Echoes in Different Tempo: Long-Term Monitoring of Crab Echoes with CHIME

Abstract

The Crab Pulsar is known to feature plasma lensing events known as echoes. These events are characterized by additional components in the pulse profile which are produced by additional images formed when the pulsar's radio emission is deflected by ionized nebular material. These components are therefore delayed relative to the primary emission. We observed the Crab pulsar with Canadian Hydrogen Intensity Mapping Experiment (CHIME) during its daily transits, creating an archive of baseband recordings of giant pulses in the 400-800 MHz band. From these, we produced daily stacks of aligned pulses between late October 2021 and March 2024. We find that in these averages, echoes are readily visible throughout the observation period, and we identify clear groups of echoes with distinct behaviour in terms of their evolution with time and frequency. Many echoes exhibit dispersive delays consistent with being observed through excess column densities relative to the unscattered rays, but we also find two events where the dispersive delays indicate column density deficits. For the first time, we also find echoes for which the line of sight never directly intersects the intervening structures, resulting in events with non-zero minimum delays, of around 0.5 ms. The frequency and diversity of the observed echoes make the Crab an excellent target for long-term studies of astrophysical plasma lensing.