Optical state changes in the X-ray-quiescent black hole A0620-00

We present optical and infrared photometry of the soft X-ray transient A0620-00, obtained by the SMARTS and YALO consortia, spanning the period 1999-2007. Although A0620-00 was X- ray quiescent throughout this period, our data show three distinct optical states, characterized by magnitude, color, and aperiodic variability. In particular, we find that in what we call the "passive" state, A0620-00 exhibits no observable aperiodic variability on any timescale longer than our exposure length. The existence of these states may explain the differences between phased light curves observed by previous authors. We suggest that in order to obtain the purest ellipsoidal light curves, passive-state data should be used in future studies of ellipsoidal variability.