LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A
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
Cite
@article{arxiv.1302.3128,
title = {LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A},
author = {Ashish Asgekar and J. B. R. Oonk and S. Yatawatta and R. J. van Weeren and J. P. McKean and G. White and N. Jackson and J. Anderson and I. M. Avruch and F. Batejat and R. Beck and M. E. Bell and M. R. Bell and I. van Bemmel and M. J. Bentum and G. Bernardi and P. Best and L. Birzan and A. Bonafede and R. Braun and F. Breitling and R. H. van de Brink and J. Broderick and W. N. Brouw and M. Bruggen and H. R. Butcher and W. van Cappellen and B. Ciardi and J. E. Conway and F. de Gasperin and E. de Geus and A. de Jong and M. de Vos and S. Duscha and J. Eisloffel and H. Falcke and R. A. Fallows and C. Ferrari and W. Frieswijk and M. A. Garrett and J-M. Griesmeier and T. Grit and A. W. Gunst and T. E. Hassall and G. Heald and J. W. T. Hessels and M. Hoeft and M. Iacobelli and H. Intema and E. Juette and A. Karastergiou and J. Kohler and V. I. Kondratiev and M. Kuniyoshi and G. Kuper and C. Law and J. van Leeuwen and P. Maat and G. Macario and G. Mann and S. Markoff and D. McKay-Bukowski and M. Mevius and J. C. A. Miller-Jones and J. D. Mol and R. Morganti and D. D. Mulcahy and H. Munk and M. J. Norden and E. Orru and H. Paas and M. Pandey-Pommier and V. N. Pandey and R. Pizzo and A. G. Polatidis and W. Reich and H. Rottgering and B. Scheers and A. Schoenmakers and J. Sluman and O. Smirnov and C. Sobey and M. Steinmetz and M. Tagger and Y. Tang and C. Tasse and R. Vermeulen and C. Vocks and R. A. M. J. Wijers and M. W. Wise and O. Wucknitz and P. Zarka},
journal= {arXiv preprint arXiv:1302.3128},
year = {2013}
}
Comments
Accepted for publication in A&A Letters; 5 pages, 4 colour figures