Timing Analysis of V404 Cyg during Its Brightest Outburst with INTEGRAL/SPI

TL;DR

This study analyzes the timing properties of V404 Cyg during its brightest outburst in 2015 using INTEGRAL/SPI data, revealing unique variability features and similarities with other black hole transients, challenging typical spectral state expectations.

Contribution

It provides the first detailed timing analysis of V404 Cyg during its peak outburst, highlighting energy-dependent variability and proposing new interpretations within the fluctuating-propagation model.

Findings

Power spectra fit with broken power-law and multi-Lorentzian models.

V404 Cyg's high-flux power spectra lack flat-top features seen in typical hard states.

Similarity of V404 Cyg's variability with V4641 Sgr during high flux periods.

Abstract

The outburst of V404 Cyg during the summer of 2015 reached unparalleled intensities at X-ray and soft gamma-ray energies with fluxes \( > 50\) Crab in the \(20-50\) keV energy band. To date, studies in the hard X-ray/soft gamma-ray energy domain have focused primarily on the energy spectra. In this work, timing analysis has been performed with \textit{INTEGRAL}/SPI data in the \(20-300\) keV energy range for \textit{INTEGRAL} revolution 1557, which corresponds to the brightest flare of V404 Cyg (on June, 26). The power spectra are fit with broken power-law and multi-Lorentzian models and compared with previously reported results of V404 Cyg flaring activity from 1989 and 2015. Also, we took advantage of the good signal-to-noise ratio obtained above 70 keV to quantify the timing/fast-variability properties of the source as a function of energy. We then point out similarities of V404 Cyg with the black hole transient V4641 Sgr. Like V4641 Sgr, we found the power spectra of V404 Cyg during high flux periods did not possess the expected flat-top feature typically seen in a hard spectral state. Interpretations are proposed in the framework of the fluctuating-propagation model to explain the observed properties.