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MAXI J1803−298, a newly-discovered Galactic transient, and black hole candidate was first detected by MAXI /GSC on 2021 May 01. In this paper, we present a detailed spectral analysis of MAXI J1803−298. Utilizing the X-ray reflection fitting method, we perform a joint fit to the spectra of MAXI J1803−298, respectively, observed by NuSTAR and NICER /XTI on the same day over the energy range between 0.7–79.0 keV in SIMS state, and the observed inner radius of the accretion disc is demonstrated to have extended to the ISCO radius (R in =1.08 R ISCO). Implementing the relativistic reflection model relxillCp , we found its spin (and the inclination angle i) can be constrained to be close to an extreme value, 0.991 (i ∼ 70 °), at 68 per cent confidence interval. The results suggest that MAXI J1803−298 may be a fast-rotating black hole with a large inclination angle. [ABSTRACT FROM AUTHOR]

We present a detailed spectral analysis of the black hole candidate MAXI J1836−194. The source was caught in the intermediate state during its 2011 outburst by Suzaku and RXTE. We jointly fit the X-ray data from these two missions using the relxill model to study the reflection component, and a steep inner emissivity profile indicating a compact corona as the primary source is required in order to achieve a good fit. In addition, a reflection model with a lamp-post configuration (relxilllp), which is normally invoked to explain the steep emissivity profile, gives a worse fit and is excluded at 99 per cent confidence level compared to relxill. We also explore the effect of the ionization gradient on the emissivity profile by fitting the data with two relativistic reflection components, and it is found that the inner emissivity flattens. These results may indicate that the ionization state of the disc is not constant. All the models above require a supersolar iron abundance higher than ∼4.5. However, we find that the high-density version of reflionx can describe the same spectra even with solar iron abundance well. A moderate rotating black hole (a * = 0.84–0.94) is consistently obtained by our models, which is in agreement with previously reported values. [ABSTRACT FROM AUTHOR]