MAXI J1535-571 2017 Outburst Seen by INTEGRAL/SPI and Investigating the Origin of Its Hard Tail

On 2017 September 2 MAXI J1535-571 went into outburst and peaked at similar to 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 similar to 150 keV. CompTT fits to the SPectrometer on INTEGRAL (SPI) data found electron temperatures (kT(e)) evolve from similar to 31 keV to 18 keV with a tied optical depth (tau similar to 0.85) or tau evolving from similar to 1.2-0.65 with a tied kT(e) (similar to 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.

Automated summary

This study investigates the temporal and spectral evolution of the outburst from MAXI J1535-571 using data from MAXI/GSC and INTEGRAL/SPI. The early spectra show a composite spectrum with a dominant high-energy component above 150 keV. The results suggest that these two components may originate from different locations.