Calibration of the Swift-UVOT ultraviolet and visible grisms

Abstract

We present the calibration of the Swift Ultraviolet and Optical Telescope (UVOT) grisms, of which there are two, providing low-resolution field spectroscopy in the ultraviolet and optical bands, respectively. The UV grism covers the range lambda 1700-5000 angstrom with a spectral resolution (lambda/Delta lambda) of 75 at lambda 2600 angstrom for source magnitudes of u= 10-16 mag, while the visible grism covers the range lambda 2850-6600 angstrom with a spectral resolution of 100 at lambda 4000 angstrom for source magnitudes of b=12-17 mag. This calibration extends over all detector positions, for all modes used during operations. The wavelength accuracy (1 sigma) is 9 angstrom in the UV grism clocked mode, 17 angstrom in the UV grism nominal mode and 22 angstrom in the visible grism. The range below lambda 2740 angstrom in the UV grism and lambda 5200 angstrom in the visible grism never suffers from overlapping by higher spectral orders. The flux calibration of the grisms includes a correction we developed for coincidence loss in the detector. The error in the coincidence loss correction is less than 20 per cent. The position of the spectrum on the detector only affects the effective area (sensitivity) by a fewper cent in the nominal modes, but varies substantially in the clocked modes. The error in the effective area is from 9 per cent in the UV grism clocked mode to 15 per cent in the visible grism clocked mode