Empirical extinction coefficients for the Swift-UVOT optical-through-ultraviolet passbands

Abstract: We calculated empirical reddening and extinction coefficients with respect to the dust reddening map of Schlegel et al. for the Swift-UVOT passbands, using the 'star pair' method and photometric data from the UVOT Serendipitous Source Catalogue and spectroscopic data from LAMOST Data Release 7 and 2MASS. The reddening coefficients for the UVW2-UVM2, UVM2-UVW1, UVW1-U, U-B, and B-V colors are -1.39, 2.08, 0.78, 0.72, and 0.84, respectively.The extinction coefficients for the UVW2, UVM2, UVW1, U, B, and V bands are 5.60, 6.99, 4.91, 4.13, 3.41 and 2.57, respectively. The numbers are consistent with predictions by the Fitzpatrick's extinction law of R(V)=3.0. Temperature-dependent variations of the coefficients are found and discussed, particularly for the ultraviolet passbands. We recommend using the new reddening and extinction coefficients in future when dereddening the Swift-UVOT data.