arXiv subjects

Ryo Sawada

Publications and source records attributed to Ryo Sawada.

At least 19 recordsLinked to original sources

arXiv2026

Interaction-powered Type Ibn Supernovae as a Transient PeVatron Candidate: The Case of SN 2023uqf

We investigate whether the Type Ibn supernova SN 2023uqf, reported close in time and direction to the $\sim$442 TeV IceCube alert IC-231004A, is physically consistent with a shock--circumstellar medium (CSM) interaction scenario. One-dimensional radiation-hydrodynamics calculations with {\tt STELLA} reproduce the ZTF optical light curves with a dense helium-rich CSM following $\rho_\mathrm{CSM} \propto r^{-3}$ and a CSM density parameter $D'\approx 50$. Using the shock evolution and CSM conditions obtained from the optical-fit RHD model, we model time-dependent cosmic-ray acceleration and hadronic neutrino production during the interaction phase. In our fiducial model, the inferred shock and CSM properties open a short-lived window in which multi-PeV hadron acceleration and efficient hadronic interactions can coexist. Additional loss-limited effects could reduce the high-energy cutoff, but the model provides a useful optical-to-neutrino consistency test for a transient PeVatron-like phase. After folding the predicted neutrino emission through the IceCube effective area, we obtain an expected number of $\sim10^{-5}-10^{-4}$ track-like events at $d = 723$ Mpc, depending on the alert selection. In the low-count regime, the model predicts a detection-time weighting for a rare event, and the detection time of IC-231004A falls within the high-weight interval while its energy scale is compatible with the modeled spectrum. Although a single event cannot establish a definitive association, our results show that the optically inferred environment of SN 2023uqf can satisfy the basic timing and energy requirements for a transient PeVatron-like phase in the fiducial model and illustrate how interaction-powered Type Ibn supernovae can be tested as high-energy neutrino sources.

astro-ph.HE↗

arXiv2026

Unifying Circumstellar Environment in Broad-Lined Type Ic Radio Supernovae Towards Off-axis Gamma-Ray Burst Exploration

Decades have passed since the first confirmed association between a broad-lined Type Ic supernova (Type IcBL SN) and a long gamma-ray burst (GRB), and the number of known GRB-SN associations has steadily increased. However, it is important to note that the radiation from GRB afterglows and the radio emission from SNe may be both produced by outflows evolving within the same ambient medium. In this study, we present the first comprehensive theoretical predictions of radio emission from a Type IcBL supernova associated with a GRB jet, explicitly accounting for the structure of the ambient medium. We model each component of the radio emission, with particular emphasis on exploring wide ranges of isotropic explosion energy and viewing angle in our GRB afterglow calculations. We show that, within specific regions of parameter space, the composite radio light curve exhibits a characteristic double-peaked structure. This clear double-peaked feature emerges when either (1) the isotropic explosion energy is small (low-luminosity GRB) or (2) the viewing angle is large (off-axis GRB). Our results demonstrate that follow-up radio observations carried out within a few years of the optical discovery of nearby Type IcBL SNe (-100 Mpc) can provide a unique diagnostic of off-axis GRBs that would otherwise remain undetected in Type IcBL SNe. This represents a step toward revealing the nature of long GRB progenitors and clarifying their connection to Type IcBL SNe.

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arXiv2025

Impacts of hydrogen envelope on supernova fallback and the resulting compact remnant masses

Fallback in core-collapse supernovae plays a central role in setting compact-remnant masses and may produce late-time emission. In hydrogen rich progenitors, the reverse shock arising at the hydrogen-helium interface has the potential to dramatically enhance fallback, yet its overall impact across a broad explosion-energy range has not been systematically quantified. Using one-dimensional hydrodynamic simulations for metal-poor progenitors with $M_{\rm ZAMS}=18$-$28\,M_\odot$ and models with and without hydrogen envelopes, we explore fallback over explosion energies of $10^{48}$-$10^{52}\,{\rm erg}$. We find a robust and universal mass-transition behaviour: when the explosion energy reaches only $2$-$3$ times the binding energy of the hydrogen envelope, the reverse shock returns to the centre and sharply increases the remnant mass by $\gtrsim 2\,M_\odot$. Above this threshold, the reverse shock escapes and hydrogen-rich and stripped-envelope progenitors yield nearly identical remnant masses. By normalizing the results with the envelope binding energy, we show that all progenitor models converge to a common fallback relation. We further provide a simple analytic prescription that connects explosion energy, hydrogen-envelope binding energy, and final compact-remnant mass. This relation provides an important link between progenitor properties and compact-remnant masses, and is useful for population-synthesis and galactic chemical-evolution studies.

