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

TL;DR

This study uses combined Chandra HRC-I, ACIS-I, and RXTE PCA data to analyze the X-ray dust scattering halo around GX13+1, providing detailed measurements from 2 to 1000 arcseconds and evaluating dust models.

Contribution

It introduces a method combining multiple instruments to improve sensitivity near bright X-ray sources and tests various dust models against the observed halo.

Findings

No evidence for a dense cloud near GX13+1.

Standard dust models like Weingartner & Draine (2001) fit poorly.

Combined data extends halo measurements from 2 to 1000 arcseconds.

Abstract

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 done by Smith, Edgar & Shafer (2002) 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 2-1000''. 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 this data for a dense cloud near the source, as suggested by Xiang et al. (2005). Finally, although no model leads to formally acceptable results, the Weingartner & Draine (2001) and nearly all of the composite grain models from Zubko, Dwek & Arendt (2004) give poor fits.