High Energy Emission of V404 Cygni during 2015 outburst with INTEGRAL/SPI: Spectral analysis issues and solutions

A strong outburst of the X-ray transient V404 Cygni (= GS2023-338) was observed in 2015 June/July up to a level of 50 Crab in the hard X-ray domain. At this level of photon flux, an instrument's behavior may be severely tested and some instrumental artifacts could affect the data analysis. We are interested in the SPI instrument aboard the INTEGRAL mission and have performed thorough checks to ensure a correct handling of the data. By analyzing the observations throughout the outburst, we have observed that the high energy domain (above 500 keV) sometimes exhibits unexpected features which are worth careful examination. Spurious triggers are known to affect the MeV region and we suspected that this phenomenon could be accentuated by the huge photon flux. We have investigated this issue, specifically during high flux periods and actually found that artificial high energy bumps may appear with the current standard analysis procedure. However, if the specific selection events usually used in the 650-2200 keV energy is applied down to 450 keV, the spurious noise and the associated spectral features are removed. We present how to obtain reliable spectral results on the high energy emission of V404 Cyg at extreme flux levels and demonstrate that with the correct configuration, the hard X-ray emission, up to a few MeV, is modeled by a two component model as observed in Cyg X-1 and for V404 Cygni itself at lower flux levels.