Low-frequency radio absorption in Cassiopeia A^★

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
e-mail: maria.arias.de.saavedra@gmail.com
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Helmholtz-Zentrum Potsdam, GFZ, Department 1: Geodesy and Remote Sensing, Telegrafenberg, A17, 14473 Potsdam, Germany
⁴ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ University of Technology Sydney, 15 Broadway, Ultimo NSW 2007, Australia
⁶ ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁷ Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands
⁸ Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁰ Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
¹¹ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹² Research School of Astronomy and Astrophysics, Australian National University, Canberra ACT 2611 Australia
¹³ Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
¹⁴ SmarterVision BV, Oostersingel 5, 9401 JX Assen, The Netherlands
¹⁵ Centre for Astrophysics & Supercomputing, Swinburne University of Technology John St., Hawthorn, VIC 3122, Australia
¹⁶ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹⁷ Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
¹⁸ LPC2E – Université d’Orléans/CNRS, Orléans, France
¹⁹ Station de Radioastronomie de Nançay, Observatoire de Paris – CNRS/INSU, USR 704 – Univ. Orléans, OSUC, route de Souesmes, 18330 Nançay, France
²⁰ CSIRO Astronomy and Space Science, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
²¹ Department of Astrophysics, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²² Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC 20052, USA
²³ Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
²⁴ University of Warmia and Mazury in Olsztyn, Oczapowskiego 1, 10-957 Olsztyn, Poland
²⁵ Department of Physics and Technology, University of Tromsø, Tromsø, Norway
²⁶ STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK
²⁷ Department of Physics and Astronomy, University of California Irvine, Irvine, CA 92697, USA
²⁸ Center for Information Technology (CIT), University of Groningen, Groningen, The Netherlands
²⁹ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 Av. Charles André, 69561 Saint Genis Laval Cedex, France
³⁰ Poznan Supercomputing and Networking Center (PCSS), Poznan, Poland
³¹ Space Research Center PAS, Bartycka 18 A, 00-716 Warsaw, Poland
³² Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
³³ Department of Physics and Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
³⁴ SKA South Africa, 3rd Floor, The Park, Park Road, Pinelands 7405, South Africa
³⁵ Jagiellonian University, Astronomical Observatory, Orla 171, 30-244 Krakow, Poland
³⁶ Department of Physics and Electrical Engineering, Linnaeus University, 35195, Vaexjoe, Sweden
³⁷ LESIA & USN, Observatoire de Paris, CNRS, Place J. Janssen, 92195 Meudon, France
³⁸ GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³⁹ SRON, Netherlands Institute for Space Research, Utrecht, The Netherlands

Received: 4 December 2017
Accepted: 13 January 2017

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 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 ~100–500 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 M_⊙ 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.