J/ApJ/456/234 Cas A radio knots spectral indices (Anderson+ 1996)
Sites of relativistic particle acceleration in supernova remnant Cassiopeia A
Anderson M.C., Rudnick L.
<Astrophys. J. 456, 234 (1996)>
=1996ApJ...456..234A 1996ApJ...456..234A (SIMBAD/NED BibCode)
ADC_Keywords: Supernova remnants
Keywords: acceleration of particles - cosmic rays -
ISM: individual (Cassiopeia A) - radiation mechanisms: nonthermal -
radio continuum: ISM - supernova remnants
Abstract:
We have determined the synchrotron spectral indices of 304 compact
radio knots in the supernova remnant Cassiopeia A. A comparison of
these data with the dynamical and brightness properties of these knots
tabulated by Anderson & Rudnick (Cat. J/ApJ/441/307) indicates that
spectral index shows a significant correlation with projected radius
from the center of the remnant. Spectrally flat knots reside in a
shell coincident with the bright radio ring, while steeper knots
occupy a shell coincident with the diffuse radio plateau surrounding
the ring. To a lesser extent, we find spectral index to be correlated
also with the radio brightness of the knot, in the sense that brighter
knots tend to have steeper spectra. No significant correlation is
found between spectral index and knot deceleration or rate of
brightness change.
As the synchrotron spectral index traces the distribution of energy
among relativistic particle populations, we use these results to study
the nature of particle acceleration mechanisms active in Cas A. Given
the dual shell nature of the spectral index distribution in Cas A and
the lack of strong correlation between spectral index and dynamical
properties of the knots, we conclude that radio-bright compact
features are not sites of currently active particle acceleration in
Cas A. This conclusion is in agreement with models of supersonic
gaseous bullets constructed by Jones, Kang, & Tregillis
(1994ApJ...432..194J 1994ApJ...432..194J). In these models, the marked synchrotron
brightening which accompanies bullet deceleration is due primarily to
preexisting relativistic particles radiating in rapidly amplifying
shear-layer magnetic fields, rather than a large infusion of new
relativistic particles accelerated in situ. Spectral variations
between compact features in Cas A are more likely to reflect
modulations in the background particle energy spectra within the
remnant, perhaps instilled by temperature variations in the underlying
thermal material. This interpretation requires that the diffuse
synchrotron emission show the same spectral variations as seen in
compact features; preliminary indications suggest that this is the
case.
Objects:
-----------------------------------------
RA (2000) DE Designation(s)
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23 23 12.6 +58 48 48 Cas A = SN 1680
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1 77 304 Radio knot spectral indices (epoch 1987)
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See also:
J/ApJ/441/307 : Proper motions and brightness of Cas A (Anderson+ 1995)
Byte-by-byte Description of file: table1
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Bytes Format Units Label Explanations
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1- 3 I3 --- Knot Knot number
7- 9 I3 --- KnotT ? Designation number of Tuffs
(1986MNRAS.219...13T 1986MNRAS.219...13T)
12- 17 F6.1 arcsec Xpos Epoch 1987 knot position (1)
21- 26 F6.1 arcsec Ypos Epoch 1987 knot position (1)
30- 35 F6.1 arcsec BoxX1 Box defining the regression region used for
each knot (1)
39- 44 F6.1 arcsec BoxY1 Box defining the regression region used for
each knot (1)
48- 53 F6.1 arcsec BoxX2 Box defining the regression region used for
each knot (1)
57- 62 F6.1 arcsec BoxY2 Box defining the regression region used for
each knot (1)
66- 69 F4.2 --- alpha Spectral index
73- 76 F4.2 --- e_alpha rms uncertainty on alpha
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Note (1): Positions measured with respect to the optical expansion center at
alpha(1950)= 23h21m11s90, delta(1950) = +58deg32'17.6"
(Van den Bergh & Kamper 1983ApJ...268..129V 1983ApJ...268..129V)
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History: Prepared via OCR at CDS.
(End) James Marcout, Patricia Bauer [CDS] 25-Oct-1996