Journal articles

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 ¹⁶ S. Markoff ³³ D. Mckay-Bukowski ³⁴ M. Mevius ¹ J. C. A. Miller-Jones ³³ J. D. Mol ¹ R. Morganti ^{2, 1} D. D. Mulcahy ¹⁰ H. Munk ¹ M. J. Norden ¹ E. Orru ¹ H. Paas ¹ M. Pandey-Pommier ³⁵ V. N. Pandey ³⁶ R. Pizzo ¹ A. G. Polatidis ¹ W. Reich ⁶ H. Rottgering ¹³ B. Scheers ³³ A. Schoenmakers ¹ J. Sluman ¹ O. Smirnov ³⁷ C. Sobey ⁶ M. Steinmetz ¹⁶ Michel Tagger ²⁴ Y. Tang ¹ C. Tasse ³⁸ R. Vermeulen ¹ C. Vocks ¹⁶ R. A. M. J. Wijers ³³ M. W. Wise ^{1, 33} O. Wucknitz ³⁹ P. Zarka ³⁸

1 ASTRON - Netherlands Institute for Radio Astronomy

2 Kapteyn Astronomical Institute [Groningen]

3 CfA - Harvard-Smithsonian Center for Astrophysics

4 SLAC - SLAC National Accelerator Laboratory

5 Jodrell Bank Cent. for Astrophysics

6 MPIFR - Max-Planck-Institut für Radioastronomie

7 SRON - SRON Netherlands Institute for Space Research

8 OSO - Onsala Space Observatory

9 LCPM - Laboratoire de Chimie Physique Moléculaire

10 University of Southampton

11 LPNHE - Laboratoire de Physique Nucléaire et de Hautes Énergies

12 University of Edinburgh

13 Leiden Observatory [Leiden]

14 Jacobs University [Bremen]

15 TU Dresden - Technische Universität Dresden = Dresden University of Technology

16 AIP - Leibniz-Institut für Astrophysik Potsdam

17 Max Planck Institute for Astrophysics

18 Hamburger Sternwarte/Hamburg Observatory

19 Medstar Research Institute

20 TLS - Thüringer Landessternwarte Tautenburg

21 Radboud university [Nijmegen]

22 Institute of Mathematical and Physical Sciences

23 LAGRANGE - Joseph Louis LAGRANGE

24 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace

25 USN - Unité Scientifique de la Station de Nançay

26 Jodrell Bank Centre for Astrophysics

27 UvA - University of Amsterdam [Amsterdam]

28 Sapienza University of Rome - Dipartimento di Matematica "Guido Castelnuovo" [Roma I]

29 CMLS - Centre de Mathématiques Laurent Schwartz

30 Ruhr-Universität Bochum [Bochum]

31 Oxford Astrophysics

32 KIT - Karlsruhe Institute of Technology

33 AI PANNEKOEK - Astronomical Institute Anton Pannekoek

34 University of Oulu

35 CRAL - Centre de Recherche Astrophysique de Lyon

36 NRAO - National Radio Astronomy Observatory [Charlottesville]

37 Rhodes University, Grahamstown

38 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique

39 AlfA - Argelander-Institut für Astronomie

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.

Keywords : ISM: clouds radio lines : ISM ISM: individual objects: Cas siopeia A

Document type :

Journal articles

Domain :

Sciences of the Universe [physics] / Astrophysics [astro-ph] / Galactic Astrophysics [astro-ph.GA]

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