NICMOS Observations of the Transiting Hot Jupiter XO-1b

We refine the physical parameters of the transiting hot Jupiter planet XO-1b and its stellar host XO-1 using Hubble Space Telescope (HST) NICMOS observations. XO-1b has a radius R_p = 1.21 ± 0.03 R_J , and XO-1 has a radius R _sstarf = 0.94 ± 0.02 R _sun, where the uncertainty in the mass of XO-1 dominates the uncertainty of R_p and R _sstarf. There are no significant differences in the XO-1 system properties between these broadband NIR observations and previous determinations based upon ground-based optical observations. We measure two transit timings from these observations with 9 s and 15 s precision. As a residual to a linear ephemeris model, there is a 2.0σ timing difference between the two HST visits that are separated by three transit events (11.8 days). These two transit timings and additional timings from the literature are sufficient to rule out the presence of an Earth mass planet orbiting in 2:1 mean motion resonance coplanar with XO-1b. We identify and correct for poorly understood "gain-like" variations present in NICMOS time series data. This correction reduces the effective noise in time series photometry by a factor of 2 for the case of XO-1.