Driessen, L. N., G. H. Janssen, C. G. Bassa, B. W. Stappers, and D. R. Stinebring. 2019. Scattering features and variability of the Crab pulsar. Monthly Notices of the Royal Astronomical Society 483:1224-1232.
"We report on Westerbork Synthesis Radio Telescope observations of the Crab pulsar at 350MHz from 2012 November 24 until 2015 June 21. During this period we consistently observe variations in the pulse profile of the Crab. Both variations in the scattering width of the pulse profile as well as delayed copies, also known as echoes, are seen regularly. These observations support the classification of two types of echoes: those that follow the truncated exponential shape expected for the thin-screen scattering approximation, and echoes that show a smoother, more Gaussian shape."
Oberlin's online access to this journal was through the OhioLINK Electronic Journal Center; that ceased in 2012 due to publisher changes. Access the pre-print version of the article at ArXiv.org.