The Cross Spectral Time lags evolution along branches in XTE J1701-462

TL;DR

This study examines the evolution of cross spectral time lags in XTE J1701-462 across different neutron star binary states, revealing how Comptonization influences lag behavior and its relation to spectral features during an outburst.

Contribution

It provides a detailed analysis of time lag evolution along Z tracks in XTE J1701-462, linking spectral states with lag behavior and proposing a physical interpretation involving Comptonization.

Findings

Hard lags accompany HBOs and soft lags dominate low-frequency noise.

Time lags decrease along the normal branch in Cyg-like phase.

RMS amplitude of NBO increases with energy then drops at high energies.

Abstract

We investigate the cross spectrum of XTE J1701-462 in various types of Neutron Star-Low Mass X-ray Binary subclasses, during its 2006-2007 outburst. We analyze the relation between the time lags and temporal variabilities. We find that the hard time lags accompany with the horizontal branch oscillations (HBOs) and the soft time lags dominate the noise in the low frequency range 0.1-10 Hz on HB. In the Cyg-like phase, the time lags decrease on the middle normal branch (NB) from HB/NB vertex to NB/FB vertex, whereas the time lags are roughly invariant in the Sco-like source. We discuss that the Compton upscattering by the corona introduces the soft lag in low frequency noise. We interpret that the variation of the Comptonization component from the disk emission lead to the HBOs' time lags evolution along the Z tracks. We also report the rms amplitude spectrum and phase lag spectrum for the NBO. The $\sim$ $160^{\circ}$ phase lag is found. We present that the rms amplitude of both the Cyg-like and the Sco-like NBO linearly increase with the photon energy in low energy bands, and it will drop in the highest energy band.