J/A+A/612/A110 Cas A LOFAR and VLA images (Arias+, 2018)
Low frequency radio absorption in Cassiopeia A.
Arias M., Vink J., de Gasperin F., Salas P., Oonk J.B.R., van Weeren R.J.,
van Amesfoort A.S., Anderson J., Beck R., Bell M.E., Bentum M.J., Best P.,
Blaauw R., Breitling F., Broderick J.W., Brouw W.N., Brueggen M.,
Butcher H.R., Ciardi B., de Geus E., Deller A., van Dijk P.C.G., Duscha S.,
Eisloel J., Garrett M.A., Griessmeier J.M., Gunst A.W., van Haarlem M.P.,
Heald G., Hessels J., Hoerandel J., Holties H.A., van der Horst A.J.,
Iacobelli M., Juette E., Krankowski A., van Leeuwen J., Mann G.,
McKay-Bukowski D., McKean J.P., Mulder H., Nelles A., Orru E., Paas H.,
Pandey-Pommier M., Pandey V.N., Pekal R., Pizzo R., Polatidis A.G.,
Reich W., Roettgering H.J.A., Rothkaehl H., Schwarz D.J., Smirnov O.,
Soida M., Steinmetz M., Tagger M., Thoudam S., Toribio M.C., Vocks C.,
van der Wiel M.H.D., Wijers R.A.M.J., Wucknitz O., Zarka P., Zucca P.
<Astron. Astrophys. 612, A110 (2018)>
=2018A&A...612A.110A 2018A&A...612A.110A (SIMBAD/NED BibCode)
ADC_Keywords: Supernova remnants ; Radio sources ; Ultraviolet
Keywords: supernovae: individual: Cas A - ISM: supernova remnants -
radiation mechanisms: general - radio continuum: general
Abstract:
Cassiopeia A is one of the best-studied supernova remnants. Its bright
radio and X-ray emission is due to shocked ejecta. Cas A is rather
unique in that the unshocked ejecta can also be studied: through
emission in the infrared, the radio-active decay of 44Ti, and the
low-frequency free-free absorption caused by cold ionised gas, which
is the topic of this paper.
Free-free absorption processes are acted by the mass, geometry,
temperature, and ionisation conditions in the absorbing gas.
Observations at the lowest radio frequencies can constrain a
combination of these properties.
We used Low Frequency Array (LOFAR) Low Band Antenna observations at
30-77MHz and Very Large Array (VLA) L-band observations at 1-2GHz to
fit for internal absorption as parametrised by the emission measure.
We simultaneously fit multiple UV-matched images with a common
resolution of 17" (this corresponds to 0.25pc for a source at the
distance of Cas A). The ample frequency coverage allows us separate
the relative contributions from the absorbing gas, the unabsorbed
front of the shell, and the absorbed back of the shell to the emission
spectrum. We explored the effects that a temperature lower than the
∼100-500K proposed from infrared observations and a high degree of
clumping can have on the derived physical properties of the unshocked
material, such as its mass and density. We also compiled integrated
radio flux density measurements, fit for the absorption processes that
occur in the radio band, and considered their ect on the secular
decline of the source.
We find a mass in the unshocked ejecta of M=2.95±0.48M☉ for an
assumed gas temperature of T=100K. This estimate is reduced for colder
gas temperatures and, most significantly, if the ejecta are clumped.We
measure the reverse shock to have a radius of 114±6" and be centred
at 23:23:26, +58:48:54 (J2000).We also find that a decrease in the
amount of mass in the unshocked ejecta (as more and more material
meets the reverse shock and heats up) cannot account for the observed
low-frequency behaviour of the secular decline rate.
To reconcile our low-frequency absorption measurements with models
that reproduce much of the observed behaviour in Cas A and predict
little mass in the unshocked ejecta, the ejecta need to be very
clumped or the temperature in the cold gas needs to be low (∼10K).
Both of these options are plausible and can together contribute to the
high absorption value that we find.
Description:
LOFAR LBA and VLA L-band images of supernova remnant Cassiopeia A.
Data taken in 2015 (LOFAR) and 2017 (VLA).
- VLAlband2017.fits: images made from one night (August 13th)
combining the two available continuum spectral windows (1750MHz and
1378) (the spectral window at 1122 MHz was completely flagged) and
nterms=2.
LOFAR images (these are in general better than the narrow band images
I used in the paper):
- LOFARLBA2015.fits: full bandwidth LOFAR LBA image centered at
58MHz (image 1)
- *MHz_nb.fits: narrow band (1 MHz bandwidth images) made with LOFAR
as described in the paper.
Objects:
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RA (2000) DE Designation(s)
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23 23 24.00 +58 48 54.0 Cas A = NAME Cassiopeia A
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 145 11 List of fits images
fits/* . 11 Individual fits images
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See also:
J/ApJ/441/307 : Proper motions and brightness of Cas A (Anderson+ 1995)
J/ApJ/456/234 : Cas A radio knots spectral indices (Anderson+ 1996)
J/A+A/368/267 : Spatially resolved spectroscopy of Cas A (Maccarone+, 2001)
J/ApJ/613/343 : Cas A knot and filament proper motions (Delaney+, 2004)
J/A+A/484/755 : Oxygen recombination lines from Cas A knots (Docenko+, 2008)
J/ApJS/179/195 : A catalog of outer ejecta knots in Cas A (Hammell+, 2008)
J/ApJ/837/118 : NIR knots in the Cas A supernova remnant (Lee+, 2017)
Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 48 A21 --- Obs.Date Observation date (YYYY-MM-DDThh:mm:ss.s)
50- 56 F7.4 MHz Freq ? Central frequency for narrow band images
58- 61 I4 Kibyte size Size of FITS file
63- 81 A19 --- FileName Name of FITS file, in subdirectory fits
83-145 A63 --- Title Title of the FITS file
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Acknowledgements:
Maria Arias, maria.arias.de.saavedra(at)gmail.com
(End) Patricia Vannier [CDS] 23-Jan-2018