The time until black-hole formation in a binary neutron-star (NS) merger contains invaluable information about the nuclear equation of state (EOS) but has thus far been difficult to measure. We propose a new way to constrain the merger remnant's NS lifetime, which is based on the tendency of the NS-remnant neutrino-driven winds to enrich the ejected material with helium. Based on the He I lambda 1083.3 nm line, we show that the feature around 800-1200 nm in AT2017gfo at 4.4 days seems inconsistent with a helium mass fraction of XHe greater than or similar to 0.05 in the polar ejecta. Our recent neutrino-hydrodynamic simulations of merger remnants are only compatible with this limit if the NS remnant collapses within 20-30 ms. Such a short lifetime implies that the total binary mass of GW170817, Mtot, lay close to the threshold binary mass for direct gravitational collapse, Mthres, for which we estimate Mthres less than or similar to 2.93Mo. This upper bound on Mthres yields upper limits on the radii and maximum mass of cold, nonrotating NSs, which rule out simultaneously large values for both quantities. In combination with causality arguments, this result implies a maximum NS mass of Mmax less than or similar to 2.3Mo. We include an updated constraint yielding lower limits on NS radii from a previous argument that the remnant did not promptly collapse, which is independent of the consideration of the helium content. The combination of all limits constrains the radii of 1.6Mo NSs to about 12 + 1 km for Mmax = 2.0Mo and 11.5 + 1 km for Mmax = 2.15Mo. This similar to 2 km allowable range tightens significantly for Mmax above approximate to 2.15Mo. This rules out a significant number of current EOS models. The short NS lifetime also implies that a black-hole torus, not a highly magnetized NS, was the central engine powering the relativistic jet of GRB170817A. Our work motivates future developments to further corroborate and improve uncertainties in our chain of arguments regarding non-local-thermodynamic-equilibrium spectral modeling, helium production in merger outflows, and the dependence of the remnant lifetime on the binary mass, with the potential to tighten our constraints from existing data and in particular from future events. This novel method may provide a powerful tool to get a handle on the poorly constrained remnant lifetime, the still debated central engine of short gamma-ray bursts, and the high-density EOS.

Models of sub-Chandrasekhar mass double detonations for Type Ia supernovae (SNe Ia) suggest a distinguishing property of this scenario is unburnt helium in the outer ejecta. However, modern explosion simulations suggest there may be significant variations in its mass and velocity distribution. We recently presented a NLTE (non local thermodynamic equilibrium) radiative transfer simulation for one realization of the double detonation scenario with a modest He mass (0.018 M-circle dot) present in the ejecta at relatively high velocities (similar to 18000kms(-1)). That simulation predicted a He i 10 830 & Aring; feature blueward of Mg ii 10 927 & Aring; consistent with near-infrared observations of 'transitional' SNe Ia. To demonstrate the expected diversity in the helium signature, here we present a calculation for a double detonation model with a higher He mass (similar to 0.04 M-circle dot) ejected at lower velocities (similar to 13000kms(-1)). Despite our simulation predicting no clear optical or 2 mu m helium features, a strong and persistent He i 10 830 & Aring; absorption is present. The feature appears at wavelengths consistent with the extended blue wing of the Mg ii 10 927 & Aring; feature sometimes present in observations, suggesting this is a helium spectral signature (although for this particular model it is too strong and persistent to be consistent with normal SNe Ia). The significant differences in He i 10 830 & Aring; predicted by the two simulations suggest helium spectral signatures likely show significant variation throughout the SNe Ia population. This motivates further work to use this observable signature to test the parameter space for double detonation models.

The double detonation is a widely discussed explosion mechanism for Type Ia supernovae, whereby a helium shell detonation ignites a secondary detonation in the carbon/oxygen core of a white dwarf. Even for modern models that invoke relatively small He shell masses, many previous studies have found that the products of the helium shell detonation lead to discrepancies with normal Type Ia supernovae, such as strong Ti ii absorption features, extremely red light curves, and too large a variation with viewing direction. It has been suggested that non-local thermodynamic equilibrium (non-LTE) effects may help reduce these discrepancies between observations. Here we carry out full non-LTE radiative transfer simulations for a recent double detonation model with a relatively small helium mass of 0.05(circle dot). We construct 1D models representative of directions in a 3D explosion model to give an indication of viewing angle dependence, and show that at early times up to around maximum light, this gives a reasonable approximation of the different directions in the 3D model. This approximation breaks down once the ejecta start to become optically thin. The full non-LTE treatment leads to improved agreement between the models and observations. The light curves become less red, due to reduced absorption by the helium shell detonation products, since these species are more highly ionized. Additionally, the expected variation with observer direction is reduced. The full non-LTE treatment shows promising improvements and reduces the discrepancies between the double detonation models and observations of normal Type Ia supernovae.

