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

Abstract

Spectral analysis of X-ray emission from ejecta in supernova remnants (SNRs) is hampered by the low spectral resolution of CCD cameras, which creates a degeneracy between the best-fit values of abundances and emission measure. The combined contribution of shocked ambient medium and ejecta to the X-ray emission complicates the determination of the ejecta mass and chemical composition, leading to big uncertainties in mass estimates and it can introduce a bias in the comparison between the observed ejecta composition and the yields predicted by explosive nucleosynthesis. We explore the capabilities of present and future spectral instruments with the aim of identifying a spectral feature which may allow us to discriminate between metal-rich and pure-metal plasmas in X-ray spectra of SNRs. We studied the behavior of the most common X-ray emission processes of an optically thin plasma in the high-abundance regime. We investigated spectral features of bremsstrahlung, radiative recombination continua (RRC) and line emission, by exploring a wide range of chemical abundances, temperatures and ionization parameters. We synthesized X-ray spectra from a 3D hydrodynamic (HD) simulation of Cas A, by using the response matrix from the Chandra/ACIS-S CCD detector and that of the XRISM/Resolve X-ray calorimeter. We found that a bright RRC shows up when the plasma is made of pure-metal ejecta, and a high spectral resolution is needed to identify this ejecta signature. We verified the applicability of our novel diagnostic tool and we propose a promising target for the future detection of such spectral feature: the southeastern Fe-rich clump of Cas A. While there is no 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 and to recover correctly absolute mass and the chemical composition of the ejecta.

arXiv comment13 pages, 12 figures, accepted for publication in A&A

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• X-ray polarization detection of Cassiopeia A with IXPE
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• Three-dimensional morphological asymmetries in the ejecta of Cassiopeia A using a component separation method in X-rays
• 3D modeling from the onset of the SN to the full-fledged SNR: Role of an initial ejecta anisotropy on matter mixing
• Evidence for thermal X-ray emission from the synchrotron dominated shocks in Tycho's supernova remnant
• A global view of shocked plasma in the supernova remnant Puppis A provided by SRG/eROSITA
• X-ray emitting structures in the Vela SNR: ejecta anisotropies and progenitor stellar wind residuals
• Bayesian insights in Tycho supernova remnant : a detailed mapping of ejecta properties
• Spatially resolved X-ray spectra of the Galactic SNR G18.95-1.1: SRG/eROSITA view