Optical Spectrum of Main-, Inter- and Off-pulse Emission from Crab Pulsar

Abstract

A dedicated stroboscopic device was used to obtain optical spectra of the Crab main-pulse and inter-pulse as well as the spectrum of the underlying nebula when the pulsar is turned off. As the nebular emission is very inhomogeneous, our ability to effectively subtract the nebular background signal is crucial. No spectral lines intrinsic to the pulsar are detected. The main-pulse and the inter-pulse behave as power laws, both with the same de-reddened index Alpha = +0.2 +- 0.1. This value was obtained by subtracting the nebular spectrum at the exact position of the pulsar. The underlying nebula is redder, Alpha = -0.4 +- 0.1. Its emission lines are split into approaching (sim. -1200 km/s) and receding (sim. +600 km/s) components. The strength of emission line components and the flux in nebular continuum vary on arcsec scale. The nebular line and continuum intensities along the N-S slit are given.