell by a factor of only 3$-$20, as measured by Chandra. Such narrow radio shells can be naturally explained if the magnetic field responsible for the radio emissions is $B_{\rm R}$ $\ge$ 2 mG. Assuming that the magnetic field is locally enhanced by a factor of approximately $a$ = 100 along the NE/SW shells, we argue that the filling factor, which is the volume ratio of such a magnetically enhanced region to that of the entire shell, must be as low as approximately $k$ = 2.5$\times$10$^{-5}$.