Photometric Calibration of the Swift Ultraviolet/Optical Telescope

TL;DR

This paper details the comprehensive photometric calibration of the Swift UVOT, including effective area, zero points, and response, ensuring accurate UV/optical measurements for diverse star types.

Contribution

It provides an updated, detailed calibration framework for the Swift UVOT, improving accuracy over previous calibrations and covering a wide spectral range.

Findings

Photometric zero points accurate to better than 4%

Calibration covers 1600-8000 Å spectral range

Guidelines for handling coincidence loss in data analysis

Abstract

We present the photometric calibration of the Swift UltraViolet/Optical Telescope (UVOT) which includes: optimum photometric and background apertures, effective area curves, colour transformations, conversion factors for count rates to flux, and the photometric zero points (which are accurate to better than 4 per cent) for each of the seven UVOT broadband filters. The calibration was performed with observations of standard stars and standard star fields that represent a wide range of spectral star types. The calibration results include the position dependent uniformity, and instrument response over the 1600-8000A operational range. Because the UVOT is a photon counting instrument, we also discuss the effect of coincidence loss on the calibration results. We provide practical guidelines for using the calibration in UVOT data analysis. The results presented here supersede previous calibration results.