Jeong-Sook Kim — Page 2

arXiv2013

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

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.

astro-ph.HE↗

arXiv2013

Evolution of the Water Maser Expanding Shell in W75N VLA 2

We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz H$_2$O masers in the high-mass star-forming region of \objectname{W75N}, carried out with VLBI Exploration of Radio Astrometry (VERA) for three-epochs in 2007 with an angular resolution of $\sim$ 1 mas. We detected H$_2$O maser emission toward the radio jet in VLA 1 and the expanding shell-like structure in VLA 2. We have made elliptical fits to the VLA 2 H$_2$O maser shell-like structure observed in the different epochs (1999, 2005, and 2007), and found that the shell is still expanding eight years after its discovery. From the difference in the size of the semi-major axes of the fitted ellipses in the epochs 1999 ($\simeq$ 71$\pm$1 mas), 2005 ($\simeq$ 97$\pm$3 mas), and 2007 ($\simeq$ 111$\pm$1 mas), we estimate an average expanding velocity of $\sim$ 5 mas yr$^{-1}$, similar to the proper motions measured in the individual H$_2$O maser features. A kinematic age of $\sim$ 20 yr is derived for this structure. In addition, our VERA observations indicate an increase in the ellipticity of the expanding shell around VLA 2 from epochs 1999 to 2007. In fact, the elliptical fit of the VERA data shows a ratio between the minor and major axes of $\sim$ 0.6, in contrast with a most circular shape for the shell detected in 1999 and 2005 (b/a $\sim$ 0.9). This suggests that we are probably observing the formation of a jet-driven H$_2$O maser structure in VLA2, evolving from a non-collimated pulsed-outflow event during the first stages of evolution of a massive young stellar object (YSO). This may support predictions made earlier by other authors on this issue, consistent with recent magnetohydrodynamical simulations. We discuss possible implications of our results in the study of the first stages of evolution of massive YSOs.

astro-ph.GA↗