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LOFAR Detections of Low-Frequency Radio Recombination Lines Towards Cassiopeia A

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LOFAR Detections of Low-Frequency Radio Recombination Lines Towards Cassiopeia A A&A 551, L11 (2013) Astronomy DOI: 10.1051/0004-6361/201221001 & c ESO 2013 Astrophysics Letter to the Editor LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A A. Asgekar1,J.B.R.Oonk1,S.Yatawatta1,2, R. J. van Weeren3,1,8, J. P. McKean1,G.White4,38, N. Jackson25, J. Anderson32,I.M.Avruch15,2,1, F. Batejat11, R. Beck32,M.E.Bell35,18,M.R.Bell29, I. van Bemmel1,M.J.Bentum1, G. Bernardi2,P.Best7,L.Bîrzan3, A. Bonafede6,R.Braun37, F. Breitling30,R.H.vandeBrink1, J. Broderick18, W. N. Brouw1,2, M. Brüggen6,H.R.Butcher1,9, W. van Cappellen1, B. Ciardi29,J.E.Conway11,F.deGasperin6, E. de Geus1, A. de Jong1,M.deVos1,S.Duscha1,J.Eislöffel28, H. Falcke10,1,R.A.Fallows1, C. Ferrari20, W. Frieswijk1, M. A. Garrett1,3, J.-M. Grießmeier23,36, T. Grit1,A.W.Gunst1, T. E. Hassall18,25, G. Heald1, J. W. T. Hessels1,13,M.Hoeft28, M. Iacobelli3,H.Intema3,31,E.Juette34, A. Karastergiou33 , J. Kohler32, V. I. Kondratiev1,27, M. Kuniyoshi32, G. Kuper1,C.Law24,13, J. van Leeuwen1,13,P.Maat1,G.Macario20,G.Mann30, S. Markoff13, D. McKay-Bukowski4,12,M.Mevius1,2, J. C. A. Miller-Jones5,13 ,J.D.Mol1, R. Morganti1,2, D. D. Mulcahy32, H. Munk1,M.J.Norden1,E.Orru1,10, H. Paas22, M. Pandey-Pommier16,V.N.Pandey1,R.Pizzo1, A. G. Polatidis1,W.Reich32, H. Röttgering3, B. Scheers13,19, A. Schoenmakers1,J.Sluman1,O.Smirnov1,14,17, C. Sobey32, M. Steinmetz30,M.Tagger23,Y.Tang1, C. Tasse21,R.Vermeulen1,C.Vocks30,R.A.M.J.Wijers13, M. W. Wise1,13, O. Wucknitz32,26,andP.Zarka21 (Affiliations can be found after the references) Received 24 December 2012 / Accepted 11 February 2013 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 α RRLs were detected in absorption between 40 and −1 50 MHz with a signal-to-noise ratio of >5 from two independent LOFAR data sets. The derived line velocities (vLSR ∼−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 data set 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 percent) integrated optical depths, indicating that there is small-scale angular structure of the order of ∼1 pc in the absorbing gas distribution over the face of the remnant. We also place an upper limit of 3 × 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. Key words. ISM: clouds – radio lines: ISM – ISM: individual objects: Cassiopeia A 1. Introduction LOFAR comprises thousands of antennas arranged in (currently) 41 stations spread across the Netherlands and other European Radio recombination lines (RRLs) arise from ions recombining ff countries. By design, LOFAR provides excellent baseline cov- with electrons in di use, partially ionised gas. At high princi- erage, particularly in the inner 3 km core-region, where over ∼> ∼< pal quantum numbers (i.e. n 200 at frequencies 1 GHz), half of the collecting area is contained. The low-band antennas RRLs are an important probe of the temperature and density < (LBAs) of LOFAR, although optimised for frequencies above of the cool (T ∼ 100 K) interstellar medium (e.g., Payne et al. 30 MHz, cover the frequency range from 10 to 90 MHz. The 1989; Konovalenko 1984). RRL measurements have distinct ad- high-band antennas (HBAs) cover the frequency range from 110 vantages over other tracers of the ionised gas; the extinction to 250 MHz. This large observable frequency window makes caused by dust and the contamination from other sources are LOFAR an ideal instrument for carrying out studies of the ISM negligible. However, there have been only a limited number of with low-frequency RRLs. surveys that have studied RRLs from carbon at low radio fre- quencies (∼<330 MHz; e.g. Erickson et al. 1995; Kantharia & In this Letter, we present the first spectral line measurements Anantharamaiah 2001; Roshi et al. 2002). Also, the low angular made using the LBA system of LOFAR. We target carbon RRLs resolution of these early surveys, from several degrees to around in absorption towards the supernova remnant Cassiopeia A (Cas 6 arcmin, did not allow the location of the line-emitting gas to be A), which is one of the brightest continuum sources in the sky determined, and also limited their capability to detect faint RRL and has a sightline that is already known to