astro-ph.HE↗

arXiv2025

Cosmic-Ray Bath in a Past Supernova Gives Birth to Earth-Like Planets

A key question in astronomy is how ubiquitous Earth-like rocky planets are. The formation of terrestrial planets in our solar system was strongly influenced by the radioactive decay heat of short-lived radionuclides (SLRs), particularly $^{26}$Al, likely delivered from nearby supernovae. However, current models struggle to reproduce the abundance of SLRs inferred from meteorite analysis without destroying the protosolar disk. We propose the `immersion' mechanism, where cosmic-ray nucleosynthesis in a supernova shockwave reproduces estimated SLR abundances at a supernova distance ($\sim$1 pc), preserving the disk. We estimate that solar-mass stars in star clusters typically experience at least one such supernova within 1 pc, supporting the feasibility of this scenario. This suggests solar-system-like SLR abundances and terrestrial planet formation are more common than previously thought.

astro-ph.EP↗

arXiv2025

Inhomogeneous stellar mixing in the final hours before the Cassiopeia A supernova

Understanding stars and their evolution is a key goal of astronomical research and has long been a focus of human interest. In recent years, theorists have paid much attention to the final interior processes within massive stars, as they can be essential for revealing neutrino-driven supernova mechanisms and other potential transients of massive star collapse. However, it is challenging to observe directly the last hours of a massive star before explosion, since it is the supernova event that triggers the start of intense observational study. Here we report evidence for a final phase of stellar activity known as a ``shell merger'', an intense shell burning in which the O-burning shell swallows its outer C-/Ne-burning shell, deep within the progenitor's interior moments before the supernova explosion. In the violent convective layer created by the shell merger, Ne, which is abundant in the stellar O-rich layer, is burned as it is pulled inward, and Si, which is synthesized inside, is transported outward. The remnant still preserves some traces of such Ne-rich downflows and Si-rich upflows in the O-rich layer, suggesting that inhomogeneous shell-merger mixing began just hours ($\lesssim 10^4$ s) before its gravitational collapse. Our results provide the first observational evidence that the final stellar burning process rapidly alters the internal structure, leaving a pre-supernova asymmetry. This breaking of spherical symmetry facilitates the explosion of massive stars and influences various supernova and remnant characteristics, including explosion asymmetries and the neutron star's kick and spin.

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arXiv2025

Expectation for the MeV Gamma-Ray Emission from Pair-Instability Supernovae

Pair-instability supernovae (PISNe) are predicted thermonuclear explosions of massive stars with helium core masses exceeding $\sim 65M_\odot$ and synthesize substantial amounts of radioactive $\mathrm{^{56}Ni}$ ($M(\mathrm{^{56}Ni})\sim60M_\odot$ in extreme cases). To investigate their observational signatures, we developed a multi-D Monte Carlo radiation transport code, assuming spherical symmetry in the background medium and the photon sources distribution, and performed simulations of gamma-ray and hard X-ray emissions from the decay chain $\mathrm{^{56}Ni}\to\mathrm{^{56}Co}\to\mathrm{^{56}Fe}$. We find that key gamma-ray lines (847 and 1238 keV) from $\mathrm{^{56}Co}$ decay in the $130M_\odot$ helium core model can be detected up to 300-400 Mpc by next-generation MeV gamma-ray telescopes. In contrast, the signals from the $100M_\odot$ model remain below the detection limits. Our results provide the template for gamma-ray follow-up observations of PISNe. Considering theoretical predictions and observational constraints, we estimate PISN event rates within 300 Mpc to be approximately 0.01-0.1 events per year, highlighting their rarity but also emphasizing their feasibility as targets for future gamma-ray observations over the decade.

