LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A

Ashish Asgekar 1 J. B. R. Oonk 1 S. Yatawatta 2 R. J. Van Weeren 1, 3 J. P. Mckean 1 G. White 4 N. Jackson 5 J. Anderson 6 I. M. Avruch 7 F. Batejat 6 R. Beck 8 M. E. Bell 9 M. R. Bell 9 I. Van Bemmel 1 M. J. Bentum 1 G. Bernardi 10 P. Best 11 L. Birzan 12 A. Bonafede 13 R. Braun 14 F. Breitling 15 R. H. Van de Brink 1 J. Broderick 9 W. N. Brouw 2 M. Bruggen 13 H. R. Butcher 1 W. Van Cappellen 16 B. Ciardi 6 J. E. Conway 17 F. De Gasperin 18 E. De Geus 19 A. De Jong 1 M. De Vos 1 S. Duscha 1 J. Eisloffel 20 H. Falcke 21, 22, 1 R. A. Fallows 23 C. Ferrari 24 W. Frieswijk 1 M. A. Garrett 12, 1 Jean-Mathias Grießmeier 25, 26 T. Grit 1 A. W. Gunst 1 T. E. Hassall 27 G. Heald 1 J. W. T. Hessels 28 M. Hoeft 20 M. Iacobelli 29, 30 H. Intema 12 E. Juette 31 A. Karastergiou 32 J. Kohler 33 V. I. Kondratiev 1 M. Kuniyoshi 34 G. Kuper 1 C. Law 35 J. Van Leeuwen 28 P. Maat 1 G. Macario 24 G. Mann 36 S. Markoff 35 D. Mckay-Bukowski 37 M. Mevius 1 J. C. A. Miller-Jones 35 J. D. Mol 1 R. Morganti 38, 39 D. D. Mulcahy 40 H. Munk 1 M. J. Norden 1 E. Orru 1 H. Paas 1 M. Pandey-Pommier 41 V. N. Pandey 42 R. Pizzo 1 A. G. Polatidis 1 W. Reich 34 H. Rottgering 12 B. Scheers 35 A. Schoenmakers 1 J. Sluman 1 O. Smirnov 43 C. Sobey 34 M. Steinmetz 36 Michel Tagger 25 Y. Tang 1 C. Tasse 44, 43 R. Vermeulen 1 C. Vocks 45 R. A. M. J. Wijers 35 M. W. Wise 1, 35 O. Wucknitz 46 P. Zarka 47
Abstract : 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-alpha RRLs were detected in absorption between 40 and 50 MHz with a signal-to-noise ratio of > 5 from two independent LOFAR datasets. The derived line velocities (v_LSR ~ -50 km/s) and integrated optical depths (~ 13 s^-1) of the RRLs in our spectra, extracted over the whole supernova remnant, are consistent within each LOFAR dataset 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 %) integrated optical depths, indicating that there is small-scale angular structure on the order of ~1 pc in the absorbing gas distribution over the face of the remnant. We also place an upper limit of 3 x 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.
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Astronomy and Astrophysics - A&A, EDP Sciences, 2013, 551 (L11), 5 p. 〈10.1051/0004-6361/201221001〉
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Ashish Asgekar, J. B. R. Oonk, S. Yatawatta, R. J. Van Weeren, J. P. Mckean, et al.. LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A. Astronomy and Astrophysics - A&A, EDP Sciences, 2013, 551 (L11), 5 p. 〈10.1051/0004-6361/201221001〉. 〈insu-01286281〉

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