Revised wavelength and spectral response calibrations for AKARI near-infrared grism spectroscopy: Cryogenic phase

摘要

We perform revised spectral calibrations for the AKARI near-infrared grism to correct quantitatively for the effect of the wavelength-dependent refractive index. The near-infrared grism covering the wavelength range of 2.5-5.0 mu m, with a spectral resolving power of 120 at 3.6 mu m, is found to be contaminated by second-order light at wavelengths longer than 4.9 mu m, which is especially serious for red objects. First, we present the wavelength calibration considering the refractive index of the grism as a function of the wavelength for the first time. We find that the previous solution is positively shifted by up to 0.01 mu m compared with the revised wavelengths at 2.5-5.0 mu m. In addition, we demonstrate that second-order contamination occurs even with a perfect order-sorting filter owing to the wavelength dependence of the refractive index. Secondly, the spectral responses of the system from the first-and second-order light are simultaneously obtained from two types of standard objects with different colors. The response from the second-order light suggests leakage of the order-sorting filter below 2.5 mu m. The relations between the output of the detector and the intensities of the first- and second-order light are formalized by a matrix equation that combines the two orders. The removal of the contaminating second-order light can be achieved by solving the matrix equation. The new calibration extends the available spectral coverage of the grism mode from 4.9 mu m up to 5.0 mu m. The revision can be used to study spectral features falling in these extended wavelengths, e. g., the carbon monoxide fundamental ro-vibrational absorption within nearby active galactic nuclei.