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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 (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: ------------------------------------------------------ RA (2000) DE Designation(s) ------------------------------------------------------ 23 23 24.00 +58 48 54.0 Cas A = NAME Cassiopeia A ------------------------------------------------------ File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file list.dat 145 11 List of fits images fits/* 0 11 Individual fits images
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
Bytes Format Units Label Explanations
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
Acknowledgements: Maria Arias,
(End) Patricia Vannier [CDS] 23-Jan-2018
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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