Revealing the accelerating wind in the inner region of the colliding-wind binary WR 112

TL;DR

This study presents the first X-ray detection of WR 112, revealing an accelerating wind in its colliding-wind binary system, with phase-locked variability and implications for future hydrodynamic modeling of such systems.

Contribution

It provides the first X-ray observations of WR 112 and introduces a model with an accelerating post-shock flow, advancing understanding of wind dynamics in colliding-wind binaries.

Findings

First X-ray detection of WR 112 from Chandra and Swift.

Evidence of an accelerating post-shock wind reaching ~1350 km/s.

Phase-locked radio and X-ray variability consistent with orbital geometry.

Abstract

Colliding winds in massive binaries generate X-ray-bright shocks, synchrotron radio emission, and sometimes even dusty "pinwheel" spirals. We report the first X-ray detections of the dusty WC+O binary system WR 112 from Chandra and Swift, alongside 27 years of VLA/ATCA radio monitoring and new diffraction-limited Keck images. Because we view the nearly circular orbit almost edge-on, the colliding-wind zone alternates between heavy Wolf-Rayet wind self-absorption and a near-transparent O-star wind foreground each 20-yr orbit, producing phase-locked radio and X-ray variability. This scenario leads to a prediction that the radio spectral index is flatter from a larger non-thermal contribution around the radio intensity maximum, which is indeed observed. Existing models that assume a single dust-expansion speed fail to reproduce the combined infrared geometry and radio light curve. Instead, we require an accelerating post-shock flow that climbs from near-stationary to ~1350 km/s in about one orbital cycle, naturally matching the infrared spiral from about 5" down to within 0.1", while also fitting the phase of the radio brightening. These kinematic constraints supply critical boundary conditions for future hydrodynamic simulations, which can link hot-plasma cooling, non-thermal radio emission, X-ray spectra, and dust formation in a self-consistent framework. WR 112 thus joins WR 140, WR 104, and WR 70-16 (Apep) as a benchmark system for testing colliding-wind physics under an increasingly diverse range of orbital architectures and physical conditions.