The θ–θ diagram: transforming pulsar scintillation spectra to coordinates on highly anisotropic interstellar scattering screens

ABSTRACT We introduce a novel analysis technique for pulsar secondary spectra. The power spectrum of pulsar scintillation (referred to as the ‘secondary spectrum’) shows differential delays and Doppler shifts due to interference from multipath propagation through the interstellar medium. We develop a transformation that maps these observables to angular coordinates on a single thin screen of phase-changing material. This transformation is possible without degeneracies in the case of a one-dimensional distribution of images on this screen, which is often a successful description of the phenomenon. The double parabolic features of secondary spectra are transformed into parallel linear features, whose properties we describe in detail. Furthermore, we introduce methods to measure the curvature parameter and the field amplitude distribution of images by applying them to observations of PSR B0834+06. Finally, we extend this formalism to two-dimensional distributions of images on the interstellar screen.

Is anomalous scattering typical for pulsars?

Recent studies of interstellar scattering frequency evolution indicate, that for a large number of pulsars the measured scatter time to frequency scaling does not follow predictions based upon simple scattering models. The number of such deviations suggests, that what was previously known as “anomalous scattering” is actually quite normal for a large subset of the pulsar population. New observations are needed, especially at low frequencies, to study the scattering of low-DM objects. In this regard even small telescopes, such as single LOFAR stations - like PL-612 in Bałdy, near Olsztyn, Poland - can be extremely helpful.