Low-frequency radio absorption in Cassiopeia A

M. Arias, J. Vink, F. De Gasperin, P. Salas, J.B.R. Oonk, R.J. van Weeren, A.S. van Amesfoort, J. Anderson, R. Beck, M.E. Bell, M.J. Bentum, P. Best, R. Blaauw, F. Breitling, J.W. Broderick, W.N. Brouw, M. Brüggen, H.R. Butcher, B. Ciardi, E. de GeusA. Deller, P.C.G. van Dijk, S. Duscha, J. Eislöffel, M.A. Garrett, J.M. Grießmeier, A.W. Gunst, M.P. van Haarlem, G. Heald, J. Hessels, J. Hörandel, H.A. Holties, A.J. Van Der Horst, M. Iacobelli, E. Juette, A. Krankowski, J. van Leeuwen, G. Mann, D. McKay-Bukowski, J.P. McKean, H. Mulder, A. Nelles, E. Orru, H. Paas, M. Pandey-Pommier, V.N. Pandey, R. Pekal, R. Pizzo, A.G. Polatidis, W. Reich

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Abstract

Context. 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. Aims. Free-free absorption processes are affected 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. Methods. We used Low Frequency Array (LOFAR) Low Band Antenna observations at 30-77 MHz and Very Large Array (VLA) L-band observations at 1-2 GHz 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 0.25 pc for a source t 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 ~100-500 K 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 effect on the secular decline of the source. Results. We find a mass in the unshocked ejecta of M = 2.95 ± 0.48 MȮ for an assumed gas temperatureof T = 100 K. 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. Conclusions. 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 (~10 K). Both of these options are plausible and can together contribute to the high absorption value that we find.

Original languageEnglish
Article numberA110
Number of pages16
JournalAstronomy and Astrophysics
Volume612
DOIs
Publication statusPublished - 1 Apr 2018

Funding

Acknowledgements. We thank T. Delaney for her VLA images of Cas A, and R. Perley and A. Kraus for making their recent flux density measurements with the VLA and Effelsberg available to us. We also thank the internal referee from the LOFAR builder’ list and the anonymous referee from A&A. Their helpful comments and suggestions improved the quality of this paper. The work of MA and JV is supported by a grant from the Netherlands Research School for Astronomy (NOVA). PS and JBRO acknowledge financial support from the Dutch Science Organisation (NWO) through TOP grant 614.001.351. RJvW acknowledges support from the ERC Advanced Investigator programme NewClusters 321271. This paper is based (in part) on data obtained with the International LOFAR Telescope (ILT). LOFAR (van Haarlem et al. 2013) is the Low Frequency Array designed and constructed by ASTRON. It has facilities in several countries, that are owned by various parties (each with their own funding sources), and that are collectively operated by the ILT foundation under a joint scientific policy. LOFAR data reduction used the DRAGNET GPU cluster (at the CIT in Gronin-gen), which was funded by the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 337062 (PI: Hessels). The LOFAR software and dedicated reduction packages on https://github.com/apmechev/GRID_LRT were deployed on the e-infrastructure by the LOFAR e-infragroup, consisting of R. Oonk (ASTRON & Leiden Observatory), A. P. Mechev (Leiden Observatory) and T. Shimwell (Leiden Observatory) with support from N. Danezi (SURFsara) and C. Schrijvers (SURFsara). This work has made use of the Dutch national e-infrastructure with the support of SURF Cooperative through grant e-infra160022. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Keywords

  • ISM: supernova remnants
  • Radiation mechanisms: general
  • Radio continuum: general
  • Supernovae: individual: Cas A
  • supernovae: individual: Cas A
  • radio continuum: general
  • radiation mechanisms: general

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  • Cas A LOFAR and VLA images

    Arias, M. (Creator), Vink, J. (Creator), De Gasperin, G. F. (Creator), Salas, P. (Creator), Oonk, J. B. R. (Contributor), van Weeren, R. .. J. (Creator), Amesfoort, van, A. S. (Creator), Anderson, J. (Creator), Beck, R. (Creator), Bell, M. E. (Creator), Bentum, M. J. (Creator), Best, P. (Creator), Blaauw, R. (Creator), Breitling, F. (Creator), Broderick, J. W. (Creator), Brouw, W. N. (Creator), Brüggen, M. (Contributor), Butcher, H. R. (Creator), Ciardi, B. (Creator), de Geus, E. (Creator), Deller, A. (Creator), van Dijk, P. C. G. (Contributor), Duscha, S. (Creator), Eislöffel, J. (Contributor), Garrett, M. A. (Creator), Grießmeier, J. M. (Contributor), Gunst, A. W. (Creator), van Haarlem, M. P. (Creator), Heald, G. (Creator), Hessels, J. (Creator), Hörandel, J. (Contributor), Holties, H. A. (Creator), van der Horst, A. J. (Creator), Iacobelli, M. (Creator), Juette, E. (Creator), Krankowski, A. (Creator), Van Leeuwen, J. (Creator), Mann, G. (Creator), McKay-Bukowski, D. (Creator), McKean, J. P. (Creator), Mulder, H. (Creator), Nelles, A. (Creator), Orru, E. (Creator), Paas, H. (Creator), Pandey-Pommier, M. (Creator), Pandey, V. N. (Creator), Pekal, R. (Creator), Pizzo, R. (Creator), Polatidis, A. G. (Creator), Reich, W. (Creator), Röttgering, H. J. A. (Contributor), Rothkaehl, H. (Creator), Schwarz, D. J. (Creator), Smirnov, O. (Creator), Soida, M. (Creator), Steinmetz, M. (Creator), Tagger, M. (Creator), Thoudam, S. (Creator), Toribio, M. C. (Creator), Vocks, C. (Creator), van der Wiel, M. H. D. (Contributor), Wijers, R. A. M. J. (Contributor), Wucknitz, O. (Creator), Zarka, P. (Creator) & Zucca, P. (Creator), Centre de Donnees Strasbourg (CDS), 8 May 2018

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