Calibration of AstroSat/UVIT gratings and spectral responses

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 calibration of these gratings using observations of UV standards NGC 40 and HZ 4. We perform wavelength and flux calibration and derive effective areas for different grating orders. We find peak effective areas of ∼18.7 cm2 at 2325Å for the -1 order of NUV-Grating, ∼4.5 cm2 at 1390Å for the -2 order of FUV-Grating1, and ∼4.3 cm2 at 1500Å for the -2 order of FUV-Grating2. The FWHM spectral resolution of the FUV gratings is ≈14.6 Å in the -2 order. The -1 order of NUV grating has an FWHM resolution of ≈33Å. We find excellent agreement in flux measurements between the FUV/NUV gratings and all broadband filters. We have generated spectral response 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.