arXiv subjects

Salvatore Orlando

Publications and source records attributed to Salvatore Orlando.

At least 19 recordsLinked to original sources

arXiv2026

Deciphering the Remnants of Core-Collapse Supernovae: Reconstructing Progenitor Star Properties and Explosion Mechanisms

(Abridged) Recent JWST observations of Cassiopeia A (Cas A) reveal unprecedented ejecta substructure, including a web of filaments and the enigmatic "Green Monster" (GM), characterized by nearly circular holes and rings. These features provide new constraints on supernova (SN) explosion physics and ejecta-circumstellar medium (CSM) interactions. We present high-resolution three-dimensional hydrodynamic and magnetohydrodynamic simulations of a neutrino-driven SN explosion tailored to Cas A, following the system from core collapse to an age of $\sim 1000$ yr. The models include key physical processes such as hydrodynamic instabilities, Ni-bubble effects, radiative cooling, non-equilibrium ionization, and electron-ion temperature equilibration. Our results show that the filamentary ejecta network naturally forms during the early explosion due to the interaction of neutrino-driven bubbles and instabilities, retaining a memory of the initial conditions before being progressively modified by the reverse shock. The GM morphology is reproduced by the interaction of dense ejecta clumps with an asymmetric, forward-shocked CSM shell, with radiative cooling enhancing fragmentation and generating the observed holes and rings. Overall, our study demonstrates that Cas A's complex morphology reflects both the imprint of the explosion mechanism and subsequent ejecta-CSM interactions.

astro-ph.HE↗

arXiv2025

Continuous Wide-Field Optical Monitoring for Very Early-Phase Transient Discovery

The study of transient phenomena in a multimessenger context is expected to remain a major pillar of astrophysical discovery in the decades ahead. Supernovae, Kilonovae, Black-Hole formation, Novae, GRBs, and tidal disruption events are prime examples, as their earliest phases link electromagnetic radiation to gravitational waves, neutrinos, and high-energy emission. Yet, the physics connecting these messengers unfolds within minutes to hours, while traditional surveys revisit the same region of the sky on the scale of days/weeks, missing when the event begins. Current survey facilities excel at answering what happened and how often, but essentially fail in addressing how it happened and how it couples to gravitational waves, neutrinos, or high-energy emission. Continuous wide-area optical monitoring, as proposed here, removes this limitation. The traditional approach, where a GW or neutrino alert triggers electromagnetic follow-up, is now complemented, and sometimes reversed: early electromagnetic discoveries can prompt searches for weaker gravitational waves or neutrino signals that would otherwise be missed. In the Einstein Telescope era, wide-field optical monitoring will allow us to find the optical counterparts of gravitational-wave events and understand their physics. At the same time, a telescope capable of continuous monitoring provides immediate scientific value for planetary defense, space-debris tracking, stellar variability, exoplanets transit monitoring, accretion-driven activity, and when we step into a new observational territory, the true discoveries are often the ones we did not expect. In this vision, continuous time-domain astronomy does not replace classical surveys: it completes them by supplying the missing temporal dimension. Follow-up observations remain essential, but they now begin at the physical onset of the event rather than after its evolution is underway.

astro-ph.IM↗

arXiv2025

The Sun as an X-ray star V.: A new method to retrieve coronal filling factors

Context. Stellar coronae are unresolved in X-rays, so inferences about their structure rely on spectral analysis. The "Sun-as-an-X-ray-star" (SaXS) approach uses the Sun as a spatially resolved template to interpret stellar spectra, but previous SaXS implementations were indirect and computationally heavy. Aims. We present a new SaXS implementation that converts solar emission measure distributions (EMDs) of distinct coronal region types into XSPEC spectral components and test whether broad-band X-ray spectra alone can recover their filling factors. Methods. We built XSPEC multi-temperature spectral models for four solar region types (background/quiet corona, active regions, cores, and flares) by using EMDs derived from Yohkoh/SXT data and translating each EMD bin into an isothermal apec component. These models were fit (using PyXspec) to two one-hour DAXSS spectra representative of quiescent (2022-06-29) and flaring (2022-04-25) states. Best-fit normalizations were converted into projected areas and filling factors and compared with near-coincident Hinode/XRT full-disk images. Results. Using the Yohkoh/SXT EMDs, the quiescent Sun spectrum is dominated by active region emission (filling factor ~22%), with the background corona poorly constrained. The flaring Sun spectrum is best described by a combination of active regions, cores, and flares with filling factors of ~47.5%, ~4.1%, and ~0.062%, respectively. The dominant components match spatial features seen in Hinode/XRT images. Limitations include the DAXSS low-energy cutoff (~0.7 keV) and the small, non-uniform Yohkoh EMD sample. Conclusions. Our SaXS implementation enables direct retrieval of coronal filling factors from broad-band X-ray spectra and provides a physically motivated alternative to ad hoc few-temperature fits, suitable for stellar X-ray analyses.

