Flaring activity in 2008 VERA observation and classification of intraday radio flares in microquasar Cyg X-3

Abstract

We present the intraday flaring activity of Cyg X-3 in the rise phase of a major flare observed in November-December 2008 at 22 GHz with the VLBI Exploration of Radio Astrometry (VERA). The two consecutive intraday flares with a time scale of ~ 3 - 4 hours are similar in their light curve morphologies. The intraday flares observed in 2008 are compared to other interferometric observations of intraday flares in Cyg X-3, as well as to those observed in GRS 1915+105. The intraday flares observed in Cyg X-3 can be classified into three types: flares with a fast rise and exponential decay (type A), flares with an almost equal time scale in the rise and decay (type B), and asymmetric flares with a slow rise and rapid decay (type C). Such types of intraday flares have also been observed in another representative microquasar GRS 1915+105, and are, therefore, a common characteristic in both microquasars. The long-term multiwavelengh light curves in 2007 and 2008 are similar except during a few-day-long transition from an unltrasoft to harder states. In the transition state, the intraday flares of type A were observed in the 2008 VERA observation whereas, in the 2007 VERA observation, a type C flare was observed, opposite to the flares observed in 2008. The 2007 observation is the only observation of a type C flare in Cyg X-3. A possible cause of the different types of intraday flares is briefly discussed in the context of the jet propagation process.