XMM-Newton and NICER Measurement of the Rms Spectrum of the Millihertz Quasiperiodic Oscillations in the Neutron-star Low-mass X-Ray Binary 4U 1636–53

Abstract

We used two XMM-Newton and six Neutron Star Interior Composition Explorer observations to investigate the fractional rms amplitude of the millihertz quasiperiodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary 4U 1636?53. We studied, for the first time, the fractional rms amplitude of the mHz QPOs versus energy in 4U 1636?53 down to 0.2 keV. We find that, as the energy increases from ∼0.2 to ∼3 keV, the rms amplitude of the mHz QPOs increases, different from the decreasing trend that has been previously observed above 3 keV. This finding has not yet been predicted by any current theoretical model; however, it provides an important observational feature to speculate whether a newly discovered mHz oscillation originates from the marginally stable nuclear burning process on the neutron-star surface.