The SPIRou wavelength calibration for precise radial velocities in the near infrared

Abstract

SPIRou is a near-infrared (nIR) spectropolarimeter at the CFHT, covering the YJHK nIR spectral bands ($980-2350\,\mathrm{nm}$). We describe the development and current status of the SPIRou wavelength calibration in order to obtain precise radial velocities (RVs) in the nIR. We make use of a UNe hollow-cathode lamp and a Fabry-P\'erot \'etalon to calibrate the pixel-wavelength correspondence for SPIRou. Different methods are developed for identifying the hollow-cathode lines, for calibrating the wavelength dependence of the Fabry-P\'erot cavity width, and for combining the two calibrators. The hollow-cathode spectra alone do not provide a sufficiently accurate wavelength solution to meet the design requirements of an internal error of $\mathrm{<0.45\,m\,s^{-1}}$, for an overall RV precision of $\mathrm{1\,m\,s^{-1}}$. However, the combination with the Fabry-P\'erot spectra allows for significant improvements, leading to an internal error of $\mathrm{\sim 0.15\,m\,s^{-1}}$. We examine the inter-night stability, intra-night stability, and impact on the stellar RVs of the wavelength solution.