We studied the harmonics of the millihertz quasi-periodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary 4U 1636-53 using the Rossi X-ray Timing Explorer observations. We detected the harmonics of the mHz QPOs in 73 data intervals, with most of them in the transitional spectra state. We found that the ratio between the rms amplitude of the harmonic and that of the fundamental remains constant in a wide range of the fundamental frequency. More importantly, we studied, for the first time, the rms amplitude of the harmonics vs. energy in 4U 1636-53 in the 2-5 keV range. We found that the rms amplitude of both the harmonic and the fundamental shows a decreasing trend as the energy increases, which is different from the behaviors reported in QPOs in certain black hole systems. Furthermore, our results suggest that not all observations with mHz QPOs have the harmonic component, although the reason behind this is still unclear.
XMM-Newton and NICER measurement of the rms spectrum of the millihertz quasi-periodic oscillations in the neutron-star low-mass X-ray binary 4U 1636-53
A Phase-resolved View of Millihertz Quasi-periodic Oscillations in the Ultraluminous X-ray Source M51 ULX-7: Evidence for a Magnetically Truncated Disk and Geometrical Beaming
Persistence of the Millihertz X-ray Quasi-Periodic Oscillation in the Active Galactic Nucleus 1ES 1927+654
MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology（麻省理工学院）；Black Hole Initiative at Harvard University（哈佛大学黑洞倡议）；JILA, University of Colorado and National Institute of Standards and Technology（科罗拉多大学与美国国家标准与技术研究院）；Department of Astrophysical and Planetary Sciences, University of Colorado（科罗拉多大学）；Institute of Astronomy, University of Cambridge（剑桥大学天文学研究所）；Centro de Astrobiología (CAB), CSIC-INTA（西班牙国家航空航天技术研究所与西班牙国家科学研究委员会天体生物学中心）；School of Mathematics, Statistics, and Physics, Newcastle（纽卡斯尔大学）
Discovery of a correlation between the frequency of the mHz quasi-periodic oscillations and the neutron-star temperature in the low-mass X-ray binary 4U 1636-53
Broadband Properties of the Harmonic of Type-C Quasi-periodic Oscillation in MAXI J1348-630
Jinggangshan University（井冈山大学）；Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）；SHAO-XMU Joint Center for Astrophysics（上夏天体物理联合中心）；Xiamen University（厦门大学）
