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

Akira Arai, Makoto Uemura, Mahito Sasada, Sergei A. Trushkin, Yoshihiro Ueda, Hiromitsu Takahashi, Koji S. Kawabata, Masayuki Yamanaka, Osamu Nagae, Yuki Ikejiri, Kiyoshi Sakimoto, Risako Matsui, Takashi Ohsugi, Takuya Yamashita, Mizuki Isogai, Yasushi Fukazawa, Tsunefumi Mizuno, Hideaki Katagiri, Kiichi Okita, Michitoshi Yoshida, Kenshi Yanagisawa, Shuji Sato, Masaru Kino, Kozo Sadakane

Published 2009-04-06Version 1

We report detailed, long term near-infrared (NIR) light curves of GRS 1915+105 in 2007-2008, covering its long "soft state" for the first time. From our NIR monitoring and the X-ray data of the All Sky Monitor (ASM) onboard Rossi X-ray Timing Explorer (RXTE), we discovered that the NIR flux dropped by > 1 mag during short X-ray flares with a time-scale of days. With the termination of the soft state, the H-Ks color reddened and the anti-correlation pattern was broken. The observed H-Ks 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, hence, implies a sudden decrease of the contribution of the accretion disk when the jet is ejected.