Calibration of AstroSat/UVIT Gratings and Spectral Responses

Abstract: AstroSat/UVIT carries two gratings in the FUV channel and a single grating in the NUV channel. These gratings are useful for low resolution, slitless spectroscopy in the far and near UV bands of a variety of cosmic sources such as hot stars, interacting binaries, active galactic nuclei, etc. We present the calibration of these gratings using observations of UV standards NGC40 and HZ4. We perform wavelength and flux calibration and derive effective areas for different grating orders. We find peak effective areas of 18.7cm^2 at 2325 Angstrom for the -1 order of NUV-Grating, 4.5cm^2 at 1390 Angstrom for the -2 order of FUV-Grating1, and 4.3cm^2 at 1500 Angstrom for the -2 order of FUV-Grating2. The FWHM spectral resolution of the FUV gratings is 14.6 Angstrom in the -2 order. The -1 order of NUV grating has an FWHM resolution of 33 Angstrom. We find excellent agreement in flux measurements between the FUV/NUV gratings and all broadband filters. We have generated spectral responses of the UVIT gratings and broadband filters that can directly be used in the spectral fitting packages such as XSPEC, Sherpa, and ISIS, thus allowing spectral analysis of UVIT data either separately or jointly with X-ray data from AstroSat or other missions.