display strong carbon sources due to beam dilution effects. RRLs in low frequency spectra (see Stepkin et al. 2007, and ref- The LOw Frequency ARray (LOFAR; Stappers et al. 2011; erences therein). In Sect. 2, we present our LOFAR commission- van Haarlem et al. 2013) is a new-generation radio telescope, ing observations and data analysis steps. Our carbon RRL detec- which given its unique design, can be used to make sensi- tions are presented in Sect. 3, and we discuss these detections tive surveys for RRLs with high spectral and spatial resolution. and present our conclusions in Sect. 4. Article published by EDP Sciences L11, page 1 of 5 A&A 551, L11 (2013) Table 1. Summary of the LOFAR data sets used for our analysis. Data set LOFAR id Start date Time Duration Processed resolution (UT) (h) Int. time (s) Channel width (kHz) Channel width (km s−1) Data-1 (continuum) L25937 2011 May 4 23:00:01.5 18 5 48.768 325 Data-1 (RRL) L25937 2011 May 4 23:00:01.5 18 5 0.762 5.1 Data-2 (RRL) L31848 2011 Oct. 15 12:00:02.5 15 5 3.1 21 Notes. Raw visibilities from Data-1 were processed twice to create two separate intermediate data sets for i) continuum imaging and ii) the RRL detections (see Sect. 2). The velocity resolutions were computed at 45 MHz. 2. Observations and data reduction The data sets for our RRL study are taken from two LOFAR commissioning observations of Cas A in the 30 to 90 MHz (LBA) band. The first data set (Data-1) was taken on 2011 May 4 using the 16 core stations and 7 remote stations that were con- structed at the time, giving baselines up to about 24 km. These data were taken with 1-s visibility integrations and 244 sepa- rate frequency subbands, each with a bandwidth of 195 kHz and 256 spectral channels. The second data set (Data-2) was ob- served on 2011 October 15 using 24 core stations and 8 remote stations, and had a maximum baseline length of about 83 km. Data-2 was taken in a multi-beam mode that is used for standard continuum observations, with one beam on Cas A and a sec- ond beam on Cygnus A. This resulted in only half of the avail- able bandwidth being used for the observation of Cas A. Also, a coarser spectral resolution of 64 spectral channels within each of the 122 subbands of 195 kHz bandwidth was used. The visibility integration time was again 1 s. After the first step of interference removal using the AOFlagger routine (Offringa et al. 2010), the data were averaged in time and/or frequency as required. Fig. 1. LOFAR image of Cas A at 52 MHz (single subband of 195 kHz bandwidth). The image was made using Briggs weighting (robust = 0) In total, we produced 3 data sets for our work. First, to and was restored using a beam size of 40 arcsec. The hotspot region make a continuum image of Cas A, we produced an interme- used to extract spectra from Data-2 is marked with the green square. diate data set from Data-1 that had a coarse spectral resolu- tion of ∼50 kHz channel−1 in order to speed up the calibra- tion process later. Next, we created two RRL data sets. For a shapelet model. We then imaged the source using the casa Data-1, we included only those baselines between the separate imager1 and constructed a composite sky model from the im- core-stations (<2.6 km) and retained the full spectral resolution age by fitting for point and shapelet sources (see e.g., Yatawatta −1 of 762 Hz channel . Data-2 was processed to retain a spectral et al. 2011). We iterated over this scheme (bbs calibration → −1 resolution of 3.1 kHz channel and to also include longer base- imaging → skymodel → calibration) to obtain high-resolution lines (<24 km) so that spectra could be extracted from individual continuum images and improved source models. In Fig. 1,we regions across the supernova remnant. For these two RRL data present a 40 arcsec resolution continuum image of the super- sets, we decided to concentrate our search to those frequencies nova remnant at 52 MHz (single subband). Given the excellent between 40 to 50 MHz. This was because the RRL absorption spatial resolution of LOFAR, a number of complex features are was expected to be stronger at the lower end of the LBA spec- clearly resolved (c.f., Kantharia et al. 1998; Lane et al. 2005). trum due to the steep broadband spectral energy distribution of Further details of the calibration strategy and a full discussion Cas A.
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