X-Ray Dust Scattering at Small Angles: The Complete Halo around GX13+1

The exquisite angular resolution available with Chandra should allow precision measurements of faint diffuse emission surrounding bright sources, such as the X-ray scattering halos created by interstellar dust. However, the ACIS CCDs suffer from pileup when observing bright sources, and this creates difficulties when trying to extract the scattered halo near the source. The initial study of the X-ray halo around GX13+1 using only the ACIS-I detector performed by Smith, Edgar, & Shafer suffered from a lack of sensitivity within 50″ of the source, limiting what conclusions could be drawn. To address this problem, observations of GX13+1 were obtained with the Chandra HRC-I and simultaneously with the RXTE PCA. Combined with the existing ACIS-I data, this allowed measurements of the X-ray halo between 2″ and 1000″. After considering a range of dust models, each assumed to be smoothly distributed with or without a dense cloud along the line of sight; the results show that there is no evidence in these data for a dense cloud near the source, as suggested by Xiang et al. Finally, although no model leads to formally acceptable results, the Weingartner & Draine model and nearly all of the composite grain models from Zubko, Dwek, & Arendt give poor fits.