Physical Sciences: Scattering of X-Rays from the Crab Nebula

Abstract

THE X-ray source in the Crab Nebula appears as a composite system consisting of a diffuse source about 2 minutes of arc in diameter1,2, and a pulsating source synchronous with the radio and optical pulsar NP 0532 (refs. 3–6). It has been suggested that the X-rays received from the Crab could in fact all be released by the central star7, and that the apparent extended source could be due to the scattering of X-rays by interstellar dust particles which are also believed to be responsible for stellar extinction and reddening. The typical scattering angle for a photon of wavelength λ impinging on a dielectric obstacle of size a is of order λ/a, that is, minutes of arc for photons of a few keV and micron sized grains, and close to the angular extension of the diffuse source. Following this idea, one can assume that any X-ray photon received from the Crab was emitted during the pulses, but scattering by an angle as low as 0.1 second is sufficient to increase the travel time to the Earth by more than one pulsar period (0.03 s), thus hiding the pulsating character of the source8. (The photon detected within the pulses would have suffered no scattering at all.) A typical amount of interstellar dust could indeed explain the relatively low fraction (0.05 to 0.015 per cent, refs. 3 to 6) of the power received within the pulses.