Interstellar Seeing. I. Superresolution Techniques Using Radio Scintillation

Abstract

Interstellar scintillation can be used to probe transverse sizes of radio sources on scales inaccessible to the nominal resolution of any terrestrial telescope, e.g. $\lesssim 10^{-6}$ arc sec. Methodology is presented that exploits this superresolution phenomenon for both single aperture and interferometer observations. The treatment applies to the saturated (strong-scattering) regime and holds for both thin screens and extended media. A general signal model for radio sources is presented, {\it scintillated amplitude modulated noise}, which applies to compact, incoherent synchrotron sources such as AGNs and gamma-ray burst sources and also to known, coherent sources such as masers and pulsars. The exact probability density function for measured intensities and interferometric visibilities is obtained by solving a general Fredholm problem. An approximate density function is also obtained by using the equivalent number of degrees of freedom in scintillation modulations. The scintillation modulation variance is presented, which includes the effects of source structure and time-bandwidth averaging in the signal processing. Two Bayesian methods are outlined for inferring the sizes of emission regions that use first order statistics of the intensity and visibility. Intensity cross-correlation methods for inferring source sizes are also given. Intensity interferometry in the the radio context is compared to the optical intensity interferometry of Hanbury-Brown and Twiss.