Scattering study of Pulsars below 100 MHz using LWA1

Abstract

Interstellar scattering causes pulsar profiles to grow asymmetrically, thus affecting the pulsar timing residuals, and is strongest at lower frequencies. Different Interstellar medium models predict different frequency ($\nu$) and dispersion measure (DM) dependencies for the scattering time-scale $\tau_{sc}$. For Gaussian inhomogeneity the expected scaling relation is $\tau_{sc} \propto \nu^{-4}\ DM^{2}$, while for a Kolmogorov distribution of irregularities, the expected relation is $\tau_{sc} \propto \nu^{-4.4}\ DM^{2.2}$. Previous scattering studies show a wide range of scattering index across all ranges of DM. A scattering index below 4 is believed to be either due to limitations of the underlying assumptions of the thin screen model or an anisotropic scattering mechanism. We present a study of scattering for seven nearby pulsars (DM $< 50$ pc cm$^{-3}$) observed at low frequencies ($10-88$ MHz), using the first station of the Long Wavelength Array (LWA1). We examine the scattering spectral index and DM variation over a period of about three years. The results yield insights into the small-scale structure of ISM as well as the applicability of the thin screen model for low DM pulsars.