Evolution Of Supernova Remnants Research Articles

Simulating the peculiar periphery of the Cygnus Loop

Abstract The middle-aged Galactic supernova remnant (SNR) the Cygnus Loop (CL) displays a peculiar morphology in X-rays, featuring a blowout in the southern region. The underlying process accounting for the formation of the peculiar periphery remains a mystery. To this end, we conduct hydrodynamical simulations to investigate the SNR evolution coupled with a tailored stellar-wind model: a bipolar stellar wind emanating from a runaway red supergiant progenitor, excavating a wind-blown cavity elongated along the $-z$-direction. Our simulation results reveal that the forward shock of the consequent SNR sweeps up the modified ambient media, shaping the overall morphology with a blowout comparable to that of CL. Besides, a series of simulation runs are performed to assess the impacts of different model parameters and the projection effect (observational angle $\theta_{\mathrm{obs}}$) on the final SNR profile. Three physical quantities are extracted from simulation results to characterize the simulated SNR and make a direct comparison with the X-ray observations of CL. We find that the final SNR morphology is sensitive to both stellar-wind properties and $\theta_{\mathrm{obs}}$. A Cygnus-Loop-like SNR could be reproduced under appropriate parameter combinations at $\theta_{\mathrm{obs}}=0^{\circ}$. While for $\theta_{\mathrm{obs}}\lesssim30^{\circ}$, the projected morphology akin to CL could be also generated under specific conditions.

Nonthermal Signatures of Radiative Supernova Remnants

The end of supernova remnant (SNR) evolution is characterized by a so-called “radiative” stage, in which efficient cooling of the hot bubble inside the forward shock slows expansion, leading to eventual shock breakup. Understanding SNR evolution at this stage is vital for predicting feedback in galaxies, since SNRs are expected to deposit their energy and momentum into the interstellar medium at the ends of their lives. A key prediction of SNR evolutionary models is the formation at the onset of the radiative stage of a cold, dense shell behind the forward shock. However, searches for these shells via their neutral hydrogen emission have had limited success. We instead introduce an independent observational signal of shell formation arising from the interaction between nonthermal particles accelerated by the SNR forward shock (cosmic rays) and the dense shell. Using a semi-analytic model of particle acceleration based on state-of-the-art simulations coupled with a high-resolution hydrodynamic model of SNR evolution, we predict the nonthermal emission that arises from this interaction. We demonstrate that the onset of the radiative stage leads to nonthermal signatures from radio to gamma rays, including radio and gamma-ray brightening by nearly 2 orders of magnitude. Such a signature may be detectable with current instruments, and will be resolvable with the next generation of gamma-ray telescopes (namely, the Cherenkov Telescope Array).

Star Formation by Supernova Implosion

Recent hydrodynamical simulations of the late stages of supernova remnant (SNR) evolution have revealed that as they merge with the ambient medium, SNRs implode, leading to the formation of dense clouds in their center. While being highly chemically enriched by their host SNR, these clouds appear to have similar properties as giant molecular clouds, which are believed to be the main site of star formation. Here, we develop a simple model in order to estimate the efficiency of the star formation that might be triggered by the implosion of SNRs. We separately consider two cases: cyclic star formation, maintained by the episodic driving of feedback from new generations of stars, and a single burst of star formation, triggered by a single explosion. We find that in the cyclic case, star formation is inefficient, with implosion-triggered star formation contributing a few percent of the observed star formation efficiency per freefall timescale. In the single-burst case, higher star formation efficiencies can be obtained. However, while the implosion-triggered process might not contribute much to the overall star formation, due to the high chemical enrichment of the birth clouds, it can explain the formation of a significant fraction of metal-rich stars.

