X-ray monitoring of the Galactic microquasar V4641 Sgr

V4641 Sgr is the black hole (BH) candidate which exhibits a giant outburst with an X-ray intensity up to 12 Crab on September 1999. Since then, several small outbursts with 1-100 mCrab level have been observed almost every year. However, it remains unclear whether this source has typical spectral states such low/hard and high/soft states seen in BH candidates and what X-ray emission mechanisms are. Large-scale radio jets have also benn observed from this source, hence it is very important source to study the relation between jets and accretion flows. Since 2008, we have monitored this source in an outburst with MAXI/GSC and Swift/XRT, and carried out detailed observations with Suzaku and NuSTAR in 2014. The observed X-ray flux ranges up to 40 mCrab. The spectra of this source look like high/soft state, and can be well explained by multi-color disk model with an innermost temperature of 1.0-1.5 keV, but the innermost radii, 8-21% of Schwarzchild radii, are too small to explain its emission region and does not keep constant. We also detected iron emission lines at 6.8 keV due to highly ionized irons from Swift/XRT, NuSTAR, and Suzaku spectra, and first revealed the presence of photo-ionized plasma in this source. In this paper, we will discuss the origin of X-ray continuum and iron emission lines, and similarity to the peculiar microquasr V404 Cyg.