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

TL;DR

This study analyzes the 2017 outburst of MAXI J1535-571 using INTEGRAL/SPI and MAXI/GSC data, revealing a high-energy component likely originating from a jet, and examining spectral evolution during state transitions.

Contribution

It provides the first detailed spectral analysis of MAXI J1535-571's outburst, identifying a distinct high-energy component and its likely jet origin, along with spectral evolution insights.

Findings

High-energy component remains constant during the hard state.

Electron temperature decreases from ~31 keV to 18 keV during the outburst.

Spectral decomposition suggests different origins for the high-energy component and the Comptonized emission.

Abstract

On 2 September 2017 MAXI J1535-571 went into outburst and peaked at ~5 Crab in the 2-20 keV energy range. Early in the flare 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 SPI data found electron temperatures (kTe) evolves from ~31 keV to 18 keV with a tied optical depth (tau ~ 0.85) or tau evolving from ~1.2-0.65 with a tied kTe (~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.