An Updated Ultraviolet Calibration for the Swift/UVOT
  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. 
A. A. Breeveld, W. Landsman, S. T. Holland, P. Roming, N. P. M. Kuin,   M. J. Page
astro-ph.IM