astro-ph.SR↗

arXiv2025

Extreme anisotropies in deep layers of an exploding star: overabundance of Cr in the northeastern jet of Cassiopeia A

Core-collapse supernovae drive nucleosynthesis under extreme thermodynamic conditions, and complex mechanisms are at work prompting the transport of heavy elements from deep stellar interiors into outer layers. We present spatially resolved X-ray spectroscopy of Cassiopeia A's (Cas A) northeastern (NE) jet using the archival 1 Ms Chandra/ACIS observations, and focusing on three fingers of the jet. We report the highest Cr/Fe mass ratio (Cr/Fe $\sim0.14$) ever observed in Cas A, localized in a compact region within the southernmost finger in the NE jet. Comparisons with nucleosynthesis models indicate that the NE jet originated approximately at the boundary separating the complete Si burning layer from the incomplete Si-burning layer. We also find that mixing from different layers is needed to explain the chemical composition of the three fingers in the NE jet. We also detect significant differences in the physical and chemical properties among the three fingers analyzed of the NE jet. In particular, we find that, unlike the other two, the southernmost finger originated from a slightly more peripheral region of the explosion. Moreover, while the northern and central fingers lie almost in the plane of the sky, the southernmost finger is moving in a different direction, showing a velocity along the line of sight of $\sim2100$ km s$^{-1}$ towards the observer, with a tilt angle of $\sim16$\textdegree. These findings highlight the NE jet's role in ejecting material from the deepest explosive burning layers, providing new insights into the asymmetries originating in the inner layers of core-collapse supernovae.

astro-ph.HE↗

arXiv2025

Multi-wavelength study of the high Galactic latitude supernova remnant candidate G118.4+37.0 associated with the Calvera pulsar

The candidate supernova remnant (SNR) G118.4+37.0 (Calvera's SNR), discovered as a faint radio ring at high Galactic latitude and coincident with extended Fermi/LAT gamma-ray emission, is likely associated to the X-ray pulsar 1RXS J141256.0+792204 (Calvera). Previous XMM-Newton data hinted at soft diffuse X-ray emission inside the ring but lacked sufficient exposure for detailed characterisation. We obtained new XMM-Newton observations, and produced count-rate images, equivalent width and median photon energy maps to identify optimal regions for spectral analysis. We complemented these observations with a reanalysis of Fermi/LAT gamma-ray data and new Telescopio Nazionale Galileo observations aimed to search for Halpha emission. The X-ray diffuse emission is well described by a model of shock-heated plasma with temperature kT ~ 0.15 keV, mildly under-solar N and O abundances and densities ne=0.1-0.7 cm-3. According to our estimates, Calvera's SNR is 10-20 kyr old and lies at a distance of 4-5 kpc. A distinct "Clump" region shows hard emission equally well described by a thermal (kT ~ 1.7 keV) or a non thermal model (Gamma ~ 2.7). The brightest X-ray area is close to the gamma-ray peak and to an isolated Halpha filament. G118.4+37.0 is a middle-aged remnant which expands in a tenuous medium and encountered a denser phase, likely the relic of the wind activity of the massive progenitor star. The estimated SNR distance is consistent within the uncertainties with that estimated for Calvera, confirming that this peculiar pulsar was born in the explosion of a massive star high above the Galactic disk. Our measured ambient density, together with the patchy morphology of the gamma-ray emission and the detection of Halpha filaments indicates that a hadronic origin is compatible with the gamma-ray flux, though a mixed leptonic-hadronic cannot be excluded.