Type Iax supernovae (SNe Iax) are proposed to arise from deflagrations of Chandrasekhar mass white dwarfs (WDs). Previous deflagration simulations have achieved good agreement with the light curves and spectra of intermediate-luminosity and bright SNe Iax. However, the model light curves decline too quickly after peak, particularly in red optical and near-infrared (NIR) bands. Deflagration models with a variety of ignition configurations do not fully unbind the WD, leaving a remnant polluted with $^{56}\mathrm{Ni}$. Emission from such a remnant may contribute to the luminosity of SNe Iax. Here we investigate the impact of adding a central energy source, assuming instantaneous powering by $^{56}\mathrm{Ni}$ decay in the remnant, in radiative transfer calculations of deflagration models. Including the remnant contribution improves agreement with the light curves of SNe Iax, particularly due to the slower post-maximum decline of the models. Spectroscopic agreement is also improved, with intermediate-luminosity and faint models showing greatest improvement. We adopt the full remnant $^{56}\mathrm{Ni}$ mass predicted for bright models, but good agreement with intermediate-luminosity and faint SNe Iax is only possible for remnant $^{56}\mathrm{Ni}$ masses significantly lower than those predicted. This may indicate that some of the $^{56}\mathrm{Ni}$ decay energy in the remnant does not contribute to the radiative luminosity but instead drives mass ejection, or that escape of energy from the remnant is significantly delayed. Future work should investigate the structure of remnants predicted by deflagration models and the potential roles of winds and delayed energy escape, as well as extend radiative transfer simulations to late times.

Recent analysis of the kilonova, AT2017gfo, has indicated that this event was highly spherical. This may challenge hydrodynamics simulations of binary neutron star mergers, which usually predict a range of asymmetries, and radiative transfer simulations show a strong direction dependence. Here we investigate whether the synthetic spectra from a 3D kilonova simulation of asymmetric ejecta from a hydrodynamical merger simulation can be compatible with the observational constraints suggesting a high degree of sphericity in AT2017gfo. Specifically, we determine whether fitting a simple P-Cygni line profile model leads to a value for the photospheric velocity that is consistent with the value obtained from the expanding photosphere method. We would infer that our kilonova simulation is highly spherical at early times, when the spectra resemble a blackbody distribution. The two independently inferred photospheric velocities can be very similar, implying a high degree of sphericity, which can be as spherical as inferred for AT2017gfo, demonstrating that the photosphere can appear spherical even for asymmetrical ejecta. The last-interaction velocities of radiation escaping the simulation show a high degree of sphericity, supporting the inferred symmetry of the photosphere. We find that when the synthetic spectra resemble a blackbody the expanding photosphere method can be used to obtain an accurate luminosity distance (within 4-7 per cent).

The spectral features observed in kilonovae have revealed the elemental composition and the velocity structures of matter ejected from neutron star mergers. In the spectra of the kilonova AT2017gfo, a P Cygni line at about 1 mu m has been linked to Sr II, providing the first direct evidence of freshly synthesised r-process material. An alternative interpretation of this feature has been proposed - He I lambda 1083.3 nm under certain non-local thermodynamic equilibrium conditions. A key way to robustly discriminate between these identifications, and indeed other proposed identifications, is to analyse the temporal emergence and evolution of the feature. In this analysis, we trace the earliest appearance of the observed feature and detail its spectro-temporal evolution, which we compare with a collisional-radiative model of helium. We show that the 1 mu m P Cygni line is inconsistent with a He I interpretation both in emergence time and in subsequent spectral evolution. Self-consistent helium masses cannot reproduce the observed feature due to the diminishing strength of radiative pathways out of triplet helium.

