2006PASP..118...37L

The Ultraviolet/Optical Telescope (UVOT) on board Swift has the capability to provide critical insight into the physics of the early afterglows of gamma-ray bursts (GRBs). But without precise calibration of the UVOT to standard photometric systems, it is impossible to leverage late-time, ground-based follow-up data to the early-time UVOT observations. In this paper, we present a calibration of the Swift UVOT photometry to the standard Johnson UBV system for the UVOT U, B, and V filters, and a step-by-step photometry recipe for analyzing these data. We base our analysis on aperture photometry performed on the ground-based and UVOT observations of the local standard stars in the fields of supernovae (SNe) 2005am and 2005cf and a number of Landolt standard stars. We find that the optimal photometry aperture radius for UVOT data is small (2".5 for unbinned data, 3".0 for 2x2 binned data), and we show that the coincidence-loss (C-loss) correction is important even for relatively faint magnitudes (16-19 mag). Based on a theoretically motivated model, we fit the C-loss correction with two parameters, the photometric zero point (ZP) and the saturation magnitude (m_∞), and derive tight constraints for both parameters [σ(ZP)=0.01 mag and σ(m_∞)=0.02 mag)]. We find that the color-term correction is not necessary for the UVOT B and V filters but that it is necessary for the U filter for blue objects [(U-V)<0.4 mag]. We analyze the UVOT UBV photometry of SN 2005am and find that the UVOT photometry is generally consistent with the ground-based observations but that a difference of up to 0.5 mag is found when the SN became faint. We also apply our calibration results to the UVOT observations of GRB 050603. There is a scatter of ∼0.04-0.08 mag in our final UVOT photometry, the cause of which is unclear, but it may be partly due to the spatial variation in the pixel sensitivity of the UVOT detector.