Characterization of the optical and X-ray properties of the northwestern wisps in the Crab Nebula

TL;DR

This study investigates the optical and X-ray wisps near the Crab pulsar, revealing their spatial differences and implications for particle distributions and Doppler-boosted models in pulsar wind nebulae.

Contribution

It provides the first detailed multi-wavelength analysis of the northwestern wisps, comparing their properties in optical and X-ray bands and testing Doppler-boosted ring models against MHD simulations.

Findings

X-ray wisps are closer to the pulsar than optical wisps.

Optical and X-ray wisps are spatially similar but not coincident.

Optical Doppler boosting factors exceed MHD predictions by about three times.

Abstract

We have studied the wisps to the north-west of the Crab pulsar as part of a multi-wavelength campaign in the visible and in X-rays. Optical observations were obtained using the Nordic Optical Telescope in La Palma and X-ray observations were made with the Chandra X-ray Observatory. The observing campaign took place from 2010 October until 2012 September. About once per year we observe wisps forming and peeling off from (or near) the region commonly associated with the termination shock of the pulsar wind. We find that the exact locations of the northwestern wisps in the optical and in X-rays are similar but not coincident, with X-ray wisps preferentially located closer to the pulsar. This suggests that the optical and X-ray wisps are not produced by the same particle distribution. Our measurements and their implications are interpreted in terms of a Doppler-boosted ring model that has its origin in magne- tohydrodynamic (MHD) modelling. While the Doppler boosting factors inferred from the X-ray wisps are consistent with current MHD simulations of pulsar wind nebulae (PWN), the optical boosting factors are not, and typically exceed values from MHD simulations by about a factor of 3.