Modeling the precession of the warped inner accretion disk in the pulsars LMC X-4 and SMC X-1 with NuSTAR and XMM-Newton

TL;DR

This study analyzes how superorbital periods caused by warped, precessing accretion disks affect X-ray spectra and pulse profiles in neutron star binaries LMC X-4 and SMC X-1, using joint NuSTAR and XMM-Newton observations.

Contribution

It introduces a geometric model linking pulse profile changes to a precessing inner accretion disk, supported by broad-band spectral analysis.

Findings

Spectral shape varies periodically with superorbital cycle.

Pulse profiles change periodically, consistent with a precessing disk.

Reprocessed emission from the precessing disk explains long-term pulse shape changes.

Abstract

We present a broad-band X-ray study of the effect of superorbital periods on X-ray spectra and pulse profiles in the neutron star X-ray binaries LMC X-4 and SMC X-1. These two sources display periodic or quasi-periodic variations in luminosity on the order of tens of days which are known to be superorbital, and are attributed to warped, precessing accretion disks. Using joint NuSTAR and XMM-Newton observations that span a complete superorbital cycle, we examine the broad-band spectra of these sources and find the shape to be well described by an absorbed power law with a soft blackbody component. Changes in spectral shape and pulse profile shape are periodic with superorbital period, as expected from a precessing disk. We perform X-ray tomography using the changes in pulse profiles to model the geometry and kinematics of the inner accretion disk. Our simple beam and inner disk geometric model indicates that the long term changes in soft pulse shape and phase are consistent with reprocessed emission from a precessing inner disk.