Timing and Spectral Analysis of 4U 1626-67 Using AstroSat/LAXPC

Abstract We present a comprehensive analysis of AstroSat/LAXPC data of the second spin-up and second spin-down phases of the persistent X-ray pulsar 4U 1626-67. Flares followed by a broad dip are detected in the spin-up observations. The pulse profiles changed from a shoulder-like structure to a broad sinusoidal shape as the source underwent a torque reversal from spin-up to spin-down. Energy-resolved pulse profiles in lower energies showed a double-horned profile in the spin-up state and a flat top with multiple peaks in the spin-down state. Regardless of the torque state, the pulse profiles exhibit a broad single-peaked shape at higher energies. The observation in the spin-down era is characterized by the presence of a prominent QPO at 46.5 ± 1.0 mHz frequency. The QPO rms and center frequency show a correlation with energy. Spin-up and spin-down states show a difference in the shape of the power density spectrum. After the torque reversal, a gradual flux drop and the hardening of the spectra were observed. The difference in the shape of the pulse profiles and the presence and absence of QPOs can be explained by the change in accretion flow geometry of the pulsar, from pencil-beam to fan-beam, between spin-down and spin-up states.