A challenging view of the 2015 summer V404 Cyg outburst at high energy with INTEGRAL/SPI: The finale

TL;DR

This study analyzes the 2015 V404 Cyg outburst using INTEGRAL/SPI data, revealing complex spectral variability and evidence for two independent emission components at high energies.

Contribution

It provides a detailed spectral variability analysis during the outburst, identifying two distinct emission components and their independent behaviors, which advances understanding of high-energy processes in such transients.

Findings

Identification of two spectral components: below and above 100 keV.

Observation of flux and hardness anticorrelation at flare end.

Demonstration of organized source evolution through Hardness-Intensity diagrams.

Abstract

During its strong outburst of 2015 June/July, the X-ray transient V404 Cygni (= GS2023+338) was observed up to a level of 50 Crab in the hard X-ray domain. We focus here on a particularly intense episode preceeding a definitive decline of the source activity. We benefit from large signal-to-noise ratios to investigate the source spectral variability on a timescale of 5 minutes. A Hardness-Intensity study of three broad bands reveals clearly different behaviors at low and high energy (below and above around 100 keV). In particular, on two occasions, the source intensity varies by a factor of 3-4 in amplitude while keeping the same spectral shape. On the other hand, at the end of the major flare, the emission presents a clear anticorrelation between flux and hardness. These behaviors strongly suggest the presence of two spectral components related to emission processes varying in a largely independent way. The first component (E < 100-150 keV) is classically identified with a Comptonizing thermal electron population and requires either an unusual seed photon population or a specific geometry with strong absorbing/reflecting material. The second component is modeled by a cutoff power-law, which could correspond to a second hotter Comptonizing population or another mechanism (synchrotron, non-thermal Comptonization...). In the framework of such a model, Hardness-Intensity and Flux-Flux diagrams clearly demonstrate that the source evolution follows a well organized underlying scheme. They reveal unique information about the Hard X-ray emission processes and connections between them.