astro-ph.HE↗

arXiv2025

Measuring the initial mass of 44Ti in SN 1987A through the 44Sc emission line

Context. Deriving the mass and large-scale asymmetries of radioactive isotopes offers valuable insights into the complex phases of a supernova explosion. Important examples are $^{56}$Ni, with its decay products $^{56}$Co and $^{56}$Fe, and $^{44}$Ti, which are studied through their X-rays emission lines and provide a powerful diagnostic tool to probe the explosive nucleosynthesis processes in the inner layers of the exploding star. Aims. In this framework, SN 1987A provides a privileged laboratory being the youngest supernova remnant from which the mass of Ti has been estimated. However, some tension exists in determining the initial mass of $^{44}$Ti. Previous analysis, relying on \textit{NuSTAR} and \textit{INTEGRAL} data, report $M_{44} = (1.5 \pm 0.3) \times 10^{-4}$ $M_\odot$ and $M_{44}=(3.1 \pm 0.8) \times 10^{-4} M_\odot$, respectively. In this paper we estimate the initial mass of $^{44}$Ti via its decay product, the $^{44}$Sc emission line at 4.09 keV, using \textit{Chandra} observations. Methods. We perform multi-epoch spectral analysis focusing on the inner part of the remnant, to minimize the contamination from the X-ray emission stemming from the shocked plasma. As a result, we provide the detection of $^{44}$Sc emission line in the central part of SN 1987A with a $\sim$99.7\% (3 $σ$) significance. Results. The simultaneous fit of the spectra extracted from observations between 2016 and 2021 provides a line flux of $6.8^{+2.2}_{-2.3}\times 10^{-7}$ photons s$^{-1}$ cm$^{-2}$ corresponding to a $^{44}$Ti mass $M_{44}=(1.6\pm0.5) \times 10^{-4} M_\odot$ (errors at the $90\%$ confidence level). The results obtained with our spectral analysis seem to align with those derived with NuSTAR.

astro-ph.HE↗

arXiv2025

Measuring the asymmetric expansion of the Fe ejecta of Cassiopeia A with XRISM/Resolve

The expansion structure of supernova remnants (SNRs) is important for understanding not only how heavy elements are distributed into space, but also how supernovae explode. The ejecta expansion structure of the young core-collapse SNR Cas A is investigated, with Doppler parameter mapping of the Fe-K complex by the Resolve microcalorimeter onboard the X-ray Imaging and Spectroscopy Mission, XRISM. It is found that the Fe ejecta are blueshifted in the southeast (SE) and redshifted in the northwest (NW), indicating an incomplete shell structure, similar to the intermediate mass elements (IMEs), such as Si and S. The Fe has a velocity shift of $\sim1400$ km~s$^{-1}$ in the NW and $\sim2160$ km~s$^{-1}$ in the SE region, with the error range of a few 100s km~s$^{-1}$. These values are consistent with those for the IMEs in the NW region, whereas larger than those for the IMEs in the SE region, although the large error region prevented us from concluding which component has significantly higher velocity. The line broadening is larger in the center with values of $\sim$2000--3000~km~s$^{-1}$, and smaller near the edges of the remnant. The radial profiles of the Doppler shift and broadening of the IMEs and Fe indicate that the Fe ejecta may expand asymmetrically as IME ejacta, although the large error regions do not allow us to conclude it. Moreover, we see little bulk Doppler broadening of the Fe lines in the northeastern jet region whereas the IME lines exhibit significant broadening. No such narrow lines are detected in the NW region. These findings suggest an asymmetric expansion of the ejecta potentially driven by large-scale asymmetries originating from the supernova explosion. This interpretation aligns with the large-scale asymmetries predicted by models of neutrino-driven supernova explosions.

astro-ph.HE↗

arXiv2025

Search for $γ$-ray emission from SNRs in the Large Magellanic Cloud: a new cluster analysis at energies above 4 GeV

A search for $γ$-ray emission from SNRs in the Large Magellanic Cloud (LMC) based on the detection of concentrations in the arrival direction Fermi-LAT images of photons at energies higher than 10 GeV found significant evidence for 9 of these sources. This analysis was based on data collected in the time window since August 4 2008 to August, 4 2020 (12 years). In the present contribution, we report results of a new search extended using a 15-year long (up to August, 4 2023) data set and to a broad energy range (higher than 4 GeV). The longer baseline and the softer energy lower limit are required to further understand the relation between the X-ray and gamma ray SNRs in the LMC, and to investigate the completness of the sample at low luminosities. Two different methods of clustering analysis were applied: Minimum Spanning Tree (MST), and the combination of Density-Based Spatial Clustering of Applications with Noise (DBSCAN) and DENsity-based CLUstEring (DENCLUE) algorithms. We confirm all previous detections and found positive indications for at least 8 new clusters with a spatial correspondence with other SNRs, increasing thus the number of remnants in LMC candidate or detected in the high energy $γ$-rays to 16 sources. This study extends previous analyses of $γ$-ray emission from SNRs in the LMC by incorporating a longer observational baseline and a broader energy range. The improved dataset and advanced clustering techniques enhance our understanding of the connection between X-ray and $γ$-ray SNRs, providing new insights into their high-energy properties, and contributing to assessing the completeness of the sample at lower luminosities.

