Cross-Calibration of Chandrayaan-2 XSM with INSPIRESat-1 DAXSS and GOES-16 XRS
X-ray spectroscopic observations of the solar corona and flares provide crucial diagnostics of plasma properties and are essential for understanding the physical processes responsible for coronal heating and solar eruptive activity. The Chandrayaan-2 Solar X-ray Monitor (XSM) provides disk-integrated spectra of the Sun in the 1--15~keV soft X-ray band, enabling modeling of the thermal X-ray emission from the corona across quiet phases to intense solar flares. XSM has been operational for about seven years, starting from the last solar minimum and covering the maximum of the current Solar Cycle. The Dual-zone Aperture X-ray Solar Spectrometer (DAXSS) instrument on board INSPIRESat-1 covers the solar X-ray spectra in a similar energy range as XSM and was operational during 2022--2026. With multiple instruments simultaneously observing the Sun in X-rays, there is scope to compare measurements across instruments. Here, we present the cross-calibration of XSM with DAXSS and a broadband X-ray flux monitor, the GOES-16 X-ray Sensor (XRS). Comparisons of XSM and DAXSS spectra reveal an unaccounted attenuation in the XSM low-energy response. Supported by laboratory measurements, we attribute this difference to the effective detector beryllium window thickness being 25 microns rather than the previously assumed 8 microns. Incorporating this revision into the XSM calibration significantly improves the agreement between the two instruments, with flux measurements agreeing within ~10% in the 1--8 Angstrom band. Comparison with GOES-16 XRS measurements over a broad range of solar activity levels further demonstrates consistency, with a median flux difference of less than 10%.