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arXiv2025

Towards Multi Energy Neutrino Astronomy: Diagnosing Enhanced Circumstellar Material around Stripped-Envelope Supernovae

A novel approach is proposed to reveal a secret birth of enhanced circumstellar material (CSM) surrounding a collapsing massive star using neutrinos as a unique probe. In this scheme, non-thermal TeV-scale neutrinos produced in ejecta-CSM interactions are tied with thermal MeV neutrinos emitted from a pre-explosion burning process, based on a scenario that CSM had been formed via the pre-supernova activity. Taking a representative model of the pre-supernova neutrinos, the spectrum and light curve of the corresponding high-energy CSM neutrinos are calculated at multiple mass-loss efficiencies, which are considered as a systematic uncertainty. In addition, as a part of the method demonstration, the detected event rates along time at JUNO and IceCube, as representative detectors, are estimated for the pre-supernova and CSM neutrinos, respectively, and are compared with the expected background rate at each detector. The presented method is found to be reasonably applicable for the range up to $\sim$1 kpc and even farther with future experimental efforts. The potentialities of other neutrino detectors, such as SK-Gd, Hyper-Kamiokande and KM3NeT, are also discussed. This is a pioneering work of performing astrophysics with neutrinos from diverse energy regimes, initiating multi energy neutrino astronomy in the forthcoming era where next-generation large-scale neutrino telescopes are operating.

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arXiv2024

Progenitor Constraint Incorporating Shell Merger: The Case of Supernova Remnant G359.0-0.9

It is generally hard to put robust constraints on progenitor masses of supernovae (SNe) and remnants (SNRs) observationally, while they offer tantalizing clues to understanding explosion mechanisms and mass distribution. Our recent study suggests that ``shell merger'', which is theoretically expected for stellar evolution, can appreciably affect final yields of inter-mediate mass elements (IMEs; such as Ne, Mg, and Si). In light of this, here we report results of X-ray spectral analysis of a Galactic SNR G359.0-0.9, whose abundance pattern may possibly be anomalous according to a previous study. Our spectroscopy using all the available data taken with XMM-Newton reveals that this remnant is classified as Mg-rich SNRs because of its high Mg-to-Ne ratio (Z_Mg/Z_Ne=1.90+0.27-0.19; mass ratio 0.66+0.09-0.07) and conclude that the result cannot be explained without the shell merger. By comparing the observation with theoretical calculations, we prefer the so-called Ne-burning shell intrusion and in this case the progenitor mass M_ZAMS is likely <15M_sun. We confirm the result also by our new molecular line observations with the NRO-45 m telescope: G359.0-0.9 is located in the Scutum-Centaurus arm (2.66--2.94 kpc) and in this case the resultant total ejecta mass ~6.8M_sun is indeed consistent with the above estimate. Our method using mass ratios of IMEs presented in this paper will become useful to distinguish the type of the shell merger, the Ne-burning shell intrusion and the O-burning shell merger, for future SNR studies.

astro-ph.HE↗

arXiv2024

Impacts of the $^{12}\rm{C}\left(α,γ\right)^{16}\!\rm{O}$ reaction rate on $^{56}{\rm Ni}$ nucleosynthesis in pair-instability supernovae

Nuclear reactions are key to our understanding of stellar evolution, particularly the $^{12}\rm{C}\left(α,γ\right)^{16}\!\rm{O}$ rate, which is known to significantly influence the lower and upper ends of the black hole (BH) mass distribution due to pair-instability supernovae (PISNe). However, these reaction rates have not been sufficiently determined. We use the $\texttt{MESA}$ stellar evolution code to explore the impact of uncertainty in the $^{12}\rm{C}\left(α,γ\right)^{16}\!\rm{O}$ rate on PISN explosions, focusing on nucleosynthesis and explosion energy by considering the high resolution of the initial mass. Our findings show that the mass of synthesized radioactive nickel ($^{56}{\rm Ni}$) and the explosion energy increase with $^{12}\rm{C}\left(α,γ\right)^{16}\!\rm{O}$ rate for the same initial mass, except in the high-mass edge region. With a high (about twice the $\texttt{STARLIB}$ standard value) rate, the maximum amount of nickel produced falls below 70 $M_\odot$, while with a low rate (about half of the standard value) it increases up to 83.9 $M_\odot$. These results highlight that carbon "preheating" plays a crucial role in PISNe by determining core concentration when a star initiates expansion. Our results also suggest that the onset of the expansion, which means the end of compression, competes with collapse caused by helium photodisintegration, and the maximum mass that can lead to an explosion depends on the $^{12}\rm{C}\left(α,γ\right)^{16}\!\rm{O}$ reaction rate.

