MAXI J1820+070: A rapidly rotating black hole with a mildly truncated disk and warm corona layers in the soft state

Context. MAXI J1820+070 is a black hole X-ray binary that was discovered in 2018 during a major outburst that triggered extensive multiwavelength observations. Subsequent studies have focused on its physical properties, particularly the black hole spin. In this context, continuum fitting of the source spectra below 25 keV favors low-spin values, whereas spin estimates based on the relativistic precession model and reflection spectroscopy suggest a rapidly rotating black hole. Aims. Our study seeks to address the debate over the spin of MAXI J1820+070 through broadband spectral modeling of NICER/NuSTAR observations obtained during the soft state. We further compare our results with previous spin estimates and examine the source variability. Finally, we investigate the origin of the soft X-ray excess, which we argue does not originate from the plunging region as previously suggested. Methods. We performed a joint spectral fitting of NICER and NuSTAR data over the broadest possible energy range of 0.2 79 . To achieve high accuracy in our spin estimation, we adopted a physically motivated model combination. We employed to describe the relativistic thin accretion disk, utilizing pre-calculated tables for the color-correction factor, convolved with to model the high-energy Comptonized tail. In addition, we modified our model to account for the substantial excess emission reported below 10 keV and conducted an MCMC analysis, thereby improving the statistical robustness of our fits. To further investigate the origin of the excess emission, we calculated spin-dependent radial disk temperature profiles across all epochs. -- keV kerrbb2 bhspec thcomp Results. In this work, we show that the soft spectral state of MAXI J1820+070 is well described by the Comptonized emission from a thin relativistic accretion disk and two blackbody spectral components with kT_ bb ≈ 0.5 and kT_ keV bb ≈ 0.2 , respectively. Our results indicate that the black hole in MAXI J1820+070 is rapidly spinning (a > 0.8), potentially powering the observed relativistic jets. Our analysis reveals a significant decline in the inner disk temperature midway through the soft state, accompanied by a modest increase in the inferred inner disk radius up to 3R_ keV g . This behavior is consistent with a small disk truncation, a phenomenon recently predicted by GRMHD simulations of the plunging region. The two blackbody components are considerably cooler than the inner disk, corresponding to large emission regions of Δ R ≈ 6R_ g (75 (440 and ∼ 40R_ km) and Δ R ≈ 35R_ g km). This suggests that this emission may originate from warm corona layers located beyond ∼ 10R_ g g , in close analogy to the warm Comptonization models proposed to explain the soft X-ray excess in active galactic nuclei.