Using Chandra-HETGS data of Cyg X-3, we have analysed the lightcurves at different radial distances from the source. While at higher energies the lightcurve remains unchanged, at low energies it varies strongly within the inner 10-15 arcsec until all intensity variations completely vanish beyond about 20 arcsec. This effect can be interpreted as the result of light that is scattered by dust having to travel a longer distance to the observer. The resulting additional time causes the (intrinsic) lightcurve also to be delayed and smeared out. From the measured delays, the distance of the source could be determined to be approximately 9 kpc. The distance obtained here is consistent with those obtained using other methods. However, the ability of the method, particularly when using Chandra with its high angular resolution, could be convincingly demonstrated, despite of the statistical uncertainties inherent in the present observation.
The observation was, unfortunately, shorter than a complete 4.8 h orbital cycle of Cyg X-3, and the countrate was reduced by a factor of four because the grating was inserted and most of the radiation was blocked or dispersed. Limited by the low quality of the data, the result could not be improved even when using the real dust distribution (if it were known) rather than our simplified model. The simple model leads to wrong results only for the case of an extreme dust distribution, e.g. if all the dust were concentrated in a `thin layer' near the source. This would produce a relatively weak halo due to the large scattering angles involved. However, the ratio between optical depth in scattering and X-ray absorption for Cyg X-3 is in agreement with the general law (Predehl and Schmitt 1994) excluding such an extreme dust distribution. Actually we note that a longer observation covering 2 to 3 orbital periods, of course without the grating, would allow not only a more precise distance measurement but in principle also a determination of the dust distribution along the line of sight.
© European Southern Observatory (ESO) 2000
Online publication: May 3, 2000