Variable 21-cm absorption at z=0.3127

We report multi-epoch Giant Metrewave Radio Telescope (GMRT) H I observations of the z=0.3127 damped absorber towards the quasar PKS 1127-145, which reveal variability in both the absorption profile and the flux of the background source, over a time-scale of a few days. The observed variations cannot be explained by simple interstellar scintillation (ISS) models where there are only one or two scintillating components and all of the ISS occurs in the Galaxy. More complicated models, where either there are more scintillating components or some of the ISS occurs in the interstellar medium of the z = 0.3127 absorber, may be acceptable. However, the variability can probably be best explained in models incorporating motion (on sub-VLBI scales) of a component of the background continuum source, with or without some ISS. All models producing the variable 21-cm absorption profile require small-scale variations in the 21-cm optical depth of the absorber. The length-scale for the opacity variations is similar to0.1pc in pure superluminal motion models, and similar to 10pc in pure ISS models. Models involving subluminal motion, combined with scintillation of the moving component, require opacity variations on far smaller scales of similar to 10-100 au.