Rapid infrared flares in GRS 1915+105: evidence for infrared synchrotron emission

We report imaging photometry of the radio jet black hole candidate source GRS 1915+105 in the infrared K band. The observations reveal rapid infrared flare events on time-scales of less than an hour. These events are strikingly similar to those regularly observed in radio monitoring at 15 GHz. Furthermore, when dereddened, the infrared events have comparable amplitudes to the radio oscillations, and observations at 15 GHz made ~8 h after our infrared observations reveal that the source was indeed displaying radio oscillations at this time. We suggest that we have observed infrared synchrotron emission from this source. We estimate the equipartition magnetic field and power required to accelerate the particles for the repeated radio events, and find both to be orders of magnitude greater than those estimated for any other X-ray binary.