Mid-infrared observations of GRS 1915+105 during plateau and flaring states

We present mid-infrared (4-18 (mu)m) observations of the microquasar GRS 1915+105 obtained with ISOCAM, the camera on board the Infrared Space Observatory (ISO), in 1996 April and 1997 October. The first observation probably occurred during a flaring event with oscillating synchrotron emission. The 1997 observation occurred a few days before a major relativistic ejection, during a plateau state of inverted-spectrum radio emission and hard quasi-stable X-ray emission. The K-M giant donor star in GRS 1915+105 cannot account for the mid-IR emission and we discuss the possible additional components depending on two absorption laws. Thermal emission from dust seems unlikely. The flat mid-IR spectrum obtained during the plateau state is likely to be synchrotron emission. It would be the first evidence of the infrared extension of the radio synchrotron emission from the compact jets, although optically thin free-free emission from an X-ray driven-wind from the accretion disc cannot be excluded.