Anti-Correlation of Near-Infrared and X-Ray Variations of the Microquasar GRS 1915+105 in the Soft State

We present detailed, long term near-infrared (NIR) light curves of GRS 1915+105 obtained in 2007--2008, covering its long ``soft state'' for the first time. From our NIR monitoring and X-ray data of the All Sky Monitor onboard Rossi X-ray Timing Explorer, we discovered that the NIR flux dropped by >1 mag during short X-ray flares with a time-scale of days. Upon termination of the soft state, the H - K_s color reddened and the anti-correlation pattern was broken. The observed H - K_s color variation suggests that the dominant NIR source was an accretion disk during the soft state. The short X-ray flares during the soft state were associated with spectral hardening in X-rays and increasing radio emission, indicating jet ejection. The temporal NIR fading during the X-ray flares thus implies a sudden decrease in the contribution of the accretion disk when a jet is ejected.