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

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 $L_{\mathrm{x}}\sim (2-4)\times 10^{39}$ 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.

arXiv comment13 pages, 10 figures, accepted for publication in Astronomy Letters

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• Switches between accretion structures during flares in 4U 1901+03
• Discovery of pulsation dropout and turn-on during the high state of the accreting X-ray pulsar LMC X-4
• Disk precession to explain the super-orbital modulation of LMC X-4: results from the Swift monitoring campaign
• A Striking Confluence Between Theory and Observations of High-Mass X-ray Binary Pulsars
• Probing accretion dynamics and spin evolution in the X-ray pulsar RX J0520.5-6932 during its 2024 Outburst