Determining the Orbital Period and Wind Geometry in GRO J1655-40

During the course of its 2005 outburst, the black hole X-ray binary GRO J1655-40 launched an accretion disk wind associated with deep X-ray absorption lines and strong Compton scattering. Little is known about this apparently super-Eddington wind, but previous works have discovered optical/infrared (OIR) emission from the wind that varies on the orbital period-a possible clue to its origin and geometry. However, there is significant uncertainty in the orbital phases, and a more precise value of the orbital period is needed to accurately phase fold the wind emission. We present our analysis of the I-band photometry from observations taken with the Small and Medium Aperture Research Telescope System 1.3 m telescope between 2006 and 2016. We have implemented two methods-data-driven and model-based-to determine the orbital period, which we report as 2.62193 +/- 0.00002 days from the data-driven method and 2.621928 +/- 0.000004 days from the model-based method, a significant (25x) increase in precision over prior measurements. We discuss the possible existence of a period derivative, implications of a peculiar deep minimum in the outburst lightcurve of the system, and connections between OIR variability and the geometry of the super-Eddington wind.