VLBI Observation of Microquasar Cyg X-3 during an X-ray State Transition from Soft to Hard in the 2007 May-June Flare

TL;DR

This study reports VLBI radio observations of the microquasar Cyg X-3 during an X-ray state transition, revealing a mini-flare and structural changes that suggest a jet ejection event during the transition from ultrasoft to hard state.

Contribution

First direct VLBI evidence of a jet ejection during an X-ray state transition in Cyg X-3, supported by multiwavelength data and shock-in-jet models.

Findings

Detected a short-lived mini-flare prior to the major flare.

Observed structural changes indicating a jet ejection event.

Supported by multiwavelength observations and shock-in-jet models.

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 (VERA) at 22 GHz. During the transition, a short-lived mini-flare of ${\lesssim}3$ 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.