The giant radio flare of Cygnus X-3 in September 2016

TL;DR

This paper reports the discovery and analysis of a giant radio flare from Cygnus X-3 in September 2016, linking it to X-ray state transitions and internal shock wave models, with detailed spectral and temporal characterization.

Contribution

It presents the first prediction of a giant flare based on X-ray and radio flux behavior, and models the flare's rise with exponential growth linked to relativistic electron generation.

Findings

Giant flare occurred after 2000 days of quiescence.

Radio flux increased from 0.01 to 15 Jy during the flare.

Flare rise fitted by an exponential law indicating shock wave acceleration.

Abstract

In the long-term multi-frequency monitoring program of the microquasars with RATAN-600 we discovered the giant flare from the X-ray binary Cygnus X-3 on 13 September 2016. It happened after 2000 days of the 'quiescent state' of the source passed after the former giant flare (~18 Jy) in March 2011. We have found that during this quiet period the hard X-ray flux (Swift/BAT, 15-50 keV) and radio flux (RATAN-600, 11 GHz) have been strongly anti-correlated. Both radio flares occurred after transitions of the microquasar to a 'hypersoft' X-ray state that occurred in February 2011 and in the end of August 2016. The giant flare was predicted by us in the first ATel #9416. Indeed after dramatic decrease of the hard X-ray Swift 15-50 keV flux and RATAN 4-11 GHz fluxes (a 'quenched state') a small flare (0.7 Jy at 4-11 GHz) developed on MJD 57632 and then on MJD 57644.5 almost simultaneously with X-rays radio flux rose from 0.01 to 15 Jy at 4.6 GHz during few days. The rise of the flaring flux is well fitted by a exponential law that could be a initial phase of the relativistic electrons generation by internal shock waves in the jets. Initially spectra were optically thick at frequencies lower 2 GHz and optically thin at frequencies higher 8 GHz with typical spectral index about -0.5. After maximum of the flare radio fluxes at all frequencies faded out with the exponential law (~ exp(-(t-t0)/2d) for 21.7 GHz).