Calibrating the Swift/UVOT to Standard Optical Photometry with Observations of Landolt Standards and Supernova 2005am

The Ultraviolet-Optical Telescope (UVOT) onboard 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 an empirical determination of the photometric zero points and the optimal photometric parameters to analyze U-, B-, and V-band images obtained with Swift/UVOT. We base our analysis on aperture photometry performed on the local standard stars in the field of Supernova (SN) 2005am, of which UVOT has a series of images and we have ground-based calibrations and follow-up observations. We achieve a consistency of about 0.05 mag between the UVOT photometry and our ground-based calibration when the photometry aperture radius is small (2′′.5 for unbinned data, 3 ′′.0 for 2 × 2 binned data), and when a correction factor is applied to the measured instrumental magnitudes. Our