The detection of GW170817 and its electromagnetic counterpart AT2017gfo confirmed the expectation that a kilonova would accompany the merging of binary neutron stars, and subsequently there has been much interest in simulating kilonova emission to better understand the observations of AT2017gfo. The majority of models considered when predicting kilonova emission have been 1D, or even idealised toy models. Few simulations have been based on realistic merger simulations, and fewer have carried out full 3D simulations of the merger and subsequent kilonova emission. We present 3D radiative transfer simulations based on the dynamical ejecta from 3D smoothed-particle hydrodynamics neutron star merger simulations, including a sophisticated neutrino treatment. Nucleosynthesis calculations following the SPH trajectories provide the energy released due to radioactive decays of r-process material. We discuss the predicted light curves in different lines of sight, as well as the influence of the assumptions we make on the light curve evolution. This includes our assumption of opacities based on the electron fraction of the material, which is predominantly responsible for the distribution of r-process elements synthesised. We find that the light curves show a modest viewing angle dependence.

The double detonation is a widely discussed mechanism to explain Type Ia supernovae from explosions of sub-Chandrasekhar mass white dwarfs. In this scenario, a helium detonation is ignited in a surface helium shell on a carbon/oxygen white dwarf, which leads to a secondary carbon detonation. Explosion simulations predict high abundances of unburnt helium in the ejecta, however, radiative transfer simulations have not been able to fully address whether helium spectral features would form. This is because helium can not be sufficiently excited to form spectral features by thermal processes, but can be excited by collisions with non-thermal electrons, which most studies have neglected. We carry out a full non-local thermodynamic equilibrium (non-LTE) radiative transfer simulation for an instance of a double detonation explosion model, and include a non-thermal treatment of fast electrons. We find a clear He I λ 10830 feature which is strongest in the first few days after explosion and becomes weaker with time. Initially this feature is blended with the Mg II λ 10927 feature but over time separates to form a secondary feature to the blue wing of the Mg II λ 10927 feature. We compare our simulation to observations of iPTF13ebh, which showed a similar feature to the blue wing of the Mg II λ 10927 feature, previously identified as C I. Our simulation shows a good match to the evolution of this feature and we identify it as high velocity He I λ 10830. This suggests that He I λ 10830 could be a signature of the double detonation scenario.

The progenitor evolution and the explosion mechanism of Type Ia supernovae remain unexplained.Nonetheless, substantial progress has been made over the past years with three-dimensional hydrodynamic simulations of different scenarios.Here, we review some recent work pertaining to the leading paradigms of modeling: thermonuclear explosions of white dwarf stars near and below the Chandrasekhar mass limit.We discuss implications of the different explosion channels and their predictions of observables.

We present 3D radiative transfer calculations for the ejecta from a neutron star merger that include line-by-line opacities for tens of millions of bound–bound transitions, composition from an r -process nuclear network, and time-dependent thermalization of decay products from individual α and β − decay reactions. In contrast to expansion opacities and other wavelength-binned treatments, a line-by-line treatment enables us to include fluorescence effects and associate spectral features with the emitting and absorbing lines of individual elements. We find variations in the synthetic observables with both the polar and azimuthal viewing angles. The spectra exhibit blended features with strong interactions by Ce iii , Sr ii , Y ii , and Zr ii that vary with time and viewing direction. We demonstrate the importance of wavelength calibration of atomic data using a model with calibrated Sr, Y, and Zr data, and find major differences in the resulting spectra, including a better agreement with AT2017gfo. The synthetic spectra for a near-polar inclination show a feature at around 8000 Å, similar to AT2017gfo. However, they evolve on a more rapid timescale, likely due to the low ejecta mass (0.005 M ☉ ) as we take into account only the early ejecta. The comparatively featureless spectra for equatorial observers gives a tentative prediction that future observations of edge-on kilonovae will appear substantially different from AT2017gfo. We also show that 1D models obtained by spherically averaging the 3D ejecta lead to dramatically different direction-integrated luminosities and spectra compared to full 3D calculations.

The kilonova AT2017gfo that resulted from the merger of two neutron stars has provided new insights into the rapid neutron capture process that is responsible for producing many of the nuclei that are heavier than iron. As with supernovae, progress in understanding kilonova spectra can be achieved either by using simplified models to connect spectral features with particular elements, or by attempting to construct detailed simulations that capture all of the relevant physics. In the forward modelling approach, we require a theoretical simulations of the merger and ejection physics, r-process nucleosynthesis, radioactive energy deposition, and radiative transfer. We plan to calculate synthetic spectra for a three-dimensional merger and r-process nucleosynthesis simulation using the ARTIS Monte Carlo radiative transfer code. Here, we describe current progress in developing the code to handle energy deposition from beta- and alpha-decay reactions and thermalisation of decay particles.

