Vlbi Observation of Microquasar Cyg X-3 During an X-Ray State Transition From Soft to Hard in the 2007 May-June Flare

Abstract

We present a radio observation of microquasar Cyg X-3 during an X-ray state transition from ultrasoft to hard state in the 2007 May-June flare using the VLBI Exploration of Radio Astrometry at 22 GHz. During the transition, a short-lived mini-flare of 3 hr was detected prior to the major flare. In such a transition, a jet ejection is believed to occur, but there have been no direct observations to support it. An analysis of Gaussian fits to the observed visibility amplitudes shows a time variation of the source axis, or a structural change, during the mini-flare. Our model fits, together with other multiwavelength observations in the radio, soft, and hard X-rays, and the shock-in-jet models for other flaring activities at GHz wavebands, suggest a high possibility of synchrotron flares during the mini-flare, indicative of a predominant contribution from jet activity. Therefore, the mini-flare with an associated structural change is indicative of a jet ejection event in the state transition from ultrasoft to hard state.

Single-dish and VLBI observations of Cygnus X-3 during the 2016 giant flare episode

“…The short-duration flare occurred close to the core, with an evidence of a very slight extension of the wind or jet. Similar small radio flares have often been observed prior to giant flare events of Cyg X-3 (Waltman et al 1994(Waltman et al , 1995Newell et al 1998;Kim et al 2013). In particular, a VLBI observation performed in 2007 during a short-live flare (3 hours) with a flux density of 1.6 Jy gives similar results.…”

“…In particular, a VLBI observation performed in 2007 during a short-live flare (3 hours) with a flux density of 1.6 Jy gives similar results. The analysis of the Gaussian fits to the visibility amplitudes with time bin of 10 min have shown an increase of the source size or a structural change during the mini-flare (Kim et al 2013). In both cases, we note that the emitting size slightly increases from the peak of the mini-flare until the end of the flare.…”

The innermost jet in the hidden ultra-luminous X-ray source Cygnus X-3

“…The angular sizes of the major and minor axes in Table 1 are significantly larger than the prediction (∼0.7 mas) from the scattering broadening model (Mioduszewski et al 2001) and also larger than the minimum size of ∼0.7 mas observed by Molnar et al (1988), Kim et al (2013) and Egron et al (2017) at the same observing frequency during outbursts. Therefore, the jet base of Cyg X-3 is intrinsically resolved not only in the radial direction, but also in the transverse direction.…”

“…In the literature, there were a few reports of minor flares resulting from transient ejection activity on timescales up to a few hours (e.g. Molnar et al 1988;Newell et al 1998;Kim et al 2013;Egron et al 2017). Future EVN observations with longer on-source time and more stations (Sardinia 64-m radio telescope, Robledo 70-m radio telescope, Torun 32-m radio telescope and the enhanced Multi Element Remotely Linked Interferometer Network) would significantly improve the (𝑢, 𝑣) coverage, in particular on short baselines, and thus help to accurately measure its flux density and search for potential small structure changes on short timescales.…”

“…Frequently, it undergoes bright outbursts with peak flux densities reaching ∼20 Jy (e.g. Gregory et al 1972;Waltman et al 1994;Tudose et al 2010;Kim et al 2013;Egron et al 2017;Kim & Kim 2020;Broderick et al 2021;Spencer et al 2022). During some giant outbursts, high-resolution observations revealed episodic (mildly) relativistic or curved jets roughly in the north-south direction (e.g.…”