Evidence for the detection of X-ray scattering from interstellar dust grains

Abstract

Overbeck¹ has shown that small angle scattering of X rays by interstellar dust will tend to increase the apparent size of a point celestial X-ray source, the result being the formation of a halo around the source image. Although the exact description of this halo, which depends on the detailed properties of the dust and of the source, has been discussed^2–6 no direct observational evidence for the existence of such haloes has been presented, although dust scattering has been invoked to explain the diffuse soft X-ray emission observed around the Crab Nebula during a lunar occultation⁷.I report here data obtained by the imaging proportional counter on board the Einstein Observatory⁸ of the bright X-ray source GX339–4 (4U1658–48). These data show a faint but apparently real X-ray halo around GX339–4. The principal uncertainty in the intensity distribution of the halo arises from the poorly determined spectrum of GX339–4 and it is likely that the parameters of the halo and of the derived model of the scattering dust grains may be improved in a future analysis using revised Einstein spectral calibration data; however, the range of allowed spectra leaves the reality of this first detection of an interstellar dust halo unchanged.