astro-ph.SR↗

arXiv2024

Gravitational Collapse of White Dwarfs to Neutron Stars. I. From Initial Conditions to Explosions with Neutrino-radiation Hydrodynamics Simulations

This paper provides collapses of massive, fully convective, and non-rotating white dwarfs (WDs) formed by accretion-induced collapse or merger-induced collapse and the subsequent explosions with the general relativistic neutrino-radiation hydrodynamics simulations. We produce initial WDs in hydrostatic equilibrium, which have super-Chandrasekhar mass and are about to collapse. The WDs have masses of 1.6$M_\odot$ with different initial central densities specifically at $10^{10}$, $10^{9.6}$, $10^{9.3}$ and $10^{9.0}\,{\rm g\,cm^{-3}}$. First, we check whether initial WDs are stable without weak interactions. Second, we calculate the collapse of WDs with weak interactions. We employ hydrodynamics simulations with Newtonian gravity in the first and second steps. Third, we calculate the formation of neutron stars and accompanying explosions with general relativistic simulations. As a result, WDs with the highest density of $10^{10}\,{\rm g\,cm^{-3}}$ collapse not by weak interactions but by the photodissociation of the iron, and three WDs with low central densities collapse by the electron capture as expected at the second step and succeed in the explosion with a small explosion energy of $\sim 10^{48}$ erg at the third step. By changing the surrounding environment of WDs, we find that there is a minimum value of ejecta masses being $\sim 10^{-5}M_{\odot}$. With the most elaborate simulations of this kind so far, the value is one to two orders of magnitude smaller than previously reported values and is compatible with the estimated ejecta mass from FRB~121102.

astro-ph.HE↗

arXiv2024

Destratification in the Progenitor Interior of the Mg-rich Supernova Remnant N49B

Simulations of pre-supernova evolution suggest that some intense shell burning can be so active that, in extreme cases, it can merge with the outer shell, changing the initial conditions for the supernova explosion. However, such violent activity in the interior of stars has been difficult to confirm from observations of stars. Here we propose that the elemental composition of O-rich ejecta in supernova remnants can be a tool to test for this kind of intense shell burning activity in the final stages of progenitor evolution. As an example, we discuss the origin of "Mg-rich" ejecta in the supernova remnant N49B. A high Mg/Ne mass ratio $\gtrsim 1$ suggests that the Ne- or O-burning shell has broken into or merged with the outer shell before the collapse. Such Mg-rich (or Ne-poor) ejecta has been identified in some other supernova remnants, supporting the idea that some destratification process, such as a shell merger, does indeed occur in the interiors of some massive stars, although they may not be the majority. Our results suggest that X-ray observations of O-rich ejecta in core-collapse supernova remnants will be a unique tool to probe the shell burning activity during the final stage of a massive star's interior.

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arXiv2024

Self-consistent Conditions for $^{26}$Al Injection into Protosolar Disk from a Nearby Supernova

The early solar system contained a short-lived radionuclide, $^{26}$Al (its half-life time $t_{1/2} = 0.7$ Myr). The decay energy $^{26}$Al is thought to have controlled the thermal evolution of planetesimals and, possibly, the water contents of planets. Many hypotheses have been proposed for the origin of $^{26}$Al in the solar system. One of the possible hypotheses is the `disk injection scenario'; when the protoplanetary disk of the solar system had already formed, a nearby $(<1 \,\mathrm{pc})$ supernova injected radioactive material directly into the disk. Such a $^{26}$Al injection hypothesis has been tested so far with limited setups for disk structure and supernova distance, and treated disk disruption and $^{26}$Al injection separately. Here, we revisit this problem to investigate whether there are self-consistent conditions under which the surviving disk radius can receive enough $^{26}$Al which can account for the abundance in the early solar system. We also consider a range of disk mass and structure, $^{26}$Al yields from supernova, and a large dust mass fraction $η_\mathrm{d}$. We find that $^{26}$Al yields of supernova are required as $\gtrsim 2.1\times10^{-3}M_\odot(η_\mathrm{d}/0.2)^{-1}$, challenging to achieve with known possible $^{26}$Al ejection and dust mass fraction ranges. Furthermore, we find that even if the above conditions are met, the supernova flow changes the disk temperature, which may not be consistent with the solar-system record. Our results place a strong constraint on the disk injection scenario. Rather, we suggest that the fresh $^{26}$Al of the early solar system must have been synthesized/injected in other ways.

