Multi-epoch HI line measurements of 4 southern pulsars

We have measured 21-cm absorption spectra in the direction of three southern pulsars, PSRs B0736-40, B1451-68 and B1641-45, in three separate epochs spread over 2.5 yr. We see no evidence for any changes in the absorption spectra over this time span in spite of good velocity resolution and sensitivity. Towards PSR B1641-45 we place an upper limit of 10^19 per cm^2 on the change in the column density of the cold, neutral gas. In addition, we observed PSR B1557-50 and compared its HI absorption spectrum with spectra taken in 1980 and 1994. A prominent deep absorption feature seen at -110 km/s in 1994 is weaker in 2000 and likely was not present at all in 1980. Using the standard interpretation which links the distance traversed by the pulsar to a physical cloud size, this results in a cloud size of ~1000 AU and density of 2x10^4 per cc, parameters typical of those seen in other observations. These results are also consistent with an alternative model by Deshpande (2000) who expects the largest variations in optical depth to be seen against the longest time intervals.