Mapping the spectral index of Cassiopeia A: evidence for flattening from radio to infrared. Issue 1 (2nd February 2021)

• Record Type: Journal Article
• Authors: Domček, V; Vink, J; Hernández Santisteban, J V; DeLaney, T; Zhou, P
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 502:Issue 1(2021)
• Page Start: 1026
• Page End: 1040
• Publication Date: 2021-02-02
• Subjects: acceleration of particles -- radiation mechanisms: non-thermal -- ISM: individual objects: Cassiopeia A -- ISM: supernova remnants; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa3896
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: Synchrotron radiation from supernova remnants is 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 theory predicts that the accelerated particles change the shock structure, resulting in a spectrum that is steeper than p > 2 (α < −0.5) at low energies and flattens with energy. For Cassiopeia 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 we 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 -band (1.285 GHz) and C -band (4.64 GHz) maps. Our result shows overall spectral flattening across the remnant (αR-IR ∼ −0.5 to −0.7), to be compared with the radio spectral index of αR = −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 south-east of the remnant (αR-IR ∼ −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 and the age of the plasma, suggesting B ≲ 2 mG for an age of 100 yr, and even B ≲ 1 mG using the age of Cas A, in agreement with other estimates.