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

TL;DR

This study uses CHIME to monitor Crab pulsar echoes over two years, revealing diverse plasma lensing events with dispersive delays, including some with non-zero minimum delays, advancing understanding of astrophysical plasma structures.

Contribution

It provides the first long-term, detailed observation of Crab pulsar echoes, uncovering new behaviors and characteristics of plasma lensing events.

Findings

Echoes are visible throughout the observation period.

Distinct groups of echoes show different evolution patterns.

Some echoes exhibit dispersive delays indicating excess or deficit column densities.

Abstract

The Crab Pulsar is known to feature plasma lensing events known as echoes. These events show additional components in the pulse profile which are produced by signal that is deflected by ionized nebular material and are therefore delayed relative to the primary emission. We observed the Crab pulsar with \ac{CHIME} during its daily transits, creating an archive of baseband recordings of bright single pulses (known as giant pulses) in the 400$-800\,\unit{MHz}$ 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 ${0.5 \rm\,ms}$. The frequency and diversity of the observed echoes make the Crab an excellent target for long-term studies of astrophysical plasma lensing.