Dust scattering halo of 4U 1630-47 observed with Chandra and Swift: New constraints on the source distance

TL;DR

This study uses Chandra and Swift observations of 4U 1630-47 to analyze its dust scattering halo, constraining the source's distance and providing insights into the interstellar medium and the source's properties.

Contribution

The paper presents a new method to estimate the distance to 4U 1630-47 using dust scattering halo analysis combined with molecular cloud data.

Findings

Distance to 4U 1630-47 is estimated at 11.5 kpc if the molecular cloud is at the far distance.

The dust scattering halo reveals a bright ring caused by a specific molecular cloud along the line of sight.

Future observations can resolve the distance ambiguity and improve understanding of dust and molecular cloud distribution.

Abstract

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 $^{12}$CO $J=1-0$ map and spectrum in the line of sight to the source indicate 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 $\pm$ 0.3 kpc. If MC -79 is at the near distance, then 4U 1630-47 is at 4.7 $\pm$ 0.3 kpc. Future Chandra, Swift, and sub-mm radio observations not only can resolve this ambiguity, but also would provide information regarding properties of dust and 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 anomalously low soft state observation observed during the 2010 decay.