Extreme scattering events

Extreme scattering events (ESEs) are caused by the occultation to radio sources by AU-sized objects (or lenses). Free electrons in the lens seem to be responsible for the dramatic flux changes during the occultation but the nature of the lens is still a mystery. The lens may be a peculiar lump of interstellar medium (ISM). Recent self-gravitating cloud model is very interesting in that it gives better explanation on the dual frequency light curves (DFLs) in the quasar 0954+658 and the cloud may be a new halo population which is sufficient to explain dark-matter problem. However, still there are many things to be clarified, such as the origin and the stability of the cloud. One of the important features of ESEs is that there are multiple events. Some of them have consecutive events caused by lenses in close angular separation. Indeed, the lenses may be clumped. For simplicity, we investigate various extreme scattering events caused by a double lens. Each lens is assumed to have an electron column density in Gaussian shape whose peak is centered. Under the assumption of point source and with typical parameter values, the double-lens model has difficulty in producing the DFL. The source size effect and stronger lensing conditions are required for the reproduction of the DFL.