An Updated Ultraviolet Calibration for the Swift∕UVOT

Abstract

We present an updated calibration of the Swift/UVOT broadband ultraviolet (uvw1, uvm2, and uvw2) filters. The new calibration accounts for the ∼1% per year decline in the UVOT sensitivity observed in all filters, and makes use of additional calibration sources with a wider range of colours and with HST spectrophotometry. In this paper we present the new effective area curves and instrumental photometric zeropoints and compare with the previous calibration.