Supernova remnants of red supergiants: From barrels to loops

Core-collapse (CC) supernova remnants (SNRs) are the nebular leftovers of defunct massive stars that died during a supernova explosion, mostly while undergoing the red supergiant phase of their evolution. The morphology and emission properties of those remnants are a function of the distribution of circumstellar material at the moment of the supernova, as well as the intrinsic properties of the explosion and those of the ambient medium. By means of 2.5-dimensional (2.5D) numerical magneto-hydrodynamic (MHD) simulations, we modelled the long-term evolution of SNRs generated by runaway rotating massive stars moving into a magnetised interstellar medium (ISM). Radiative transfer calculations reveal that the projected non-thermal emission of SNRs decreases over time, namely: older remnants are fainter than younger ones. Older (80 kyr) SNRs whose progenitors were moving with a space velocity corresponding to a Mach number of M = 1 (v⋆ = 20 km s−1) in the Galactic plane of the interstellar medium (nISM = 1 cm−3) are brighter in synchrotron than when moving with a Mach number of M = 2 (v⋆ = 40 km s−1). We show that runaway red supergiant progenitors first induce an asymmetric non-thermal 1.4 GHz barrel-like synchrotron SNRs (at the age of about 8 kyr), before further evolving to adopt a Cygnus-loop-like shape (at about 80 kyr). It is conjectured that a significative fraction of SNRs are currently in this bilateral-to-Cygnus loop evolutionary sequence. Therefore, this population should be taken into account with repect to interpreting the data as part of the forthcoming Cherenkov Telescope Array (CTA) observatory.

Feedback of Efficient Shock Acceleration on Magnetic-field Structure Inside Young Type Ia Supernova Remnants

Using an effective adiabatic index γ eff to mimic the feedback of efficient shock acceleration, we simulate the temporal evolution of a young type Ia supernova remnant (SNR) with two different background magnetic field (BMF) topologies: a uniform and a turbulent BMF. The density distribution and magnetic-field characteristics of our benchmark SNR are studied with two-dimensional cylindrical magnetohydrodynamic simulations. When γ eff is considered, we find that: (1) the two-shock structure shrinks and the downstream magnetic-field orientation is dominated by the Rayleigh–Taylor instability structures; (2) there exists more quasi-radial magnetic fields inside the shocked region; and (3) inside the intershock region, both the quasi-radial magnetic energy density and the total magnetic energy density are enhanced: in the radial direction, with γ eff = 1.1, they are amplified about 10–26 times more than those with γ eff = 5/3. While in the angular direction, the total magnetic energy densities could be amplified about 350 times more than those with γ eff = 5/3, and there are more grid cells within the intershock region where the magnetic energy density is amplified by a factor greater than 100.

Cloud Formation by Supernova Implosion

The deposition of energy and momentum by supernova explosions has been subject to numerous studies in the past few decades. However, while there has been some work that focused on the transition from the adiabatic to the radiative stage of a supernova remnant (SNR), the late radiative stage and merging with the interstellar medium (ISM) have received little attention. Here, we use three-dimensional, hydrodynamic simulations, focusing on the evolution of SNRs during the radiative phase, considering a wide range of physical explosion parameters ( nH,ISM∈0.1,100cm−3 and ESN∈1,14×1051erg ). We find that the radiative phase can be subdivided in four stages: A pressure-driven snowplow phase, during which the hot overpressurized bubble gas is evacuated and pushed into the cold shell; a momentum-conserving snowplow phase that is accompanied by a broadening of the shell; an implosion phase where cold material from the back of the shell is flooding the central vacuum; and a final cloud phase, during which the imploding gas is settling as a central, compact overdensity. The launching timescale for the implosion ranges from a few 100 kyr to a few Myr, while the cloud formation timescale ranges from a few to about 10 Myr. The highly chemically enriched clouds can become massive (M cl ∼ 103–104 M ⊙) and self-gravitating within a few Myr after their formation, providing an attractive, novel pathway for supernova-induced star and planet formation in the ISM.

Reverberation of pulsar wind nebulae – III. Modelling of the plasma interface empowering a long term radiative evolution

ABSTRACT The vast majority of pulsar wind nebulae (PWNe) present in the Galaxy is formed by middle-aged systems characterized by a strong interaction of the PWN itself with the supernova remnant (SNR). Unfortunately, modelling these systems can be quite complex and numerically expensive, due to the non-linearity of the PWN–SNR evolution even in the simple one-dimensional (1D)/one-zone case when the reverse shock of the SNR reaches the PWN, and the two begin to interact (and reverberation starts). Here, we introduce a new numerical technique that couples the numerical efficiency of the one-zone thin shell approach with the reliability of a full ‘Lagrangian’ evolution, able to correctly reproduce the PWN–SNR interaction during the reverberation, and to consistently evolve the particle spectrum beyond. Based on our previous findings, we show that our novel strategy resolves many of the uncertainties present in previous approaches, as the arbitrariness in the SNR structure, and ensure a robust evolution, compatible with results that can be obtained with more complex 1D dynamical approaches. Our approach enable us for the first time to provide reliable spectral models of the later compression phases in the evolution of PWNe. While in general, we found that the compression is less extreme than that obtained without such detailed dynamical considerations, leading to the formation of less structured spectral energy distributions, we still find that a non-negligible fraction of PWNe might experience a super-efficient phase, with the optical and/or X-ray luminosity exceeding the spin-down one.

