Issue |
A&A
Volume 551, March 2013
|
|
---|---|---|
Article Number | L11 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201221001 | |
Published online | 28 February 2013 |
LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A
1 Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands
e-mail: asgekar@astron.nl
2 Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
3 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
4 STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK
5 International Centre for Radio Astronomy Research – Curtin University, GPO Box U1987, Perth, WA 6845, Australia
6 University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
7 Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
8 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
9 Research School of Astronomy and Astrophysics, Australian National University, Mt Stromlo Obs., via Cotter Road, A.C.T. 2611 Weston, Australia
10 Department of Astrophysics/IMAPP,Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
11 Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
12 Sodankylä Geophysical Observatory, University of Oulu, Tähteläntie 62, 99600 Sodankylä, Finland
13 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
14 Centre for Radio Astronomy Techniques & Technologies (RATT), Department of Physics and Elelctronics, Rhodes University, PO Box 94, 6140 Grahamstown, South Africa
15 SRON Netherlands Insitute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
16 Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 av. Charles André, 69561 Saint Genis Laval Cedex, France
17 SKA South Africa, 3rd Floor, The Park, Park Road, 7405 Pinelands, South Africa
18 School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
19 Centrum Wiskunde & Informatica, PO Box 94079, 1090 GB, Amsterdam, The Netherlands
20 Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
21 LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France
22 Center for Information Technology (CIT), University of Groningen, The Netherlands
23 Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, LPC2E UMR 7328 CNRS, 45071 Orléans Cedex 02, France
24 Radio Astronomy Lab, UC Berkeley, CA, USA
25 Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
26 Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
27 Astro Space Center of the Lebedev Physical Institute, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
28 Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
29 Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
30 Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
31 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
32 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
33 Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
34 Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
35 ARC Centre of Excellence for All-sky astrophysics (CAASTRO), Sydney Institute of Astronomy, University of Sydney, Australia
36 Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, 18330 Nancay, France
37 CSIRO Australia Telescope National Facility, PO Box 76, NSW 1710 Epping, Australia
38 Department of Physics & Astronomy, The Open University, UK
Received: 24 December 2012
Accepted: 11 February 2013
Cassiopeia A was observed using the low-band antennas of the LOw Frequency ARray (LOFAR) with high spectral resolution. This allowed a search for radio recombination lines (RRLs) along the line-of-sight to this source. Five carbon α RRLs were detected in absorption between 40 and 50 MHz with a signal-to-noise ratio of >5 from two independent LOFAR data sets. The derived line velocities (vLSR ~ − 50 km s-1) and integrated optical depths (~13 s-1) of the RRLs in our spectra, extracted over the whole supernova remnant, are consistent within each LOFAR data set and with those previously reported. For the first time, we are able to extract spectra against the brightest hotspot of the remnant at frequencies below 330 MHz. These spectra show significantly higher (15–80 percent) integrated optical depths, indicating that there is small-scale angular structure of the order of ~1 pc in the absorbing gas distribution over the face of the remnant. We also place an upper limit of 3 × 10-4 on the peak optical depths of hydrogen and helium RRLs. These results demonstrate that LOFAR has the desired spectral stability and sensitivity to study faint recombination lines in the decameter band.
Key words: ISM: clouds / radio lines: ISM / ISM: individual objects: Cassiopeia A
© ESO, 2013
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