A Melody in the Noise: Modeling Echoes of the Crab Nebula

TL;DR

This paper presents a detailed model of echoes from the Crab nebula caused by small filamentary structures, successfully explaining delay evolution and asymmetry but not magnification, supporting the filamentary nature of the nebula.

Contribution

It introduces a simplified yet detailed filament model that explains key echo properties and supports the presence of small-scale structures in the Crab nebula.

Findings

Echo delays follow near quadratic evolution.

Model reproduces asymmetry between incoming and outgoing echoes.

Magnifications of echoes are not accurately reproduced.

Abstract

Pulses from the Crab pulsar are often followed by ``echoes'', produced by radiation that was deflected by small filamentary structures in the Crab nebula and thus traveled via longer paths. We describe a simplified but detailed model that treats the filaments as cylinders of dense, neutral material with a thin ionized skin. In this picture, echoes are produced when the line of sight crosses the skin at glancing incidence, which naturally leads to the large electron column density gradients required to get the observed delays even with electron densities comparable to those inferred from optical line emission ratios. We compare the properties of the predicted echoes with those of a relatively isolated observed one identified during daily monitoring with CHIME. We find that the delays of the simulated echoes follow closely the near quadratic evolution known to be a feature of these echoes, and that, unlike in previous models, we match the characteristic observed asymmetry between incoming and outgoing arcs, with the size of the gap in between a consequence of the skin crossing time. However, our model fails to quantitatively reproduce the magnifications of the echoes. We believe this likely is because the filaments are not as smooth as envisaged, so that a given echo results from many images. Nevertheless, our results strongly support the hypothesis that the nebula is filled with small-scale filamentary structures, which may well be substructures of the larger filaments that are seen in optical images.