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

Abstract

Context. Spectral analysis of X-ray emission from ejecta in supernova remnants (SNRs) is hampered by the low spectral resolution of CCD detectors, which typically creates a degeneracy between the best-fit values of chemical abundances and the plasma emission measure. The combined contribution of shocked ambient medium and ejecta to the emerging X-ray emission further complicates the determination of the ejecta mass and chemical composition. This degeneracy leads to big uncertainties in mass estimates and can introduce a bias in the comparison between the ejecta chemical composition derived from the observations and the yields predicted by explosive nucleosynthesis models. Aims. We explore the capabilities of present and future spectral instruments with the aim of identifying a spectral feature that may allow us to discriminate between metal-rich and pure-metal plasmas in X-ray spectra of SNRs. Methods. 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, plasma temperatures, and ionization parameters. We then synthesized X-ray spectra from a state-of-the-art 3D hydrodynamic simulation of Cas A, by using the response matrix from the Chandra ACIS-S charged-coupled device detector and that of the XRISM/Resolve X-ray calorimeter spectrometer. Results. 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 actually identify this ejecta signature. We tested and 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. Conclusions. 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, opening a new window on the link between progenitor star, supernova and SNRs.

Chemical Abundances in Sgr A East: Evidence for a Type Iax Supernova Remnant

“…Pure-Fe ejecta have been found in the young SNR CassiopeiaA (Hwang & Laming 2012). It has also been speculated that pure metal exists in the middle-age SNR W49B (Vink 2012;Zhou & Vink 2018;Greco et al 2020). Similar to Sgr A East, W49B is also an Fe-rich, mixed-morphology SNR interacting with the dense ambient medium (e.g., Keohane et al 2007;Miceli et al 2010;Zhu et al 2014).…”

“…They obtained significantly larger IGE masses (M(Fe)=0.3±0.1 M e ). Moreover, the existence of pure ejecta would significantly enhance the metal masses with the same EM and cause extra uncertainties (Park et al 2005;Greco et al 2020). Considering the difficulty of obtaining accurate measurements of the metal masses using a global spectrum and current low-energy-resolution Figure 7.…”

“…We adopted a 20% systematic error for the ratios from Martínez-Rodríguez et al 2020, which have very small statistical errors. CCD spectra (Greco et al 2020), a good way is to lean on the abundance/mass ratios, which are less affected by the geometry and pure-ejecta assumptions.…”

“…The degeneracy between the best-fit abundances and emission measure leads to big uncertainties in mass estimates. Recently, Greco et al (2020) pointed out that a bright radiative recombining continuum shows up when the plasma is made of pure-metal ejecta. This introduces further difficulties in distinguishing recombining plasma and pure ejecta plasma using the current CCD detectors.…”

Three-dimensional morphological asymmetries in the ejecta of Cassiopeia A using a component separation method in X-rays

“…Picquenot et al: Morphological asymmetries in the ejecta of Cassiopeia A in X-rays At the spectral resolution of CCD type instruments, most emission lines are not resolved and the observed emission is a blurred complex of lines. The centroid energy of emission lines can shift either via Doppler effect or when the ionization timescale increases and the ions distribution in a given line complex evolves (Dewey 2010; Greco, Emanuele et al 2020). In Fig .…”