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.

arXiv commentAccepted for publication in A&A Letters; 5 pages, 4 colour figures

References in corpus (5)

• Post-correlation radio frequency interference classification methods
• Observing pulsars and fast transients with LOFAR
• Radio Interferometric Calibration Using The SAGE Algorithm
• The LOFAR radio environment
• Radio recombination lines from the largest bound atoms in space

Cited by in corpus (13)

• The search for radio emission from the exoplanetary systems 55 Cancri, $\upsilon$ Andromedae, and $\tau$ Boötis using LOFAR beam-formed observations
• Parametrising Epoch of Reionization foregrounds: A deep survey of low-frequency point-source spectra with the MWA
• Carbon and hydrogen radio recombination lines from the cold clouds towards Cassiopeia A
• Science with the Murchison Widefield Array: Phase I Results and Phase II Opportunities
• Wide-field LOFAR-LBA power-spectra analyses: Impact of calibration, polarization leakage and ionosphere
• Low Frequency Carbon Radio Recombination Lines I: Calculations of Departure Coefficients
• Discovery of Carbon Radio Recombination Lines in M82
• LOFAR observations of decameter carbon radio recombination lines towards Cassiopeia A
• Discovery of Carbon Radio Recombination Lines in absorption towards Cygnus~A
• Mapping low frequency carbon radio recombination lines towards Cassiopeia A at 340, 148, 54 and 43 MHz
• Exact Bound-Bound Gaunt Factor Values for Quantum Levels up to n=2000
• The impact of lossy data compression on the power spectrum of the high redshift 21-cm signal with LOFAR
• Observations of Carbon Radio Recombination Lines with the NenuFAR telescope. I. Cassiopeia A and Cygnus A