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

TL;DR

This study analyzes Suzaku and RXTE data of LMC X-4, revealing spectral and pulse profile variations over its superorbital cycle, and suggests a precessing disk reprocessing hard X-rays to produce the soft component.

Contribution

It provides an improved measurement of the superorbital period and detailed spectral and timing analysis, proposing a precessing disk model for the observed variations.

Findings

Superorbital period measured as 30.32 days.

Spectral modeling shows a power-law with cutoff and a blackbody component.

Pulse profiles exhibit phase shifts between soft and hard X-rays.

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/ASM 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 Gamma ~ 0.6 and a high-energy cutoff at ~ 25 keV, a blackbody with kT_BB ~ 0.18 keV, and emission lines from Fe K_alpha, O VIII, and Ne IX (X Lyalpha). Assuming a distance of 50 kpc, The source has luminosity L_X ~ 3 x 10^38 ergs s^-1 in the 2--50 keV band, and the luminosity of the soft (blackbody) component is L_BB ~ 1.5 x 10^37 ergs 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.