Geometric Distance Determination Using Type I X‐Ray Bursts

With the excellent angular resolution of the Chandra X-ray Observatory, it ispossible to geometrically determine the distance to variable Galactic sources,based on the phenomenon that scattered radiation appearing in the X-ray halohas to travel along a slightly longer path than the direct, unscatteredradiation. By measuring the delayed variability, constraints on the sourcedistance can be obtained if the halo brightness is large enough to dominate thepoint spread function (PSF) and to provide sufficient statistics. The distanceto Cyg X-3, which has a quasi-sinusoidal light curve, has been obtained withthis approach by Predehl et al. Here we examine the feasibility of using thedelayed signature of type I X-ray bursts as distance indicators. We usesimulations of delayed X-ray burst light curves in the halo to find that theoptimal annular region and energy band for a distance measurement with agrating observation is roughly 10-50" and 1-5 keV respectively, assumingChandra's effective area and PSF, uniformly distributed dust, the inputspectrum and optical depth to GX 13+1, and the Weingartner & Draineinterstellar grain model. We find that the statistics are dominated by Poissonnoise rather than systematic uncertainties, e.g., the PSF contribution to thehalo. Using Chandra, a distance measurement to such a source at 4 (8) kpc couldbe made to about 23% (30%) accuracy with a single burst with 68% confidence. Bystacking many bursts, a reasonable estimate of systematic errors limit thedistance measurement to about 10% accuracy.Comment: 7 pages, 4 figures; Accepted for publication in Ap