Quasi-periodic variability in X-ray Pulsars

X-ray pulsars are composed of rapidly rotating, highly magnetized neutron stars powered by accretion. Apart from the coherent modulations due to rotation, they also show significant aperiodic and quasi-periodic variability. I will present results from the quasi-periodic variability studies from the transient X-ray pulsar RX J0440.9+4431 and persistent X-ray pulsar 4U 1626-67. In RX J0440.9+4431, a quasi-periodic oscillation (QPO) was detected at 60 mHz, which evolved to 42 mHz at lower luminosity. The QPO rms were found to be energy dependent with an overall increasing trend with energy. For the first time, we found the QPO frequency varying with photon energy in an X-ray pulsar, which poses a challenge in explaining the QPO with current models such as the Keplarian and beat frequency model. Hence, more physically motivated models are required to understand the physical mechanism behind the mHz QPOs. In 4U 1626-67, a prominent quasi-periodic oscillation (QPO) feature at 46 mHz exclusive to the spin-down torque state was observed with ~20% rms amplitude, which was positively correlated with energy. We also report the detection of sidebands to QPO occurring at a beat frequency of 83 mHz with 8% r.m.s. amplitude, having >3 sigma detection significance. We explore various mechanisms that could possibly explain the presence of this QPO exclusively during the spin-down state.