Spectroscopic Signatures of the Superorbital Period in the Neutron Star Binary LMC X-4

TL;DR

This study uses high-resolution X-ray spectroscopy to analyze emission line variability in LMC X-4, confirming accretion disk precession as the cause of its superorbital period and revealing detailed plasma conditions.

Contribution

First high-resolution X-ray spectral analysis of LMC X-4's superorbital phase, providing evidence for accretion disk precession and detailed plasma diagnostics.

Findings

Emission lines from multiple ion species identified

Evidence of highly ionized iron with Doppler shifts

Spectral variations linked to superorbital phase

Abstract

We present the first high-resolution X-ray study of emission line variability with superorbital phase in the neutron star binary LMC X-4. Our analysis provides new evidence from X-ray spectroscopy confirming accretion disk precession as the origin of the superorbital period. The spectra, obtained with the Chandra High-Energy Transmission Grating Spectrometer (HETGS) and the XMM-Newton Reflection Grating Spectrometer (RGS), contain a number of emission features, including lines from hydrogen-like and helium-like species of N, O, Ne, and Fe, a narrow O VII RRC, and fluorescent emission from cold Fe. We use the narrow RRC and the He-alpha triplets to constrain the temperature and density of the (photoionized) gas. By comparing spectra from different superorbital phases, we attempt to isolate the contributions to line emission from the accretion disk and the stellar wind. There is also evidence for highly ionized iron redshifted and blueshifted by ~25,000 km/s. We argue that this emission originates in the inner accretion disk, and show that the emission line properties in LMC X-4 are natural consequences of accretion disk precession.