学术文献库

Synchrotron radiation from supernova remnants are caused by electrons accelerated through diffusive shock acceleration (DSA). The standard DSA theory predicts an electron spectral index of $p=2$, corresponding to a radio spectral index of $α=-0.5$. An extension of DSA predicts that the accelerated particles changes the shock structure, resulting in a spectrum that is steeper than $p>2$ ($α<-0.5$) at low energies and flattens with energy. For Cas A, a synchrotron spectral flattening was previously reported for a small part of the remnant in the mid-infrared regime. Here, we present new measurements for spectral flattening using archival radio (4.72~GHz) and mid-infrared (3.6~$μ$m) data, and produce a complete spectral index map to investigate the spatial variations within the remnant. We compare this to measurements of the radio spectral index from L-(1.285~GHz) and C-(4.64~GHz) band maps. Our result shows overall spectral flattening across the remnant ($α_\mathrm{R-IR} \sim -0.5$ to $-0.7$), to be compared to the radio spectral index of $α_{\rm R}=-0.77$. The flattest values coincide with the locations of most recent particle acceleration. In addition to overall flattening, we detect a relatively steeper region in the southeast of the remnant ($α_\mathrm{R-IR} \sim -0.67$). We explore whether these locally steeper spectra could be the result of synchrotron cooling, which provides constraints on the local magnetic-field strengths/age of the plasma, suggesting $B\lesssim 2$~mG for an age of 100~yr, and even $B\lesssim 1$~mG using the age of Cas A, in agreement with other estimates.