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

TL;DR

This study analyzes the reflection spectra of MAXI J1535-571 during different states, revealing a stable inner disk radius close to the innermost stable orbit and a high black hole spin, with coronal properties evolving during state transitions.

Contribution

It provides detailed relativistic reflection modeling across multiple states, constraining the black hole spin and inner disk radius, and highlights discrepancies in disk inclination and jet orientation.

Findings

Inner radius remains close to the innermost stable circular orbit across states.

Black hole spin is constrained to approximately 0.985.

Coronal properties evolve from dense low-temperature to tenuous high-temperature during state transition.

Abstract

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 modeling the relativistic reflection in detail, we find that the inner radius $R_{\rm{in}}$ is relatively stable with $R_{\rm{in}}\lesssim 1.55 R_{\rm{g}}$ 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 $R_{\rm{in}} = R_{\rm{ISCO}}$, the spin parameter is constrained to be $0.985_{-0.004}^{+0.002}$ at 90% confidence (statistical only). The best-fitting results reveal that the inclination of the inner accretion disc is $\sim70-74$ degrees, which notably conflicts with the apparent orientation of the ballistic jet ($\leqslant$45 degrees). 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 LHS to tenuous high-temperature after the state transition, which indicates that the state transition is accompanied by the evolution of the coronal properties.