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

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

Domains: Galactic Astrophysics [astro-ph.GA]

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, 2013, 551 (L11), 5 p. ⟨10.1051/0004-6361/201221001⟩. ⟨insu-01286281⟩