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... Show moreCassiopeia 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 50 MHz with a signal-to-noise ratio of {gt}5 from two independent LOFAR data sets. The derived line velocities (v$_{LSR}$ ~{} - 50 km s$^{-1}$) 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 { imes} 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. Show less
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... Show moreCassiopeia 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 50 MHz with a signal-to-noise ratio of {gt}5 from two independent LOFAR data sets. The derived line velocities (v$_{LSR}$ ~{} - 50 km s$^{-1}$) 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 { imes} 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. Show less
Some radio pulsars show clear ''drifting subpulses'', in which subpulses are seen to drift in pulse longitude in a systematic pattern. Here we examine how the drifting subpulses of PSR B0809+74... Show moreSome radio pulsars show clear ''drifting subpulses'', in which subpulses are seen to drift in pulse longitude in a systematic pattern. Here we examine how the drifting subpulses of PSR B0809+74 evolve with time and observing frequency. We show that the subpulse period (P$_{3}$) is constant on timescales of days, months and years, and between 14-5100 MHz. Despite this, the shapes of the driftbands change radically with frequency. Previous studies have concluded that, while the subpulses appear to move through the pulse window approximately linearly at low frequencies ({lt}500 MHz), a discrete step of ~{}180{deg} in subpulse phase is observed at higher frequencies ({gt}820 MHz) near to the peak of the average pulse profile. We use LOFAR, GMRT, GBT, WSRT and Effelsberg 100-m data to explore the frequency-dependence of this phase step. We show that the size of the subpulse phase step increases gradually, and is observable even at low frequencies. We attribute the subpulse phase step to the presence of two separate driftbands, whose relative arrival times vary with frequency - one driftband arriving 30 pulses earlier at 20 MHz than it does at 1380 MHz, whilst the other arrives simultaneously at all frequencies. The drifting pattern which is observed here cannot be explained by either the rotating carousel model or the surface oscillation model, and could provide new insight into the physical processes happening within the pulsar magnetosphere. Show less
