The inner flow geometry in MAXI J1820+070 during hard and hard-intermediate states

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[Abridged] Context: We present a systematic X-ray spectral-timing study of the recently discovered, exceptionally bright black hole X-ray binary system MAXI J1820+070.

• Our analysis focuses on the first part of the 2018 outburst, covering the rise throughout the hard state, the bright hard and hard-intermediate states, and the transition to the soft-intermediate state.
• Aims: We address the issue of constraining the geometry of the innermost accretion flow and its evolution throughout an outburst.
• Methods: We employed two independent X-ray spectral-timing methods applied to the NICER data of MAXI J1820+070.