On Rapid Interstellar Scintillation of Quasars: PKS 1257-326 Revisited

The line of sight towards the compact, radio loud quasar PKS 1257-326 passes through a patch of scattering plasma in the local Galactic ISM that causes large and rapid, intra-hour variations in the received flux density at centimetre wavelengths. This rapid interstellar scintillation (SS) has been occurring for at least 15 years, implying that the scattering ``screen'' is at least 100 AU in physical extent. Through observations of the ISS we have measured microarcsecond-scale ``core shifts'' in PKS 1257-326, corresponding to changing opacity during an intrinsic outburst. Recent analysis of VLA data of a sample of 128 quasars found 6 sources scintillating with a characteristic time-scale of < 2 hours, suggesting that nearby scattering screens in the ISM may have a covering fraction of a few percent. That is an important consideration for proposed surveys of the transient and variable radio sky.