A Dust-scattering Halo of 4U 1630-47 Observed with Chandra and Swift: New Constraints on the Source Distance

We have observed the Galactic black hole transient 4U 1630-47 during the decay of its 2016 outburst with Chandra and Swift to investigate the properties of the dust-scattering halo created by the source. The scattering halo shows a structure that includes a bright ring between 80" and 240" surrounding the source, and a continuous distribution beyond 250". An analysis of the (CO)-C-12 J = 1-0 map and spectrum in the line of sight to the source indicates that a molecular cloud with a radial velocity of 79 km s(-1) (denoted MC 79) is the main scattering body that creates the bright ring. We found additional clouds in the line of sight, calculated their kinematic distances, and resolved the well known "near" and "far" distance ambiguity for most of the clouds. At the favored far-distance estimate of MC -79, the modeling of the surface brightness profile results in a distance to 4U 1630 47 of 11.5 +/- 0.3 kpc. If MC 79 is at the near distance, then 4U 1630-47 is at 4.7 +/- 0.3 kpc. Future Chandra, Swift, and submillimeter radio observations not only can resolve this ambiguity, but also would provide information regarding properties of dust and the distribution of all molecular clouds along the line of sight. Using the results of this study we also discuss the nature of this source and the reasons for the observation of an anomalously low soft state during the 2010 decay.