Multi-colour optical photometry of V404 Cygni in outburst

TL;DR

This study presents multi-colour optical photometry of V404 Cygni during its 2015 outburst, revealing rapid variability, correlated colour changes, and time lags, suggesting non-thermal emission processes similar to other microquasars.

Contribution

First broad-band photometric analysis of V404 Cygni during its outburst using small telescopes, providing detailed variability and colour behaviour insights.

Findings

Detected rapid optical variability with flares every 10-30 minutes.

Observed correlated colour variations in the optical bands.

Identified time lags of up to a fraction of a minute between filters.

Abstract

Context. This observational paper has been prepared in the context of the large multi-wavelength effort by many observers with the aim of following up the transient flaring event of V404 Cygni that took place for several weeks in 2015 June. Aims. Our main original aim was to contribute to the study of this transient source by acquiring broad-band photometric observations during its most active flaring phases. Nevertheless, after a detailed analysis of the data, several interesting results were obtained that encouraged a dedicated publication. Methods. The methodology used was based on broad-band differential CCD photometry. This outburst of V404 Cygni rendered the source a very bright target easily within reach of small educational telescopes. Therefore, the 41 cm telescope available at the Astronomical Observatory of the University of Ja\'en was used in this work. Results. We detected variability at different time scales, both in amplitude and colour. Individual optical flares appear every half hour on average during our 3 h long observation, although large-amplitude (\sim 1 mag) variations are also observed to occur on intervals as short as 10 minutes. Also, colour variations appear to be highly correlated in a colour-colour diagram. Another remarkable finding is the detection of time lag, from about one to a fraction of a minute between light curves in different filters (VRcIc). Conclusions. The observed behaviour is tentatively interpreted in an scenario based on the ejection of non-thermal emitting, relativistic plasmons, with their synchrotron spectra extending up to optical wavelengths. This would render some of the V404 Cygni flares very similar to those of the well-know microquasar GRS 1915+105