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Spectral analysis of X-ray emission from ejecta in supernova remnants (SNRs)is hampered by the low spectral resolution of CCD cameras, which creates adegeneracy between the best-fit values of abundances and emission measure. Thecombined contribution of shocked ambient medium and ejecta to the X-rayemission complicates the determination of the ejecta mass and chemicalcomposition, leading to big uncertainties in mass estimates and it canintroduce a bias in the comparison between the observed ejecta composition andthe yields predicted by explosive nucleosynthesis. We explore the capabilitiesof present and future spectral instruments with the aim of identifying aspectral feature which may allow us to discriminate between metal-rich andpure-metal plasmas in X-ray spectra of SNRs. We studied the behavior of themost common X-ray emission processes of an optically thin plasma in thehigh-abundance regime. We investigated spectral features of bremsstrahlung,radiative recombination continua (RRC) and line emission, by exploring a widerange of chemical abundances, temperatures and ionization parameters. Wesynthesized X-ray spectra from a 3D hydrodynamic (HD) simulation of Cas A, byusing the response matrix from the Chandra/ACIS-S CCD detector and that of theXRISM/Resolve X-ray calorimeter. We found that a bright RRC shows up when theplasma is made of pure-metal ejecta, and a high spectral resolution is neededto identify this ejecta signature. We verified the applicability of our noveldiagnostic tool and we propose a promising target for the future detection ofsuch spectral feature: the southeastern Fe-rich clump of Cas A. While there isno way to unambiguously reveal pure-metal ejecta emission with CCD detectors,X-ray calorimeters will be able to pinpoint the presence of pure-metal RRC andto recover correctly absolute mass and the chemical composition of the ejecta.

Unveiling pure-metal ejecta X-ray emission in supernova remnants through their radiative recombination continuum