Study of the effect of turbulent interstellar medium on the morphology of young supernova remnants

Context. Supernova remnants (SNRs) are one of the main sources of galactic cosmic-ray acceleration. This acceleration, believed to happen at the blast wave front, leads to energy loss at the shock front. This results in the apparent proximity between the blast wave and the contact discontinuity. Aims. In this article, we study the effect that turbulent-like density perturbations of the interstellar medium (ISM) have on the evolution of young SNRs. We focus on the impact these fluctuations have on the structure of SNRs and more precisely on the resulting distance between blast wave and contact discontinuity. As cosmic-ray acceleration is necessary to explain this distance, this study indirectly puts into question the cosmic-ray acceleration at the blast wave front. Methods. We performed a set of purely hydrodynamic three-dimensional simulations without cosmic-ray acceleration in a co-expanding frame. We randomly initialised the density variation of the ISM following a Kolmogorov power law. The resulting ratios of radii between blast wave, contact discontinuity, and reverse shock are then compared to astronomical observations. Results. The addition of density perturbation does not significantly change the average ratio of radii. However, the simulations show a higher growth of interfacial instabilities in the presence of a turbulent ISM. The resulting deformation of the contact discontinuity could explain the proximity between contact discontinuity and blast wave. The deformations also explain the plateau in the radial distribution of the line-of-sight velocity associated with the observations of Tycho. Conclusions. Density perturbations of the ISM should not be neglected in the simulation of young SNRs as they have an impact on the structure of these latter objects that is comparable to that of cosmic-ray acceleration.

Sensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnants

The local Cosmic Ray (CR) energy spectrum exhibits a spectral softening at energies around 3 PeV. Sources which are capable of accelerating hadrons to such energies are called hadronic PeVatrons. However, hadronic PeVatrons have not yet been firmly identified within the Galaxy. Several source classes, including Galactic Supernova Remnants (SNRs), have been proposed as PeVatron candidates. The potential to search for hadronic PeVatrons with the Cherenkov Telescope Array (CTA) is assessed. The focus is on the usage of very high energy γ-ray spectral signatures for the identification of PeVatrons. Assuming that SNRs can accelerate CRs up to knee energies, the number of Galactic SNRs which can be identified as PeVatrons with CTA is estimated within a model for the evolution of SNRs. Additionally, the potential of a follow-up observation strategy under moonlight conditions for PeVatron searches is investigated. Statistical methods for the identification of PeVatrons are introduced, and realistic Monte-Carlo simulations of the response of the CTA observatory to the emission spectra from hadronic PeVatrons are performed. Based on simulations of a simplified model for the evolution for SNRs, the detection of a γ-ray signal from in average 9 Galactic PeVatron SNRs is expected to result from the scan of the Galactic plane with CTA after 10 h of exposure. CTA is also shown to have excellent potential to confirm these sources as PeVatrons in deep observations with O(100) hours of exposure per source.

Radio Emission from Supernova Remnants: Model Comparison with Observations

Supernova remnants (SNRs) are an integral part in studying the properties of the Galaxy and its interstellar medium. For the current work, we compare the observed radio luminosities of SNRs to predictions based on a recent analytic model applied to 54 SNRs with X-ray observations. We use the X-ray data to determine the properties of shock velocities, ages and circumstellar densities for the SNRs, whereas shock radii are determined from catalogs. With this set of SNR properties, we can calculate the model radio emission and compare it to the observed radio emission for a sample of SNRs. This is the first time that this test has been carried out—previously the SNR properties were assumed instead of derived from X-ray data. With the assumption that the radio emission process depends on SNR properties in the form of power-law functions, we explore ways to improve the radio emission model. The main results of this study are (i) the model has significant deficiencies and cannot reproduce observed radio emission; and (ii) the model can be improved significantly by changing its dependence on SNR parameters, although the improved model is still not accurate. Significant work remains to improve the components of radio emission models, including changes to the SNR evolution model, the radio emitting volume, and the efficiencies for conversion of shock energy into relativistic electrons and for magnetic field amplification.

