Mapping the spectral index of Cas A: evidence for flattening from radio to infrared. (arXiv:2005.12677v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Domcek_V/0/1/0/all/0/1">V. Domček</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vink_J/0/1/0/all/0/1">J. Vink</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Santisteban_J/0/1/0/all/0/1">J.V. Hernández Santisteban</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Delaney_T/0/1/0/all/0/1">T. Delaney</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhou_P/0/1/0/all/0/1">P. Zhou</a>
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 ($alpha _mathrm{R-IR}
sim -0.5$ to $-0.7$), to be compared to the radio spectral index of
$alpha_{rm R}=-0.77$. 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 ($alpha
_mathrm{R-IR} sim -0.67$). 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 $Blesssim 2$~mG
for an age of 100~yr, and even $Blesssim 1$~mG using the age of Cas A, in
agreement with other estimates.
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 ($alpha _mathrm{R-IR}
sim -0.5$ to $-0.7$), to be compared to the radio spectral index of
$alpha_{rm R}=-0.77$. 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 ($alpha
_mathrm{R-IR} sim -0.67$). 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 $Blesssim 2$~mG
for an age of 100~yr, and even $Blesssim 1$~mG using the age of Cas A, in
agreement with other estimates.
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