Euclid Quick Data Release (Q1). From spectrograms to spectra: The SIR spectroscopic processing function

The Euclid space mission aims to investigate the nature of dark energy and dark matter by mapping the large-scale structure of the Universe. A key component of Euclid's observational strategy is slitless spectroscopy, which is conducted using the Near Infrared Spectrometer and Photometer (NISP). This technique enables the acquisition of large-scale spectroscopic data without the need for targeted apertures, thus allowing for precise redshift measurements of millions of galaxies. These data are essential for Euclid's core science objectives, including the study of cosmic acceleration and the evolution of galaxy clustering, and will enable many non-cosmological investigations. This study presents the SIR processing function, which is responsible for processing slitless spectroscopic data from Euclid's NISP instrument. The objective is to generate fully calibrated science-grade one-dimensional spectra in order to ensure high-quality spectroscopic data for cosmological or astrophysical analyses. The processing function relies on a source catalogue generated from photometric data, effectively corrects detector effects, subtracts cross-contaminations, minimises self-contamination, calibrates wavelength and flux, and produces reliable spectra for later scientific use. The first Quick Data Release (Q1) of Euclid's spectroscopic data provides approximately three million validated spectra for sources observed in the red-grism mode from a selected portion of the Euclid Wide Survey. We find that the wavelength accuracy and measured resolving power are within top-level mission requirements, thanks to the excellent optical quality of the instrument. The SIR processing function represents a significant step in processing slitless spectroscopic data for the Euclid mission. As the survey progresses, continued refinements and additional features will enhance its capabilities, thus further supporting high-precision cosmological and astrophysical measurements.