Peculiarities of super-Eddington flares from the X-ray pulsar LMC X-4 with NuSTAR

TL;DR

This study analyzes super-Eddington flares from the X-ray pulsar LMC X-4 using NuSTAR, revealing energy-dependent luminosity changes and pulse profile variations, and discusses possible accretion geometry modifications.

Contribution

First detailed analysis of super-Eddington flares from LMC X-4 across a broad energy range with NuSTAR, highlighting energy-dependent spectral and pulse profile changes.

Findings

Luminosity increase mainly below 25-30 keV during flares

Pulse profile becomes approximately triangular during flares

Pulsed fraction increases with energy, reaching 60-70%

Abstract

We present results of the analysis of super-Eddington flares registered from the X-ray pulsar LMC X-4 by the NuSTAR observatory in the broad energy range 3-79 keV. The pulsar spectrum is well described by the thermal comptonization model both in a quiescent state and during flares, when the peak luminosity reaches values $L_{\rm x} \sim (2-4)\times10^{39}$ erg s$^{-1}$. An important feature, found for the first time, is that the order of magnitude increase in the luminosity during flares is observed primarily at energies below 25-30 keV, whereas at higher energies (30-70 keV) the shape of the spectrum and the source flux remain practically unchanged. The luminosity increase is accompanied by changes in the source pulse profile -- in the energy range of 3-40 keV it becomes approximately triangular, and the pulsed fraction increases with increasing energy, reaching 60-70% in the energy range of 25-40 keV. The paper discusses possible changes in the geometry of the accretion column, which can explain variations in spectra and pulse profiles.