Accretion geometry of the black hole binary MAXI J1820+070 probed by frequency-resolved spectroscopy

The geometry of the inner accretion flow in hard and hard-intermediate states X-ray binaries remains controversial. Using NICER observations black hole binary MAXI J1820+070 during rising phase its 2018 outburst, we study evolution timing properties, particular characteristic variability frequencies prominent iron K$\alpha$ line. frequency-resolved spectroscopy, find that reflection occurs at large distances from Comptonizing region bright state. During hard- to soft transition, properties suggest reflector moves closer source. In parallel, peak line shifts 6.5 ~7 keV, becoming consistent with expected a highly inclined disc extending close hole. We additionally significant changes dependence root-mean-square (rms) on both energy Fourier frequency as source softens. rms-energy dependence, profile, traced by spectroscopy all support picture truncated disc/inner geometry.