The detection of GW170817 and the accompanying electromagnetic counterpart, AT2017gfo, have provided an important set of observational constraints for theoretical models of neutron star mergers, nucleosynthesis, and radiative transfer for kilonovae. We apply the 3D Monte Carlo radiative transfer code ARTIS to produce synthetic light curves of the dynamical ejecta from a neutron star merger, which has been modelled with 3D smooth-particle hydrodynamics (SPH) and included neutrino interactions. Nucleosynthesis calculations provide the energy released from radioactive decays of r-process nuclei, and radiation transport is performed using grey opacities given as functions of the electron fraction. We present line-of-sight dependent bolometric light curves, and find the emission along polar lines of sight to be up to a factor of ~2 brighter than along equatorial lines of sight. Instead of a distinct emission peak, our bolometric light curve exhibits a monotonic decline, characterised by a shoulder at the time when the bulk ejecta becomes optically thin. We show approximate band light curves based on radiation temperatures and compare these to the observations of AT2017gfo. We find that the rapidly declining temperatures lead to a blue to red colour evolution similar to that shown by AT2017gfo. We also investigate the impact of an additional, spherically symmetric secular ejecta component, and we find that the early light curve remains nearly unaffected, while after about 1 day the emission is strongly enhanced and dominated by the secular ejecta, leading to the shift of the shoulder from 1-2 to 6-10 days.

Dataset containing simulated spectra presented in the paper "Double detonations: variations in Type Ia supernovae due to different core and He shell masses – II. Synthetic observables" (ADS).

The majority of existing results for the kilonova (or macronova) emission from material ejected during a neutron-star (NS) merger is based on (quasi-)one-zone models or manually constructed toy-model ejecta configurations. In this study we present a kilonova analysis of the material ejected during the first ~10ms of a NS merger, called dynamical ejecta, using directly the outflow trajectories from general relativistic smoothed-particle hydrodynamics simulations including a sophisticated neutrino treatment and the corresponding nucleosynthesis results, which have been presented in Part I of this study. We employ a multi-dimensional two-moment radiation transport scheme with approximate M1 closure to evolve the photon field and use a heuristic prescription for the opacities found by calibration with atomic-physics based reference results. We find that the photosphere is generically ellipsoidal but augmented with small-scale structure and produces emission that is about 1.5-3 times stronger towards the pole than the equator. The kilonova typically peaks after 0.7-1.5days in the near-infrared frequency regime with luminosities between 3-7x10^40erg/s and at photospheric temperatures of 2.2-2.8x10^3K. A softer equation of state or higher binary-mass asymmetry leads to a longer and brighter signal. Significant variations of the light curve are also obtained for models with artificially modified electron fractions, emphasizing the importance of a reliable neutrino-transport modeling. None of the models investigated here, which only consider dynamical ejecta, produces a transient as bright as AT2017gfo. The near-infrared peak of our models is incompatible with the early blue component of AT2017gfo.

A leading model for Type Ia supernovae involves the double-detonation of a sub-Chandrasekhar mass white dwarf. Double-detonations arise when a surface helium shell detonation generates shockwaves that trigger a core detonation; this mechanism may be triggered via accretion or during the merger of binaries. Most previous double-detonation simulations only included the primary white dwarf; however, the fate of the secondary has significant observational consequences. Recently, hydrodynamic simulations accounted for the companion in double-degenerate double-detonation mergers. In the merger of a 1.05M_⊙ primary white dwarf and 0.7M_⊙ secondary white dwarf, the primary consistently detonates while the fate of the secondary remains uncertain. We consider two versions of this scenario, one in which the secondary survives and another in which it detonates. We present the first 3D radiative transfer calculations for these models and show that the synthetic observables for both models are similar and match properties of the peculiar 02es-like subclass of Type Ia supernovae. Our calculations show angle dependencies sensitive to the companion's fate, and we can obtain a closer spectroscopic match to normal Type Ia supernovae when the secondary detonates and the effects of helium detonation ash are minimised. The asymmetry in the width-luminosity relationship is comparable to previous double-detonation models, but the overall spread is increased with a secondary detonation. The secondary detonation has a meaningful impact on all synthetic observables; however, multidimensional nebular phase calculations are needed to support or rule out either model as a likely explanation for Type Ia supernovae.