astro-ph.HE↗

arXiv2025

Unusual X-ray Oxygen Line Ratios of SN 1987A Arising From the Absorption of Galactic Hot Interstellar Medium

Recent high-resolution X-ray spectroscopic studies have revealed unusual oxygen line ratios, such as the high O VII forbidden-to-resonance ratio, in several supernova remnants. While the physical origin is still under debate, for most of them, it has been suggested that this phenomenon arises from either charge exchange (CX) or resonant scattering (RS). In this work, we report the high O VII G-ratio ($\gtrsim1$) and high O VIII Ly$β$/Ly$α$ ratio ($\gtrsim0.2$) found in multiepoch XMM-Newton RGS observations of SN 1987A. The line ratios cannot be fully explained by non-equilibrium ionization effects, CX, or RS. We suggest the absorption of foreground hot gas as the most likely origin, which plays the major role in modifying line fluxes and line ratios. Based on this scenario, we introduced two Gaussian absorption components at the O VII resonance line and the O VIII Ly$α$ line and constrained the optical depth of the two lines as $τ_{\rm OVII}\sim0.6$ and $τ_{\rm OVIII}\sim0.2$. We estimated the temperature as $kT_{\rm e}\sim0.15$ keV and the oxygen column density as $N_{\rm O}\sim0.5\times10^{16}$ cm$^{-2}$ for the absorbing gas, which is consistent with the hot interstellar medium in the Galactic halo. Neglecting this absorption component may lead to an underestimation of the O abundance. We revised the O abundance of SN 1987A, which is increased by $\sim20\%$ compared with previous results. The N/O ratio by number of atoms is revised to be $\sim1.2$.

astro-ph.HE↗

arXiv2025

Evolution of X-ray Gas in SN 1987A from 2007 to 2021: Ring Fading and Ejecta Brightening Unveiled through Differential Emission Measure Analysis

As the nearest supernova (SN) observed since Kepler's SN of 1604, SN 1987A provides an unprecedented opportunity to study in detail the early evolution of supernova remnants (SNRs). Despite extensive studies through both observations and simulations, there is still an urgent need for a more effective approach to integrate the results from two sides. In this study, we conducted a detailed differential emission measure (DEM) analysis on the XMM-Newton observations taken in 2007 to 2021 to characterize the continuous temperature structure of SN 1987A, which can be better compared with simulations. The X-ray plasma exhibit a temperature distribution with a major peak at $\sim0.5$-$1$ keV and a high-temperature tail extending to $\gtrsim5$ keV. The emission measure (EM) of the major peak started to decline around 2014, while the EM of the tail continued increasing and appears to have formed a secondary peak at $\sim3$-$5$ keV in recent years. Our DEM results consistent well with simulations, which help to further identify the major peak as originating from the equatorial ring and the secondary peak as arising from the newly shocked ejecta. Together with the simulations, our DEM analysis reveals recent fading of the ring and brightening of the ejecta in X-rays from SN 1987A. Additionally, we observed a recent decrease in the centroid energy of Fe K line, providing further evidence of newly shocked ejecta.

astro-ph.HE↗

arXiv2024

The Green Monster hiding in front of Cas A: JWST reveals a dense and dusty circumstellar structure pockmarked by ejecta interactions

