Infrared observations of an energetic outburst in GRS 1915+105

In the course of an intense X-ray and radio outburst of GRS 1915+105, we observed a pair of radio-emitting clouds emerging from the compact core in opposite directions at relativistic speeds. At near-infrared wavelengths we observed the time-delayed reverberation of this radio flare/ejection event. Five days after the radio outburst, the source became redder as it brightened by ∼1 mag in K(2.2 μm), which suggests the appearance of a warm dust component. The enhanced infrared emission was close to 10% of the X-ray luminosity of the source and amounts to about 0.1% of the typical kinetic energy in the bulk motion of the relativistic ejecta in GRS 1915+105.