Neutral Hydrogen Absorption Measurements of Four Distant Pulsars
and the Electron Density in the Inner Galaxy
J.M. WEISBERG (Carleton College, USA),
M.H. SIEGEL (Carleton College, USA),
D.A. FRAIL (NRAO, Socorro, USA),
S. JOHNSTON (RCfTA, University of Sydney, Australia)
Abstract We have used the VLA to measure neutral hydrogen absorption and emission spectra toward four distant, low-latitude pulsars in the inner Galaxy whose spectra have not been previously recorded. In two cases, there is also a second reasonably bright source in the main beam of the telescope. We use the companion spectra plus all published \HI absorption spectra in the direction of each pulsar to assist in the kinematic determination of pulsar distances. We also derive limits on the mean electron density along each line of sight, and we find that even along paths toward the inner Galaxy, the average electron density does not exceed ~0.1 cm^-3. We test the Taylor & Cordes (1993) galactic electron density model against our measured electron densities plus additional inner Galaxy values recently determined by Koribalski et al. (1994), and conclude that no major corrections to the model are indicated.
Key Words: pulsars: distances, --- pulsars: individual sources: B1648--42, B1703--40, B1718--35, B1830--08 --- interstellar: matter, electron density, --- radio sources: 21-cm absorption