2022ApJ...935...25R

On 2017 September 2 MAXI J1535-571 went into outburst and peaked at ∼5 Crab in the 2-20 keV energy range. Early in the flare, the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) performed target of opportunity pointings and monitored the source as it transitioned from the hard state to the soft state. Using quasi-simultaneous observations from MAXI/GSC and INTEGRAL/SPI, we studied the temporal and spectral evolution of MAXI J1535-571 in the 2-500 keV range. Early spectra show a Comptonized spectrum and a high-energy component dominant above ∼150 keV. CompTT fits to the SPectrometer on INTEGRAL (SPI) data found electron temperatures (kT_e ) evolve from ∼31 keV to 18 keV with a tied optical depth (τ ∼ 0.85) or τ evolving from ∼1.2-0.65 with a tied kT_e (∼24 keV). To investigate the nature of the high-energy component, we performed a spectral decomposition of the 100-400 keV energy band. The CompTT flux varies significantly during the hard state while the high-energy component flux is consistent with a constant flux. This result suggests that the two components originate from different locations, which favors a jet origin interpretation for the high-energy component over a hybrid corona interpretation. Lastly, two short rebrightenings during the hard-to-soft transition are compared to similar events reported in MAXI J1820+070.