The Evolution of LMC X‐4 Flares: Evidence for Super‐Eddington Radiation Oozing through Inhomogeneous Polar Cap Accretion Flows?

We present the results of two extensive Rossi X-ray Timing Explorerobservations of large X-ray flaring episodes from the high-mass X-ray binarypulsar LMC X-4. Light curves during the flaring episodes comprise bright peaksembedded in relatively fainter regions, with complex patterns of recurrence andclustering of flares. We identify precursors preceding the flaring activity.Pulse profiles during the flares appear to be simple sinusoids, and pulsedfractions are proportional to the flare intensities. We fit Gaussian functionsto flare peaks to estimate the mean full-width-half-maximum to be $\sim$68 s.Significant rapid aperiodic variability exists up to a few hertz during theflares, which is related to the appearance of narrow, spiky peaks in the lightcurves. While spectral fits and softness ratios show overall spectral softeningas the flare intensity increases, the narrow, spiky peaks do not follow thistrend. The mean fluence of the flare peaks is (3.1 $\pm$ 2.9) $\times$10$^{40}$ ergs in the 2.5--25 keV energy range, with its maximum at $\sim$1.9$\times$ 10$^{41}$ ergs. The flare peak luminosity reaches up to (2.1 $\pm$0.2) $\times$ 10$^{39}$ ergs s$^{-1}$, far above the Eddington luminosity of aneutron star. We discuss possible origins of the flares, and we also proposethat inhomogeneous accretion columns onto the neutron star polar caps areresponsible for the observed properties.Comment: 39 pages (including figures and tables), accepted for publication in Ap