First INTEGRAL observations of V404 Cygni during the 2015 outburst : spectral behavior in the 20 - 650 keV energy range

TL;DR

This paper reports on INTEGRAL's first observations of V404 Cygni during its 2015 outburst, analyzing its spectral behavior in the 20-650 keV range, revealing complex variability and potential jet contributions.

Contribution

It provides the first detailed spectral analysis of V404 Cygni during the 2015 outburst, highlighting the presence of multiple spectral components and proposing models involving Comptonization and jet emission.

Findings

Source exhibited extreme variability and high flux levels.

Spectral data consistent with a two-component model including Comptonization and jet emission.

No detection of annihilation line, setting upper limits.

Abstract

In June 2015, the source V404 Cygni (= GS2023+38) underwent an extraordinary outburst. We present the results obtained during the first revolution dedicated to this target by the INTEGRAL mission, and focus on the spectral behavior in the hard X-ray domain, using both SPI and IBIS instruments. The source exhibits extreme variability, and reaches fluxes of several tens of Crab. However, the emission between 20 and 650 keV can be understood in terms of two main components, varying on all the observable timescales, similar to what is observed in the persistent black hole system Cyg X-1. The low energy component (up to ~ 200 keV) presents a rather unusual shape, probably due to the intrinsic source variability. Nonetheless, a satisfactory description is obtained with a Comptonization model, if an unusually hot population of seed photons ($kT_0$ ~ 7 keV) is introduced. Above this first component, a clear excess extending up to 400-600 keV leads us to investigate a scenario where an additional (cutoff) power law could correspond to the contribution of the jet synchrotron emission, as proposed in Cyg X-1. A search for an annihilation feature did not provide any firm detection, with an upper limit of 2 x $10^{-4} ph/cm^2 s$ (2 \sigma) for a narrow line centered at 511 keV, on the averaged obtained spectrum.