2025ApJS..277...27Z

The Ultraviolet/Optical Telescope (UVOT) on board the Neil Gehrels Swift Observatory is capable of imaging with seven lenticular filters and of taking slitless spectra with two grisms. Both image and grism data have been widely used to study gamma-ray bursts (GRBs), supernovae, and other ultraviolet (UV)/optical transients, and proved UVOT is a powerful instrument in time-domain astronomy. However, the second-order contamination, for blue sources, strongly limits the red end of UV grism spectra. This, in turn, reduces the valid wavelength range to only about 33% of the total. To explore the broadband spectral energy distribution of GRBs at the early stage, a larger valid wavelength range is required. Hence, based on the uvotpy (https://github.com/PaulKuin/uvotpy) package, we propose a method to remove the second-order contamination from UV grism spectra (nominal mode) up to ∼4000 Å, i.e., about 70% of the full wavelength range. The 1σ systematic uncertainty of this method is ∼11.2%. In addition, if a source is red enough, the red end of the valid range could reach ∼5000 Å. The source code is available on GitHub at https://github.com/HaoZhou0810/cluvotpy.