astro-ph.EP↗

arXiv2023

Binary Interaction Can Yield a Diversity of Circumstellar Media around Type II Supernova Progenitors

Recent observations of supernovae (SNe) have indicated that a fraction of massive stars possess dense circumstellar medium (CSM) at the moment of their core collapses. They suggest the presence of additional activities of the SN progenitor driving the enhancement of the mass-loss rate, and some physical processes attributing to single star's activities have been considered. In this study, we carry out binary evolutionary simulations of massive stars with the aim of investigating the CSM structure. We show that the mass-transfer rate in a binary can increase at the beginning of the Roche lobe overflow, and this enhancement would be associated with the structure of the CSM before the explosion. We also illustrate that depending on the orbital period of the binary, the density structure of the CSM can have a diverse distribution including shell-like and cliff-like structures. These characteristic structures appear within the lengthscale of $\sim 10^{17}\,{\rm cm}$ and could be traced by long-term observations of SNe, if the slow velocity of the CSM is assumed ($\sim 10\,{\rm km}\,{\rm s}^{-1}$). Our results highlight the importance of binary interaction in the aspect of reproducing the diversity of the CSM configuration.

astro-ph.SR↗

arXiv2023

Updating the $^{56}$Ni Problem in Core-collapse Supernova Explosion

Details of the core-collapse supernova (CCSN) explosion mechanism still need to be fully understood. There is an increasing number of successful examples of reproducing explosions in multidimensional hydrodynamic simulations, but subsequent studies pointed out that the growth rates of the explosion energy $\dot{E}_\mathrm{expl}$ of these simulations are insufficient to produce enough $^{56}$Ni to match observations. This issue is known as the `$^{56}$Ni problem' in CCSNe. Recently, however, some studies have suggested that this $^{56}$Ni problem is derived from the simplicity of the explosion model. In response, we investigate the effect of the explosion energy growth rate $\dot{E}_\mathrm{expl}$ on the behavior of nucleosynthesis in CCSNe in a more realistic model. We employ the 1D Lagrangian hydrodynamic code, in which we take neutrino heating and cooling terms into account with the light-bulb approximation. We reiterate that, consistent with previous rebuttal studies, there is the $^{56}$Ni problem: Although $^{56}$Ni is synthesized to almost the same mass coordinate independent of $\dot{E}_\mathrm{expl}$, some of the innermost material in the low-$\dot{E}_\mathrm{expl}$ model failed to escape, leading to a shift in the innermost mass coordinate of the ejecta to the outer positions. Comparing our results with observations, we find that while modern slow explosions can, in principle, reproduce observations of standard Type II SNe, this is not possible with stripped-envelope SNe. Our finding places a strong constraint on the explosion mechanism. There are significant differences in the progenitor structures and the explosion mechanism between Type II and stripped-envelope SNe.

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arXiv2022

Constraints on Explosion Timescale of Core-Collapse Supernovae Based on Systematic Analysis of Light Curves

Explosion mechanism of core-collapse supernovae is not fully understood yet. In this work, we give constraints on the explosion timescale based on $^{56}$Ni synthesized by supernova explosions. First, we systematically analyze multi-band light curves of 82 stripped-envelope supernovae (SESNe) to obtain bolometric light curves, which is among the largest samples of the bolometric light curves of SESNe derived from the multi-band spectral energy distribution. We measure the decline timescale and the peak luminosity of the light curves and estimate the ejecta mass ($M_{\rm ej}$) and $^{56}$Ni mass ($M_{\rm Ni}$) to connect the observed properties with the explosion physics. We then carry out one-dimensional hydrodynamics and nucleosynthesis calculations, varying the progenitor mass and the explosion timescale. From the calculations, we show that the maximum $^{56}$Ni mass that $^{56}$Ni-powered SNe can reach is expressed as $M_{\rm Ni} \lesssim 0.2 \ M_{\rm ej}$. Comparing the results from the observations and the calculations, we show that the explosion timescale shorter than 0.3 sec explains the synthesized $^{56}$Ni mass of the majority of the SESNe.

