REFOS: Solar Soft X-ray Spectrophotometer on Board Nanosatellite

In this work, we describe the design and flight performance of the REFOS instrument (from Russian REntgenovskiy FOto Spektrometr; in English “X-ray Photo Spectrometer”). REFOS is a soft X-ray spectrophotometer that registers full-disk integrated solar spectra (“Sun as a star”). It operates on board the Impulse-1 nanosatellite, which was launched on 24 June 2023. REFOS has a 1.2 – 30 keV spectral range, a 0.123 keV nominal full width at half-maximum (FWHM) resolution at 5.9 keV, and a cadence of 16 s. We illustrate the instrument flight performance using the spectra of the X5.0 flare that occurred on 31 December 2023 at 21:55 UT. For this flare, REFOS registered a meaningful signal in all of its energy bins. Based on a comparison between the GOES and REFOS fluxes, we corrected the REFOS spectral sensitivity. Additionally, we assessed the quality of the calibration based on the shape of the continuum. The observed continuum allows diagnosing the plasma temperature, and the observed spectral lines allow studying abundances of the Mg, Si, S, Ar, Ca, Fe, and Ni.