LOFAR detections of low-frequency radio recombination lines towards
  Cassiopeia A
  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.
