Variable O VI and N V Emission from the X-ray Binary LMC X-3: Heating of the Black Hole Companion

TL;DR

This study detects variable UV emission lines from LMC X-3, indicating X-ray illumination causes a hot spot on the B star companion, and suggests these emissions can help determine system parameters.

Contribution

First detection of variable O VI and N V emission from LMC X-3, linking UV emission variability to X-ray illumination effects on the companion star.

Findings

UV emission velocities follow the optical velocity curve.

UV emission intensities decrease during certain binary phases.

The hot spot on the B star is likely the emission source.

Abstract

Based on high-resolution ultraviolet spectroscopy obtained with FUSE and COS, we present new detections of O VI and N V emission from the black-hole X-ray binary (XRB) system LMC X-3. We also update the ephemeris of the XRB using recent radial velocity measurements obtained with the echelle spectrograph on the Magellan-Clay telescope. We observe significant velocity variability of the UV emission, and we find that the O VI and N V emission velocities follow the optical velocity curve of the XRB. Moreover, the O VI and N V intensities regularly decrease between binary phase = 0.5 and 1.0, which suggests that the source of the UV emission is increasingly occulted as the B star in the XRB moves from superior to inferior conjunction. These trends suggest that illumination of the B-star atmosphere by the intense X-ray emission from the accreting black hole creates a hot spot on one side of the B star, and this hot spot is the origin of the O VI and N V emission. However, the velocity semiamplitude of the ultraviolet emission, K_{UV}~180 km/s, is lower than the optical semiamplitude; this difference could be due to rotation of the B star. If our hypothesis about the origin of the highly ionized emission is correct, then careful analysis of the emission occultation could, in principle, constrain the inclination of the XRB and the mass of the black hole.