A Study of Radio Knots within Supernova Remnant Cassiopeia A

The study on the dynamic evolution of young supernova remnants (SNRs) is an important way to understand the density structure of the progenitor’s circumstellar medium. We have reported the acceleration or deceleration, proper motion, and brightness changes of 260 compact radio features in the second-youngest known SNR Cas A at 5 GHz based on the Very Large Array data of five epochs from 1987 to 2004. The radio expansion center is located at α(1950) = 23h21m9.ˢ7 ± 0.ˢ29, δ(1950) = +58°32′25.″2 ± 2.″2. Three-quarters of the compact knots are decelerating; this suggests that there are significant density fluctuations in the stellar winds of the remnant’s progenitor. We have verified that the acceleration or deceleration of compact knots is not related with the distribution of brightness. The brightening, fading, disappearing, or new appearing of compact radio features in Cas A suggests that the magnetic field in the remnant is changing rapidly.

Long-term Evolution of Nonthermal Emission from Type Ia and Core-collapse Supernova Remnants in a Diversified Circumstellar Medium

The contribution of galactic supernova remnants (SNRs) to the origin of cosmic rays (CRs) is an important open question in modern astrophysics. Broadband nonthermal emission is a useful proxy for probing the energy budget and production history of CRs in SNRs. We conduct hydrodynamic simulations to model the long-term SNR evolution from explosion all the way to the radiative phase (or 3 × 105 yr at maximum) and compute the time evolution of the broadband nonthermal spectrum to explore its potential applications on constraining the surrounding environments, as well as the natures and mass-loss histories, of the SNR progenitors. A parametric survey is performed on the ambient environments separated into two main groups, namely, a homogeneous medium with a uniform gas density and one with the presence of a circumstellar structure created by the stellar wind of a massive red supergiant progenitor star. Our results reveal a highly diverse evolution history of the nonthermal emission closely correlated to the environmental characteristics of an SNR. Up to the radiative phase, the roles of CR reacceleration and ion−neutral wave damping on the spectral evolution are investigated. Finally, we make an assessment of the future prospect of SNR observations by the next-generation hard X-ray space observatory FORCE and predict what we can learn from their comparison with our evolution models.

An exploration of X-ray Supernova remnants in the Milky Way and nearby galaxies

ABSTRACT We probe the environmental properties of X-ray supernova remnants (SNRs) at various points along their evolutionary journey, especially the S-T phase, and their conformance with theoretically derived models of SNR evolution. The remnant size is used as a proxy for the age of the remnant. Our data set includes 34 Milky Way, 59 Large Magellanic Cloud (LMC), and 5 Small Magellanic Cloud (SMC) SNRs. We select remnants that have been definitively typed as either core-collapse (CC) or Type Ia supernovae, with well-defined size estimates, and a thermal X-ray flux measured over the entire remnant. A catalog of SNR size and X-ray luminosity is presented and plotted, with ambient density and age estimates from the literature. Model remnants with a given density, in the Sedov-Taylor (S-T) phase, are overplotted on the diameter-versus-luminosity plot, allowing the evolutionary state and physical properties of SNRs to be compared to each other, and to theoretical models. We find that small, young remnants are predominantly Type Ia remnants or high luminosity CCs, suggesting that many CC SNRs are not detected until after they have emerged from the progenitor’s wind-blown bubble. An examination of the distribution of SNR diameters in the Milky Way and LMC reveals that LMC SNRs must be evolving in an ambient medium which is 30 per cent as dense as that in the Milky Way. This is consistent with ambient density estimates for the Galaxy and LMC.

