GBT02B-009

AU scale HI structures: a probe using scattering of pulsar signals

Abstract

A novel probe to study AU scale structures in the cold HI gas by exploiting scattering of pulsar signal is described. We propose to investigate AU-scale HI structures using this probe toward two pulsars with the GBT. Since its discovery a few decades ago, the small angular scale ($\le$ 0.2\arcsec) HI opacity variation of $\le$ 0.1 remained a puzzle. Origin of this opacity variation is controversial. Conventional interpretation of these variations require cold \HI clouds of size in the range 5 to 100 AU and volume density $\sim$ $10^5$ \cmthree or \HI gas in sheet like geometry. On the other hand Deshpande (2000) and Gwinn (2001) argue that the observed opacity variations seen in transverse separations $<$100 AU is to be expected due to the cumulative contribution from \HI structures of a wide range of scales or due to a combination of radio-wave scattering by the interstellar plasma and subsonic gradient in the Doppler velocity of the HI. The method we propose exploits two phenomena to probe small-scale HI structure: the multi-path propagation resulting from scattering, and the pulsed nature of pulsar signals. The proposed probe is insensitive to smooth (large-scale) gradients in HI opacity as well as velocity gradient across the scattering disk, and hence holds a promise to distinguish between the actual and apparent small-scale structures in HI opacity.

Front Ends

Gregorian L(1.15 to 1.73 GHz)

Back Ends

Spectral Processor

Type of Observing

Spectroscopy
Pulsar
High Time Resolution

Switching Type

Created: Sun Sep 15 10:04:41 Eastern Daylight Time 2002