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 8 R. Beck 9 M. E. Bell 10 M. R. Bell 10 I. Van Bemmel 1 M. J. Bentum 1 G. Bernardi 11 P. Best 12 L. Birzan 13 A. Bonafede 14 R. Braun 15 F. Breitling 16 R. H. Van de Brink 1 J. Broderick 10 W. N. Brouw 2 M. Bruggen 14 H. R. Butcher 1 W. Van Cappellen 1 B. Ciardi 17 J. E. Conway 8 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, 6, 1 R. A. Fallows 22 C. Ferrari 23 W. Frieswijk 1 M. A. Garrett 13, 1 Jean-Mathias Grießmeier 24, 25 T. Grit 1 A. W. Gunst 1 T. E. Hassall 26 G. Heald 1 J. W. T. Hessels 27 M. Hoeft 20 M. Iacobelli 28, 29 H. Intema 13 E. Juette 30 A. Karastergiou 31 J. Kohler 32 V. I. Kondratiev 1 M. Kuniyoshi 6 G. Kuper 1 C. Law 33 J. Van Leeuwen 27 P. Maat 1 G. Macario 23 G. Mann 34 S. Markoff 33 D. Mckay-Bukowski 35 M. Mevius 1 J. C. A. Miller-Jones 33 J. D. Mol 1 R. Morganti 36, 37 D. D. Mulcahy 10 H. Munk 1 M. J. Norden 1 E. Orru 1 H. Paas 1 M. Pandey-Pommier 38 V. N. Pandey 39 R. Pizzo 1 A. G. Polatidis 1 W. Reich 6 H. Rottgering 13 B. Scheers 33 A. Schoenmakers 1 J. Sluman 1 O. Smirnov 40 C. Sobey 6 M. Steinmetz 34 Michel Tagger 24 Y. Tang 1 C. Tasse 41, 40 R. Vermeulen 1 C. Vocks 42 R. A. M. J. Wijers 33 M. W. Wise 1, 33 O. Wucknitz 43 P. Zarka 44
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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