The Effects of Elemental Abundances on Fitting Supernova Remnant Models to Data

Models for supernova remnant (SNR) evolution can be used to determine the energy of the explosion, the age of the SNR, and the density of the surrounding medium by matching observations. Observed SNR properties derived from the X-ray spectrum include the electron temperature (kTe) and emission measure (EM) of the shocked gas. SNR models are based on hydrodynamic solutions for density, pressure, and velocity. The relations between these and kTe or EM depend on the three inputs of composition, ionization state, and electron-ion temperature ratio (Te/TI). The standard definitions and the XSPEC definitions for kTe and EM have important differences that are not well-known. The same definition used by observers of SNRs must be used in models for correct interpretation. Here, the effects of the three inputs on standard and on XSPEC versions of kTe and EM are investigated, with examples. The ratio of standard EM to the XSPEC value ranges widely, between ∼10−3 to ∼1, with smallest ratios for gas with low hydrogen abundance. The standard kTe differs from the XSPEC value by less than a few percent. For the illustrative example SNR J0049-7314, the ejecta component is shown to be consistent with core-collapse composition and a stellar wind environment.

Long-term Evolution of a Supernova Remnant Hosting a Double Neutron Star Binary

An ultra-stripped supernova (USSN) is a type of core-collapse supernova explosion proposed to be a candidate formation site of a double neutron star (DNS) binary. We investigate the dynamical evolution of an ultra-stripped supernova remnant (USSNR), which should host a DNS at its center. By accounting for the mass-loss history of the progenitor binary using a model developed by a previous study, we construct the large-scale structure of the circumstellar medium (CSM) up to a radius ∼100 pc, and simulate the explosion and subsequent evolution of a USSN surrounded by such a CSM environment. We find that the CSM encompasses an extended region characterized by a hot plasma with a temperature ∼108 K located around the termination shock of the wind from the progenitor binary (∼10 pc), and the USSNR blast wave is drastically weakened while penetrating through this hot plasma. Radio continuum emission from a young USSNR is sufficiently bright to be detectable if it inhabits our galaxy but faint compared to the observed Galactic supernova remnants (SNRs), and thereafter declines in luminosity through adiabatic cooling. Within our parameter space, USSNRs typically exhibit a low radio luminosity and surface brightness compared to the known Galactic SNRs. Due to the small event rate of USSNe and their relatively short observable life span, we calculate that USSNRs account for only ∼0.1%–1% of the total SNR population. This is consistent with the fact that no SNR hosting a DNS binary has been discovered in the Milky Way so far.

Spectral softening in core-collapse supernova remnant expanding inside wind-blown bubble

Context. Galactic cosmic rays (CRs) are widely assumed to arise from diffusive shock acceleration, specifically at shocks in supernova remnants (SNRs). These shocks expand in a complex environment, particularly in the core-collapse scenario as these SNRs evolve inside the wind-blown bubbles created by their progenitor stars. The CRs at core-collapse SNRs may carry spectral signatures of that complexity. Aims. We study particle acceleration in the core-collapse SNR of a progenitor with an initial mass of 60 M⊙ and realistic stellar evolution. The SNR shock interacts with discontinuities inside the wind-blown bubble and generates several transmitted and reflected shocks. We analyse their impact on particle spectra and the resulting emission from the remnant. Methods. To model the particle acceleration at the forward shock of a SNR expanding inside a wind bubble, we initially simulated the evolution of the pre-supernova circumstellar medium (CSM) by solving the hydrodynamic equations for the entire lifetime of the progenitor star. As the large-scale magnetic field, we considered parameterised circumstellar magnetic field with passive field transport. We then solved the hydrodynamic equations for the evolution of a SNR inside the pre-supernova CSM simultaneously with the transport equation for CRs in test-particle approximation and with the induction equation for the magnetohydrodynamics in 1D spherical symmetry. Results. The evolution of a core-collapse SNR inside a complex wind-blown bubble modifies the spectra of both the particles and their emission on account of several factors including density fluctuations, temperature variations, and the magnetic field configuration. We find softer particle spectra with spectral indices close to 2.5 during shock propagation inside the shocked wind, and this softness persists at later evolutionary stages. Further, our calculated total production spectrum released into the interstellar medium demonstrates spectral consistency at high energy (HE) with the injection spectrum of Galactic CRs, which is required in propagation models. The magnetic field structure effectively influences the emission morphology of SNRs as it governs the transportation of particles and the synchrotron emissivity. There is rarely a full correspondence of the intensity morphology in the radio, X-ray, and gamma-ray bands.