JWST observations of the young Galactic supernova remnant Cassiopeia A revealed an unexpected structure seen as a green emission feature in colored composite MIRI F1130W and F1280W images - hence dubbed the Green Monster - that stretches across the central parts of the remnant in projection. Combining the kinematic information from NIRSpec and MIRI MRS with the multi-wavelength imaging from NIRCam and MIRI, we associate the Green Monster with circumstellar material that was lost during an asymmetric mass-loss phase. MIRI images are dominated by dust emission but its spectra show emission lines from Ne, H and Fe with low radial velocities indicative of a CSM nature. An X-ray analysis of this feature in a companion paper (Vink et al. 2024) supports its CSM nature and detects significant blue shifting, thereby placing the Green Monster on the near side, in front of the Cas A SN remnant. The most striking features of the Green Monster are dozens of almost perfectly circular 1" - 3" sized holes, most likely created by interaction between high-velocity SN ejecta material and the CSM. Further investigation is needed to understand whether these holes were formed by small 8000-10500 km/s N-rich ejecta knots that penetrated and advanced out ahead of the remnant's 5000 - 6000 km/s outer blastwave, or by narrow ejecta fingers that protrude into the forward-shocked CSM. The detection of the Green Monster provides further evidence of the highly asymmetric mass-loss that Cas A's progenitor star underwent prior to explosion.

astro-ph.HE↗

arXiv2024

Watermarking Decision Tree Ensembles

Protecting the intellectual property of machine learning models is a hot topic and many watermarking schemes for deep neural networks have been proposed in the literature. Unfortunately, prior work largely neglected the investigation of watermarking techniques for other types of models, including decision tree ensembles, which are a state-of-the-art model for classification tasks on non-perceptual data. In this paper, we present the first watermarking scheme designed for decision tree ensembles, focusing in particular on random forest models. We discuss watermark creation and verification, presenting a thorough security analysis with respect to possible attacks. We finally perform an experimental evaluation of the proposed scheme, showing excellent results in terms of accuracy and security against the most relevant threats.

cs.LG↗

arXiv2024

Early Exit Strategies for Approximate k-NN Search in Dense Retrieval

Learned dense representations are a popular family of techniques for encoding queries and documents using high-dimensional embeddings, which enable retrieval by performing approximate k nearest-neighbors search (A-kNN). A popular technique for making A-kNN search efficient is based on a two-level index, where the embeddings of documents are clustered offline and, at query processing, a fixed number N of clusters closest to the query is visited exhaustively to compute the result set. In this paper, we build upon state-of-the-art for early exit A-kNN and propose an unsupervised method based on the notion of patience, which can reach competitive effectiveness with large efficiency gains. Moreover, we discuss a cascade approach where we first identify queries that find their nearest neighbor within the closest t << N clusters, and then we decide how many more to visit based on our patience approach or other state-of-the-art strategies. Reproducible experiments employing state-of-the-art dense retrieval models and publicly available resources show that our techniques improve the A-kNN efficiency with up to 5x speedups while achieving negligible effectiveness losses. All the code used is available at https://github.com/francescobusolin/faiss_pEE

cs.IR↗

arXiv2024

Time evolution of the synchrotron X-ray emission in Kepler's SNR: the effects of turbulence and shock velocity

The maximum energy of electrons in supernova remnant (SNR) shocks is typically limited by radiative losses, where the synchrotron cooling time equals the acceleration time. The low speed of shocks in a dense medium increases the acceleration time, leading to lower maximum electron energies and fainter X-ray emissions. However, in Kepler's SNR, an enhanced electron acceleration, which proceeds close to the Bohm limit, occurs in the north of its shell, where the shock is slowed by a dense circumstellar medium (CSM). To investigate whether this scenario still holds at smaller scales, we analyzed the temporal evolution of the X-ray synchrotron flux in filamentary structures, using the two deepest Chandra/ACIS X-ray observations, performed in 2006 and 2014. We examined spectra from different filaments, we measured their proper motion and calculated the acceleration to synchrotron time-scale ratios. The interaction with the turbulent and dense northern CSM induces competing effects on electron acceleration: on one hand, turbulence reduces the electron mean free path enhancing the acceleration efficiency, on the other hand, lower shock velocities increase the acceleration time-scale. In most filaments, these effects compensate each other, but in one region the acceleration time-scale exceeds the synchrotron time-scale, resulting in a significant decrease in nonthermal X-ray emission from 2006 to 2014, indicating fading synchrotron emission. Our findings provide a coherent understanding of the different regimes of electron acceleration observed in Kepler's SNR through various diagnostics.

astro-ph.HE↗

arXiv2024

Evidence of gradients of density and magnetic field in the remnant of Tycho's supernova

By using surface brightness maps of Tycho's supernova remnant (SNR) in radio and X-rays, along with the properties of thermal and synchrotron emission, we have derived the post-shock density and magnetic field strength distributions over the projection of this remnant. Our analysis reveals a density gradient oriented towards the north-west, while the magnetic field strength gradient aligns with the Galactic plane, pointing eastward. Additionally, utilizing this magnetic field map, we have derived the spatial distributions of the cut-off frequency and maximum energy of electrons in Tycho's SNR. We further comment on the implications of these findings for interpreting the gamma-ray emission from Tycho's SNR.

