Mapping the spectral index of Cas A: evidence for flattening from radio to infrared

Abstract

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 $\alpha =-0.5$. An extension of DSA predicts that the accelerated particles changes the shock structure, resulting in a spectrum that is steeper than $p>2$ ($\alpha <-0.5$) 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~$\mu$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 (${\alpha }_{\mathrm{R}-\mathrm{IR}}\sim -0.5$ to $-0.7$), to be compared to the radio spectral index of ${\alpha }_{\mathrm{R}}=-0.77$. 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 (${\alpha }_{\mathrm{R}-\mathrm{IR}}\sim -0.67$). 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.

18 pages, 18 figures, accepted to MNRAS

References in corpus (14)

• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• Detection of the Characteristic Pion-Decay Signature in Supernova Remnants
• Primary particle acceleration above 100 TeV in the shell-type Supernova Remnant RX J1713.7-3946 with deep H.E.S.S. observations
• Teraelectronvolt Astronomy
• Freshly Formed Dust in the Cassiopeia A Supernova Remnant as Revealed by the Spitzer Space Telescope
• The dust mass in Cassiopeia A from a spatially resolved Herschel analysis
• Dynamical Feedback of Self-generated Magnetic Fields in Cosmic Ray Modified Shocks
• Characterizing the non-thermal emission of Cas A
• Turbulence Dissipation and Particle Injection in Non-Linear Diffusive Shock Acceleration with Magnetic Field Amplification
• Spitzer IRAC Images and Sample Spectra of Cassiopeia A's Explosion
• The Density and Mass of Unshocked Ejecta in Cassiopeia A through Low Frequency Radio Absorption
• Evidence for proton acceleration up to TeV energies based on VERITAS and Fermi-LAT observations of the Cas A SNR
• The influence of the Alfvénic drift on the shape of cosmic ray spectra in SNRs
• On the continuum radio-spectrum of Cas A: possible evidence of the non-linear particle acceleration

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• Spectral softening in core-collapse supernova remnant expanding inside wind-blown bubble
• A Stimulating Explanation of the Extragalactic Radio Background
• Far-infrared Polarization of the Supernova Remnant Cassiopeia A with SOFIA HAWC+
• Nonthermal processes and particle acceleration in supernova remnants