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arXiv2022

X-raying the Birth of Binary Neutron Stars and Neutron Star-Black Hole Binaries

We consider fallback accretion after an ultra-stripped supernova (USSN) that accompanies formation of a binary neutron star (BNS) or a neutron star-black hole binary (NS-BH). The fallback matter initially accretes directly to the nascent NS, while it starts to accrete to the circumbinary disk, typically $0.1\mbox{-}1\, \mathrm{day}$ after the onset of the USSN explosion. The circumbinary disk mass further accretes, forming mini disks around each compact object, with a super-Eddington rate up to a few years. We show that such a system constitutes a binary ultraluminous X-ray source (ULX), and a fraction of the X rays can emerge through the USSN ejecta. We encourage follow-up observations of USSNe within $\lesssim 100\,\rm Mpc$ and $\sim 100\mbox{-}1,000\,\mathrm{day}$ after the explosion using Chandra, XMM Newton and NuSTAR, which could detect the X-ray counterpart with time variations representing the properties of the nascent compact binary, e.g., the orbital motion of the binary, the spin of the NS, and/or the quasiperiodic oscillation of the mini disks.

astro-ph.HE↗

arXiv2022

On the energy source of ultra-stripped supernovae

Ultra-stripped supernovae (USSN) with a relatively low ejecta mass of $\sim0.1M_\odot$ (e.g., iPTF14gqr and SN2019dge) are considered to originate from ultra-stripped carbon-oxygen (CO) cores in close binary systems and are likely to be progenitors of binary neutron stars (BNSs). Here we conduct long-term simulations of USSNe from ultra-stripped progenitors with various masses ($1.45\,M_\odot \leq M_\mathrm{CO} \leq 2.0\,M_\odot$) based on results of neutrino-radiation hydrodynamics simulations, and consistently calculate the nucleosynthesis and the SN light curves. We find that a USSN from a more massive progenitor has a larger ejecta mass but a smaller $^{56}$Ni mass mainly due to the fallback, which leads to the light curve being dimmer and slower. By comparing the synthetic light curves with the observed ones, we show that SN2019dge can be solely powered by $^{56}$Ni synthesized during the explosion of a progenitor with $M_\mathrm{CO} \lesssim 1.6\,M_\odot$ while iPTF14gqr cannot be explained by the $^{56}$Ni powered model; $\sim 0.05M_\odot$ of $^{56}$Ni inferred from the light curve fitting is argued to be difficult to synthesize for ultra-stripped progenitors. We consider fallback accretion onto and rotation-powered relativistic wind from the newborn NS as alternative energy sources and show that iPTF14gqr could be powered by a newborn NS with a magnetic field of $B_p \sim 10^{15}\,\mathrm{G}$ and an initial rotation period of $P_i \sim 0.1\,\mathrm{s}$.

astro-ph.HE↗

arXiv2021

Are stripped envelope supernovae really deficient in $^{56}$Ni?

Recent works have indicated that the $^{56}$Ni masses estimated for Stripped Envelope SNe (SESNe) are systematically higher than those estimated for SNe II. Although this may suggest a distinct progenitor structure between these types of SNe, the possibility remains that this may be caused by observational bias. One important possible bias is that SESNe with low $^{56}$Ni mass are dim, and therefore they are more likely to escape detection. By investigating the distributions of the $^{56}$Ni mass and distance for the samples collected from the literature, we find that the current literature SESN sample indeed suffers from a significant observational bias, i.e., objects with low $^{56}$Ni mass - if they exist - will be missed, especially at larger distances. Note, however, that those distant objects in our sample are mostly SNe Ic-BL. We also conducted mock observations assuming that the $^{56}$Ni mass distribution for SESNe is intrinsically the same with that for SNe II. We find that the $^{56}$Ni mass distribution of the detected SESNe samples moves toward higher mass than the assumed intrinsic distribution, because of the difficulty in detecting the low-$^{56}$Ni mass SESNe. These results could explain the general trend of the higher $^{56}$Ni mass distribution (than SNe II) of SESNe found thus far in the literature. However, further finding clear examples of low-$^{56}$Ni mass SESNe ($\leq 0.01M_{\odot}$) is required to add weight to this hypothesis. Also, the objects with high $^{56}$Ni mass ($\gtrsim 0.2 M_{\odot}$) are not explained by our model, which may require an additional explanation.

astro-ph.HE↗