astro-ph.HE↗

arXiv2024

A JWST Survey of the Supernova Remnant Cassiopeia A

We present initial results from a JWST survey of the youngest Galactic core-collapse supernova remnant Cassiopeia A (Cas A), made up of NIRCam and MIRI imaging mosaics that map emission from the main shell, interior, and surrounding circumstellar/interstellar material (CSM/ISM). We also present four exploratory positions of MIRI/MRS IFU spectroscopy that sample ejecta, CSM, and associated dust from representative shocked and unshocked regions. Surprising discoveries include: 1) a web-like network of unshocked ejecta filaments resolved to 0.01 pc scales exhibiting an overall morphology consistent with turbulent mixing of cool, low-entropy matter from the progenitor's oxygen layer with hot, high-entropy matter heated by neutrino interactions and radioactivity, 2) a thick sheet of dust-dominated emission from shocked CSM seen in projection toward the remnant's interior pockmarked with small (approximately one arcsecond) round holes formed by knots of high-velocity ejecta that have pierced through the CSM and driven expanding tangential shocks, 3) dozens of light echoes with angular sizes between 0.1 arcsecond to 1 arcminute reflecting previously unseen fine-scale structure in the ISM. NIRCam observations place new upper limits on infrared emission from the neutron star in Cas A's center and tightly constrain scenarios involving a possible fallback disk. These JWST survey data and initial findings help address unresolved questions about massive star explosions that have broad implications for the formation and evolution of stellar populations, the metal and dust enrichment of galaxies, and the origin of compact remnant objects.

astro-ph.HE↗

arXiv2024

Probing Shocked Ejecta in SN 1987A with XRISM-Resolve: the effects of the gate valve closed

Supernova (SN) 1987A is widely regarded as an excellent candidate for leveraging the capabilities of the freshly launched XRISM satellite. Recent researches indicate that the X-ray emission from SN 1987A will increasingly originate from its ejecta in the years to come. In a previous study, we thoroughly examined the proficiency of XRISM-Resolve in identifying signatures of shocked ejecta in SN 1987A, synthesizing the XRISM-Resolve spectrum based on a state-of-the-art magneto-hydrodynamic simulation. However, following the satellite's launch, a technical issue arose with the XRISM instrument's gate valve, which failed to open, thereby affecting observations with the Resolve spectrometer. Here, we update our analysis, reevaluating our diagnostic approach under the assumption that the gate valve remains closed. We find that, even with the reduced instrumental capabilities, it will be possible to pinpoint the ejecta contribution through the study of the line profiles in the XRISM-Resolve spectrum of SN 1987A.

astro-ph.HE↗

arXiv2024

Latest Evolution of the X-Ray Remnant of SN 1987A: Beyond the Inner Ring

Based on our Chandra imaging-spectroscopic observations, we present the latest evolution of the X-ray remnant of SN 1987A. Recent changes in the electron temperatures and volume emission measures suggest that the blast wave in SN 1987A is moving out of the dense inner ring structure, also called the equatorial ring (ER). The 0.5-2.0 keV X-ray light curve shows a linearly declining trend (by $\sim$4.5 % yr$^{-1}$) between 2016 and 2020, as the blast wave heats the hitherto unknown circumstellar medium (CSM) outside the ER. While the peak X-ray emission in the latest 0.3-8.0 keV image is still within the ER, the radial expansion rate in the 3.0-8.0 keV images suggests an increasing contribution of the X-ray emission from less dense CSM since 2012, at least partly from beyond the ER. It is remarkable that, since 2020, the declining soft X-ray flux has stabilized around $\sim$7 $\times$ 10$^{-12}$ erg s$^{-1}$ cm$^{-2}$, which may signal a contribution from the reverse-shocked outer layers of ejecta as predicted by the 3-D magneto-hydrodynamic (MHD) models. In the latest ACIS spectrum of supernova remnant (SNR) 1987A in 2022 we report a significant detection of the Fe K line at $\sim$6.7 keV, which may be due to changing thermal conditions of the X-ray emitting CSM and/or the onset of reverse shock interactions with the Fe-ejecta.

astro-ph.HE↗