Origins of the Long-term Variability of the Near-infrared Emission of the Black Hole X-Ray Binary GRS 1915+105 in the X-Ray Low Luminous State

Since 2018 July, the black hole X-ray binary GRS 1915+105 has been in an X-ray low luminous state (XLLS), which is the faintest period ever observed in a soft X-ray band. We present here the results of near-infrared (NIR) observations from 2019 April–December in the XLLS using HONIR attached to the Kanata telescope, together with radio data obtained by the Yamaguchi Interferometer and archival X-ray data. We found the historical NIR brightening in the XLLS. For several tens of days in the XLLS, NIR flux showed a positive correlation with radio flux and negative correlation with X-ray flux. This result suggests that at least a jet contributes to the NIR bands in the NIR-bright XLLS period. An accretion disk is not likely a candidate for the NIR emission because of the decrease of X-ray flux. In contrast to the NIR band, there was no historical radio brightening in the XLLS. This implies that the NIR emission in the NIR-bright period cannot be explained by the jet alone. The X-ray obscuration suggested by X-ray data is another possible candidate for the nonjet emission component in the NIR band.