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Spectral Variability of a Soft-intermediate State QPO from MAXI J1820+070

Black Hole X-ray binaries are known to exhibit variability in their light curves on timescales as short as milliseconds and as long as months. The short-timescale variability can be in the form of quasi-periodic oscillations (QPOs), which may be produced by general relativistic effects. In this work, we looked at low-frequency QPOs from an exciting recent black hole transient, MAXI J1820+070, which first went into outburst in 2018. This source was observed in a multi-wavelength campaign that included the NICER mission, a soft X-ray telescope attached to the International Space Station. In our analysis, we applied X-ray timing and spectral-timing techniques to help place constraints on the QPO emission mechanism in the extreme environment surrounding the black hole. Although the QPO amplitude was too low to carry out phase-resolved spectroscopy, our work indicates that weak QPOs such as this one may be present in the light curves of many sources.