A Cacophony of Echoes from daily monitoring of the Crab Pulsar at Jodrell Bank

Abstract

Using archival data from the 42 foot telescope at the Jodrell Bank Observatory, we produce daily stacks of aligned giant pulses for the Crab pulsar, to study changes to the daily profiles between April 2012 to December 2016. From these, we identify echoes, where intervening material away from the line of sight causes pulsed emission to be redirected towards the observer, with delay corresponding to the increased distance of travel, resulting in additional profile components. These observations show that such echoes may be far more common than implied by the previous rate of detections. All the observed echoes are consistent with approaching zero-delay at their closest approach to the normal giant pulse emission. This indicates that the structures responsible for producing these events must be highly anisotropic, with typical lengths greater than $\sim 4\textrm{AU}$, typical widths on the sky of $\sim 0.1 \textrm{AU}$ and typical depths of $\sim 5\textrm{AU}$, given the previously observed electron densities of the nebular filaments, on the order of 1000 cm$^{-3}$. This suggests that these inhomogeneities are likely to be offshoot substructure from the larger nebular filaments of the Crab nebula.