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In order to study the interstellar medium and to derive kinematic distances, we have measured 21 cm absorption spectra in the direction of eight southern pulsars with the Parkes telescope. A low-noise receiver permitted high sensitivity observations, and a 1024-channel autocorrelator and associated hardware and software that accumulated data into several pulsar phase bins also led to significant advances in experimental technique. All spectra were recorded at a resolution of 7.8 kHz or 1.74 km/sec, which is significantly narrower than previously measured Parkes pulsar HI absorption spectra. This higher resolution also increased the sensitivity of the absorption measurements, since most HI absorption lines are narrow.
For three of these pulsars, HI absorption measurements have not previously been obtained. We were also able to improve significantly the sensitivity and resolution on four others whose 21-cm spectra were previously measured. Finally, we have successfully obtained a high-quality absorption spectrum on the eighth pulsar, whose previously published spectrum was unusable for kinematic distance determinations due to frequency structure imposed by interstellar scintillations.
We present our emission and absorption spectra for these eight pulsars, derive kinematic distances and electron densities along the lines of sight where possible; and compare our results with previous work.
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