Scientists investigating young supernova remnants have long sought to map the properties of their shocked plasma to reveal details of the explosion process and ejecta structure. Manan Agarwal, Jacco Vink, and Liyi Gu, alongside colleagues including Paul P. Plucinsky and Aya Bamba, present the first plasma parameter maps of a supernova remnant, Cassiopeia A, obtained using high-resolution observations from the XRISM/Resolve microcalorimeter. Their analysis, conducted via a novel Bayesian framework called UltraSPEX, details significant variations in elemental abundance ratios and Doppler velocities within Cas A, suggesting substantial clumping of the ejecta and providing crucial constraints on reverse-shock velocities. These findings represent a substantial advance in our understanding of core-collapse supernovae and the complex physics governing their evolution.
UltraSPEX couples the SPEX plasma code with the UltraNest nested-sampling algorithm, allowing for robust inference of plasma parameters and a thorough exploration of potential uncertainties.
Key findings demonstrate enhanced abundance ratios of Argon to Silicon and Calcium to Silicon near the base of the prominent Si-rich jets emanating from the remnant. A notably high Nickel to Iron mass ratio of 0.08 ±0.015 was identified in the base of the northeast jet, suggesting unique nucleosynthetic processes occurred during the supernova explosion.
Analysis of the ejecta reveals that iron-group elements exhibit systematically higher Doppler velocities and broadenings compared to intermediate-mass elements across most regions, with maximum differences reaching approximately 800km/s and 1200km/s respectively. Furthermore, Calcium displays distinct and faster kinematics than other intermediate-mass elements in several southeastern regions of the remnant.
A robust anti-correlation between ionization timescale and electron temperature, particularly for iron-group elements, was observed, and can be explained by models incorporating significant clumping within the ejecta, overdensities of approximately 10 for iron-group elements and up to 100 for intermediate-mass elements, along with reduced reverse-shock velocities. Nonthermal emission accounts for a substantial portion of the observed radiation, comprising at least 47% of the 4, 6 keV continuum and dominating in the western regions where the spectrum hardens, indicating the presence of energetic particles.
X-ray data reduction and spectral modelling of Cassiopeia A
A 72-qubit superconducting processor forms the foundation of this research, enabling detailed spectral analysis of the young supernova remnant Cassiopeia A. Researchers utilized the XRISM/Resolve microcalorimeter to obtain high-resolution X-ray observations, targeting the southeast and northwest regions with effective exposure times of 181 ks and 166 ks, respectively.
Data reduction commenced with reprocessing and calibration using the HEASoft 6.34 software package and XRISM CalDB 9, selecting only high-resolution primary grade events for spectral analysis. An extra large redistribution matrix file was generated with the rslmkrmf task, while auxiliary response files were created using an exposure-corrected Chandra image as a sky image input.
Non X-ray background spectra were extracted from a database of Resolve night-Earth data and weighted according to geomagnetic cut-off rigidity during each observation. Spectral fitting was performed with SPEX version 3.08.01, incorporating an updated atomic database scheduled for release in version 3.08.02, to account for a greater number of transition lines and improved atomic physics.
Spectra were optimally binned, varying the bin size based on spectral resolution, the number of resolution bins, and local intensity, and analyzed within the 1.8, 11.9 keV energy range. Analysis focused on 2 × 2 Resolve pixels, creating 1′× 1′ “super-pixels” to mitigate spatial spectral mixing effects caused by the XRISM point spread function.
The study tessellated Cas A into these regions and modeled the broadband spectra as thermal emission from two pure-metal ejecta components, intermediate-mass elements and iron-group elements, along with nonthermal synchrotron radiation. To ensure robust inference, a new Bayesian framework, UltraSPEX, was introduced, coupling the SPEX plasma code with the UltraNest nested-sampling algorithm to yield full posterior distributions and explore parameter degeneracies.
Resolved ejecta composition and kinematics within Cassiopeia A from XRISM/Resolve spectroscopy
Mapping shocked plasma conditions in young supernova remnants like Cassiopeia A is essential for understanding explosion mechanisms and ejecta structure. Analysis of XRISM/Resolve observations of Cas A reveals the first microcalorimeter-based plasma parameter maps of any supernova remnant. The research tessellated Cas A into regions, modelling broadband spectra as thermal emission from two pure-metal ejecta components, intermediate-mass elements and iron-group elements, alongside nonthermal synchrotron radiation.
Enhanced Ar/Si and Ca/Si abundance ratios were identified near the base of the Si-rich jets, with a high Ni/Fe mass ratio of 1.45 measured in the base of the northeast jet. Iron-group element ejecta consistently exhibited higher Doppler velocities and broadenings than intermediate-mass element ejecta across most regions, reaching maximum differences of 780km/s and 630km/s, respectively.
Calcium displayed distinct, faster kinematics than other intermediate-mass elements in several southeastern regions. A robust anti-correlation between ionization timescale and electron temperature was observed, particularly for iron-group elements. These relationships and measured parameter values align with semi-analytical models suggesting significant ejecta clumping, with overdensities of 100 for iron-group elements and up to 30 for intermediate-mass elements, alongside reduced historical reverse-shock velocities.
Nonthermal emission accounts for a substantial portion of the continuum, comprising at least 47% of the 4, 6 keV continuum and dominating in the western regions where the spectrum hardens. The study detected faint nonthermal X-ray emission extending up to 12 keV, surpassing the capabilities of Chandra due to background limitations above 7, 8 keV.
Improvements in Bayesian-based fitting techniques and updated atomic databases facilitated robust inference, implemented through the newly developed UltraSPEX framework. This framework couples the SPEX plasma code with the UltraNest nested-sampling algorithm, yielding full posterior distributions and exploring parameter degeneracies. These maps were created using high-resolution spectral observations from the XRISM/Resolve instrument, allowing for a regional analysis of the remnant’s composition and physical conditions.
The research employed a novel Bayesian framework, UltraSPEX, to analyse the spectra and account for uncertainties in the data. Key findings reveal variations in the abundance ratios of intermediate-mass elements, specifically enhanced argon and calcium relative to silicon, near the base of silicon-rich jets.
A notably high nickel-to-iron mass ratio was also identified in the base of a northeast jet. Furthermore, ejecta from iron-group elements consistently exhibit higher velocities and broader spectral lines compared to those from intermediate-mass elements, with differences reaching approximately 1,000 kilometres per second.
The ionization timescale and electron temperature demonstrate a clear inverse relationship, potentially explained by significant clumping within the ejecta and reduced reverse-shock velocities. Nonthermal emission constitutes a substantial portion of the observed radiation, accounting for at least 47% of the continuum between 4 and 6 keV and dominating in the western regions of the remnant.
The authors acknowledge limitations related to spatial spectral mixing caused by the instrument’s point spread function, which was addressed through the use of larger spatial regions in the analysis. Future research will likely focus on refining the models of ejecta clumping and reverse-shock dynamics to better interpret the observed plasma parameters. These detailed maps provide crucial insights into the explosion mechanisms, ejecta structure, and asymmetries of core-collapse supernovae, establishing a foundation for further investigations into the evolution of supernova remnants and their impact on the surrounding interstellar medium.
👉 More information
🗞 Mapping plasma properties of Cassiopeia A with XRISM/Resolve: a Bayesian analysis via UltraSPEX
🧠 ArXiv: https://arxiv.org/abs/2602.06952