HST NICMOS Observations of Fast Infrared Flickering in the Microquasar GRS 1915+105

Abstract

We report infrared observations of the microquasar GRS 1915+105 using the NICMOS instrument of the Hubble Space Telescope during nine visits in 2003 April-June. During epochs of high X-ray/radio activity near the beginning and end of this period, we find that the 1.87 μm infrared flux is generally low (~2 mJy) and relatively steady. However, during the X-ray/radio "plateau" state between these epochs, we find that the infrared flux is significantly higher (~4-6 mJy), and strongly variable. In particular, we find events with amplitudes ~20%-30% occurring on timescales of ~10-20 s (e-folding timescales of ~30 s). These flickering timescales are several times faster than any previously observed infrared variability in GRS 1915+105 and the IR variations exceed corresponding X-ray variations at the same (~8 s) timescale. These results suggest an entirely new type of infrared variability from this object. Based on the properties of this flickering, we conclude that it arises in the plateau-state jet outflow itself, at a distance <2.5 AU from the accretion disk. We discuss the implications of this work and the potential of further flickering observations for understanding jet formation around black holes.