Evolution of the complicated decaying branch of the very long outburst in XTE J1701-462

Abstract

I analyze the evolution of the features which occurred on the timescale of weeks in the very long (∼600d) X-ray outburst of the neutron star low-mass X-ray binary XTE J1701-462. I used RXTE/ASM (1.5–12keV) and Swift/BAT (15–50keV) observations for a time-series analysis of the long-term variations. I utilized the means (usually one-day) of intensity I to increase the signal-to-noise ratio. I applied weighted wavelet Z-transform to investigate the cyclic modulation of I. Using the combined data from both satellites, I also calculated the hardness ratio HR=I(15–50keV)/I(1.5–12keV) and investigated its evolution throughout the outburst. Despite the largely different profiles of the light curves in the ASM and the BAT bands especially in the first fourth of the outburst, the profiles of the measures of their scatter (the σresASM and the σresBAT curves) display mutual similarities. Both the σresASM and σresBAT curves achieved their maxima in the primary peak of the light curve which was present only in the ASM band. The formation of the quasi-periodic variations occurred and their approximate cycle-length PC was decided already in the surrounding of this peak when the bolometric luminosity was close to or even exceeding the Eddington limit. The modulation with PC is present in some time segments only in the ASM band, while the intensity variations are always chaotic in the BAT band. Using the results of spectroscopy of Lin et al., I suggests that only multicolor disk, dominant in the ASM band, was involved in this cycle. The existence of this cycle did not depend on the exact type of the Z, so it was independent of the exact conditions in the inner disk region. The length of PC displayed a large and gradual increase from ∼16 to ∼26days with the progress of the outburst. Using models of Foulkes et al., I interpret these variations as a superorbital cycle, caused by tilting and warping of the irradiated disk.