Dust scattering halo of 4U 1630-47: high resolution X-ray and mm observations constrain source and molecular cloud distances

Official URL: https://dx.doi.org/10.1093/mnras/staf1688

We re-investigated the distance to the black hole X-ray binary 4U 1630−47 by analysing its dust scattering halo (DSH) using high-resolution X-ray (Chandra) and millimeter (APEX) observations. Dust scattering haloes form when X-rays from a compact source are scattered by interstellar dust, creating diffuse ring-like structures that can provide clues about the source’s distance. Our previous work suggested two possible distances: 4.9 and 11.5 kpc, but uncertainties remained due to low-resolution CO maps. We developed a new methodology to refine these estimates, starting with a machine learning approach to determine 3D representation of molecular clouds from the APEX data set. The 3D maps are combined with X-ray flux measurements to generate synthetic DSH images. By comparing synthetic images with the observed Chandra data through radial and azimuthal profile fitting, we not only measure the source distance but also distinguish whether the molecular clouds are at their near- or far-distances. The current analysis again supported a distance of 11.5 kpc over alternative estimates. While the method produced a lower reduced χ2 for both the azimuthal and radial fits for a distance of 13.6 kpc, we ruled it out as it would have produced a bright ring beyond the APEX field of view, which is not seen in the Chandra image. The 4.85 kpc estimate was also excluded due to poor fit quality and cloud distance conflicts. The systematic error of 1 kpc, which arises due to errors in determining molecular cloud distances, dominates the total error.