Hung-En Hsieh(IANCU); Yi Chou(IANCU); Yi-Hao Su(IANCU); Ka-Ho Tse(IANCU); Fang-Ming Gu(IANCU)
We present the phase-resolved spectroscopy based on Hilbert-Huang Transform (HHT) for millihertz quasi-periodic oscillations (mHz QPOs) in 4 U 1636-536. This ~8mHz QPO can be detected about several thousand seconds before type-I X-rat burst. It was interpreted as marginally stable burning on neutron-star surface. We used the HHT to extract the QPOs instantaneous phases, and constructed its phase-resolved spectra for whole cycle. The spectral parameter modulations show the neutron star surface temperature is likely positively correlated with the variation of neutron star luminosity.