Physical Properties of the Supernova Remnant Population in the Small Magellanic Cloud

The X-ray emission from a supernova remnant is a powerful diagnostic of the state of its shocked plasma. The temperature and the emission measure are related to the energy of the explosion, the age of the remnant, and the density of the surrounding medium. Here we present the results of a study of the remnant population of the Small Magellanic Cloud. Progress in X-ray observations of remnants has resulted in a sample of 20 remnants in the Small Magellanic Cloud with measured temperatures and emission measures. We apply spherically symmetric supernova remnant evolution models to this set of remnants to estimate ages, explosion energies, and circumstellar medium densities. The distribution of ages yields a remnant birth rate of ∼1/1200 yr. The energies and densities are well fit with log-normal distributions, with means of 1.6 × 1051 erg and 0.14 cm−3, and 1σ dispersions of a factor of 1.87 in energy and 3.06 in density, respectively.

Investigation of the Physical Origin of Overionized Recombining Plasma in the Supernova Remnant IC 443 with XMM-Newton

Abstract The physical origin of the overionized recombining plasmas (RPs) in supernova remnants (SNRs) has been attracting attention because its understanding provides new insight into SNR evolution. However, the process of the overionization, although it has been discussed in some RP-SNRs, is not yet fully understood. Here, we report on spatially resolved spectroscopy of X-ray emission from IC 443 with XMM-Newton. We find that RPs in regions interacting with dense molecular clouds tend to have lower electron temperature and lower recombination timescale. These tendencies indicate that RPs in these regions are cooler and more strongly overionized, which is naturally interpreted as a result of rapid cooling by the molecular clouds via thermal conduction. Our result on IC 443 is similar to that on W44 showing evidence for thermal conduction as the origin of RPs at least in older remnants. We suggest that evaporation of clumpy gas embedded in a hot plasma rapidly cools the plasma as was also found in the W44 case. We also discuss if ionization by protons accelerated in IC 443 is responsible for RPs. Based on the energetics of particle acceleration, we conclude that the proton bombardment is unlikely to explain the observed properties of RPs.

Evaluation of hadronic emission in starburst galaxies and star-forming galaxies

In this work, we reanalyzed 11 years of spectral data from the Fermi Large Area Telescope (Fermi-LAT) of currently observed starburst galaxies (SBGs) and star-forming galaxies (SFGs). We used a one-zone model provided by NAIMA and the hadronic origin to explain the GeV observation data of the SBGs and SFGs. We found that a protonic distribution of a power-law form with an exponential cutoff can explain the spectra of most SBGs and SFGs. However, it cannot explain the spectral hardening components of NGC 1068 and NGC 4945 in the GeV energy band. Therefore, we considered the two-zone model to well explain these phenomena. We summarized the features of two model parameters, including the spectral index, cutoff energy, and proton energy budget. Similar to the evolution of supernova remnants (SNRs) in the Milky Way, we estimated the protonic acceleration limitation inside the SBGs to be the order of 102 TeV using the one-zone model; this is close to those of SNRs in the Milky Way.

The Post-impact Evolution of the X-Ray-emitting Gas in SNR 1987A as Viewed by XMM-Newton

Abstract Since 1996 the blast wave driven by SN 1987A has been interacting with the dense circumstellar material, which provides us with a unique opportunity to study the early evolution of a newborn supernova remnant (SNR). Based on the XMM-Newton RGS and EPIC-pn X-ray observations from 2007 to 2019, we investigated the post-impact evolution of the X-ray-emitting gas in SNR 1987A. The hot plasma is represented by two nonequilibrium ionization components with temperatures of ∼0.6 keV and ∼2.5 keV. The low-temperature plasma has a density ∼2400 cm−3, which is likely dominated by the lower-density gas inside the equatorial ring (ER). The high-temperature plasma with a density ∼550 cm−3 could be dominated by the H ii region and the high-latitude material beyond the ring. In the last few years, the emission measure of the low-temperature plasma has been decreasing, indicating that the blast wave has left the main ER. But the blast wave is still propagating into the high-latitude gas, resulting in the steady increase of the high-temperature emission measure. Meanwhile, the average abundances of N, O, Ne, and Mg are found to be declining, which may reflect the different chemical compositions between the two plasma components. We also detected Fe K lines in most of the observations, showing increasing flux and centroid energy. We interpret the Fe K lines as originating from a third hot component, which may come from the reflected shock heated gas or originate from Fe-rich ejecta clumps shocked by the reverse shock.