Friday, July 5, 2013

1307.1226 (Jeong-Sook Kim et al.)

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

Jeong-Sook Kim, Soon-Wook Kim, Tomoharu Kurayama, Mareki Honma, Tetsuo Sasao, Sang Joon Kim
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.
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