Analysis of the reflection spectra of MAXI J1535-571 in the hard and intermediate states

We report results on the joint-fit of the NuSTAR and HXMT data for the black hole X-ray binary candidate MAXI J1535-571. The observations were obtained in 2017 when the source evolved through the hard, hard-intermediate, and soft-intermediate states over the rising phase of the outburst. After subtracting continuum components, X-ray reflection signatures are clearly showed in those observations. By modelling the relativistic reflection in detail, we find that the inner radius R-in is relatively stable with R-in less than or similar to 1.55R(g) during the three states, which implies that the inner radius likely extends to the innermost stable circular orbit even in the bright hard state. When adopting R-in = R-ISCO, the spin parameter is constrained to be 0.985(-0.004)(+0.002) at 90 per cent confidence (statistical only). The best-fitting results reveal that the inclination of the inner accretion disc is similar to 70-74 deg, which notably conflicts with the apparent orientation of the ballistic jet (<= 45 deg). In addition, both the photon index and the electron temperature increase during the transition from hard to soft state. It seems that the corona evolves from dense low-temperature in the low/hard state to tenuous high-temperature after the state transition, which indicates that the state transition is accompanied by the evolution of the coronal properties.