X-ray spectral evolution of V404 Cygni in the initial phase of the 2015 outburst

TL;DR

This study analyzes the spectral evolution of V404 Cygni during its 2015 outburst, revealing complex variability patterns and suggesting a jet-related origin for seed photons in the hard X-ray emission.

Contribution

It provides detailed spectral analysis of V404 Cygni's outburst, proposing a jet-related source for seed photons and characterizing the variability of hard X-ray emission.

Findings

Hard spectra during bright phases consistent with thermal Comptonization.

Seed photon temperature (~7 keV) too high for disk blackbody emission.

Correlation between hard X-ray variability and softer flux, indicating intrinsic source variability.

Abstract

The black hole binary GS 2023+338 exhibited an unprecedently bright outburst on June 2015. Since June 17th, the high energy instruments on board INTEGRAL detected an extremely variable emission during both bright and low luminosity phases, with dramatic variations of the hardness ratio on time scales of ~seconds. The analysis of the IBIS and SPI data reveals the presence of hard spectra in the brightest phases, compatible with thermal Comptonization with temperature kTe ~ 40 keV. The seed photons temperature is best fit by kT0 ~ 7 keV, that is too high to be compatible with blackbody emission from the disk. This result is consistent with the seed photons being provided by a different source, that we hypothesize to be a synchrotron driven component in the jet. During the brightest phase of flares, the hardness shows a complex pattern of correlation with flux, with a maximum energy released in the range 40-100 keV. The hard X-ray variability for E > 50 keV is correlated with flux variations in the softer band, showing that the overall source variability cannot originate entirely from absorption, but at least part of it is due to the central accreting source.