Suzaku X-ray Spectra and Pulse Profile Variations During the Superorbital Cycle of LMC X-4

Abstract

We present results from spectral and temporal analyses of Suzaku and RXTE observations of the high-mass X-ray binary LMC X-4. Using the full 13 years of available RXTE/all-sky monitor data, we apply the ANOVA and Lomb Normalized Periodogram methods to obtain an improved superorbital period measurement of 30.32 ± 0.04 days. The phase-averaged X-ray spectra from Suzaku observations during the high state of the superorbital period can be modeled in the 0.6-50 keV band as the combination of a power law with Γ ~ 0.6 and a high-energy cutoff at ~25 keV, a blackbody with kT _BB ~ 0.18 keV, and emission lines from Fe K_α, O VIII, and Ne IX (X Lyα). Assuming a distance of 50 kpc, the source has luminosity L _X ~ 3 × 10³⁸ erg s^-1 in the 2-50 keV band, and the luminosity of the soft (blackbody) component is L _BB ~ 1.5 × 10³⁷ erg s^-1. The energy-resolved pulse profiles show single-peaked soft (0.5-1 keV) and hard (6-10 keV) pulses but a more complex pattern of medium (2-10 keV) pulses; cross-correlation of the hard with the soft pulses shows a phase shift that varies between observations. We interpret these results in terms of a picture in which a precessing disk reprocesses the hard X-rays and produces the observed soft spectral component, as has been suggested for the similar sources Her X-1 and SMC X-1.