Spectral Cross-Calibrations of ASO-S/HXI with Other X-ray Missions

Cross-calibration of X-ray instruments is crucial for stereoscopic studies of solar hard X-ray directivity and three-dimensional morphology, where spectral and imaging differences observed from multiple perspectives provide critical constraints. We present the preliminary results of the spectral calibrations of ASO-S/HXI through internal consistency checks of its three total flux detectors and cross-calibrations with Solar Orbiter/STIX, Fermi/GBM, and Konus-Wind data. Both calibration tests suggest that the response matrix of one of the HXI total flux monitors, D94 (with a thin aluminum window), needs to be modified. The new response matrix is determined by testing the series of response matrices with changing effective aluminum thickness through simulations. Post-correction analysis reveals $\sim 88\%$ of co-observed events demonstrate electron spectral index consistency within $\pm 0.5$ , while comparisons of the total counts above 30 keV show < 10% differences for STIX-HXI flares with separation angles < 20° and Fermi-observed moderate flares. Though challenges persist regarding detector response uncertainties, the pile-up effect, and other nonideal factors, the achieved inter-instrument consistency enables quantitative electron anisotropy investigations through joint spectral analysis.