Peculiarities of Super-Eddington Flares from the X-ray Pulsar LMC X-4 Based on NuSTAR Data

Abstract

We present the results of our analysis of super-Eddington flares recorded fromthe X-ray pulsar LMCX-4 by theNuSTAR observatory in the energy range 3–79 keV. The pulsar spectrumis well described by the thermal Comptonization model (COMPTT) both in quiescence and during flares, when the peak luminosity reaches L_x ~ (2−4) × 10³⁹ erg s⁻¹. An important feature that has been investigated for the first time in this paper is that an increase in luminosity during flares by more than an order of magnitude is observed at energies below 25–30 keV, while at higher energies (30–70 keV) the spectrum shape and the source flux remain virtually unchanged. The increase in luminosity is accompanied by changes in the source pulse profile—in the energy range 3–40 keV it becomes approximately triangular and the pulsed fraction increaseswith rising energy, reaching 60–70%in the energy range 25–40 keV.We discuss possible changes in the geometry of the accretion column consistent with similar changes in the spectra and pulse profiles.