Extract cleaned Swift/UVOT UV grism spectra with uvotpy package

TL;DR

This paper presents a method based on the uvotpy package to remove second order contamination from Swift UVOT UV grism spectra, significantly increasing the usable wavelength range for studying ultraviolet/optical transients.

Contribution

We developed a new contamination removal technique for UVOT UV grism spectra that extends the valid wavelength range from 33% to about 70-80%.

Findings

Removes second order contamination up to 4000Å with 11.2% systematic uncertainty.

Extends the valid spectral range to about 70% of the full wavelength coverage.

Source code is publicly available on GitHub.

Abstract

The ultraviolet/optical telescope (UVOT) onboard the Neil Gehrels Swift Observatory is capable of imaging with 7 lenticular filters and of taking slitless spectra with 2 grisms. Both image and grism data have been widely used to study gamma-ray bursts, supernovae and other ultraviolet/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 ultraviolet (UV) grism spectra. This, in turn, reduces the valid wavelength range to only about 33% of the total. However, 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 package, we propose a method to remove the second order contamination from UV grism spectra (nominal mode) up to about 4000\AA, i.e., about 70% of the full wavelength range. The 1-sigma systematic uncertainty of this method is about 11.2%. In addition, if a source is red enough, the red end of the valid range could reach about 5000\AA. The source code is available on GitHub.