We present the first results of a comprehensive supernova (SN) radiative-transfer (RT) code-comparison initiative (StaNdaRT), where the emission from the same set of standardized test models is simulated by currently-used RT codes. A total of ten codes have been run on a set of four benchmark ejecta models of Type Ia supernovae. We consider two sub-Chandrasekhar-mass ($M_\mathrm{tot} = 1.0$ M$_\odot$) toy models with analytic density and composition profiles and two Chandrasekhar-mass delayed-detonation models that are outcomes of hydrodynamical simulations. We adopt spherical symmetry for all four models. The results of the different codes, including the light curves, spectra, and the evolution of several physical properties as a function of radius and time, are provided in electronic form in a standard format via a public repository. We also include the detailed test model profiles and several python scripts for accessing and presenting the input and output files. We also provide the code used to generate the toy models studied here. In this paper, we describe in detail the test models, radiative-transfer codes and output formats and provide access to the repository. We present example results of several key diagnostic features.

Double detonations of sub-Chandrasekhar mass white dwarfs are a promising explosion scenario for Type Ia supernovae, whereby a detonation in a surface helium shell triggers a secondary detonation in a carbon-oxygen core. Recent work has shown that low mass helium shell models reproduce observations of normal SNe Ia. We present 3D radiative transfer simulations for a suite of 3D simulations of the double detonation explosion scenario for a range of shell and core masses. We find light curves broadly able to reproduce the faint end of the width-luminosity relation shown by SNe Ia, however, we find that all of our models show extremely red colours, not observed in normal SNe Ia. This includes our lowest mass helium shell model. We find clear Ti II absorption features in the model spectra, which would lead to classification as peculiar SNe Ia, as well as line blanketing in some lines of sight by singly ionised Cr and Fe-peak elements. Our radiative transfer simulations show that these explosion models remain promising to explain peculiar SNe Ia. Future full non-LTE simulations may improve the agreement of these explosion models with observations of normal SNe Ia.

The nebular spectra of Type Ia supernovae (greater than or similar to 100 d after explosion) consist mainly of emission lines from singly and doubly ionized Fe-group nuclei. However, theoretical models for many scenarios predict that non-thermal ionization leads to multiply ionized species whose recombination photons ionize and deplete Fe+, resulting in negligible [Fe II] emission. We investigate a method to determine the collisional excitation conditions from [Fe II] line ratios independently from the ionization state and find that it cannot be applied to highly ionized models due to the influence of recombination cascades on Fe+ level populations. When the ionization state is artificially lowered, the line ratios (and excitation conditions) are too similar to distinguish between explosion scenarios. We investigate changes to the treatment of non-thermal energy deposition as a way to reconcile overionized theoretical models with observations and find that a simple work function approximation provides closer agreement with the data for sub-M-ch models than a detailed Spencer-Fano treatment with widely used cross-section data. To quantify the magnitude of additional heating processes that would be required to sufficiently reduce ionization from fast leptons, we artificially boost the rate of energy loss to free electrons. We find that the equivalent of as much as an eight times increase to the plasma loss rate would be needed to reconcile the sub-M-ch model with observed spectra. Future studies could distinguish between reductions in the non-thermal ionization rates and increased recombination rates, such as by clumping.

Double detonations in sub-Chandrasekhar mass carbon-oxygen white dwarfs with helium shell are a potential explosion mechanism for a Type Ia supernova (SNe Ia). It comprises a shell detonation and subsequent core detonation. The focus of our study is on the effect of the progenitor metallicity on the nucleosynthetic yields. For this, we compute and analyse a set of eleven different models with varying core and shell masses at four different metallicities each. This results in a total of 44 models at metallicities between 0.01$Z_\odot$ and 3$Z_\odot$. Our models show a strong impact of the metallicity in the high density regime. The presence of $^{22}$Ne causes a neutron-excess which shifts the production from $^{56}$Ni to stable isotopes such as $^{54}$Fe and $^{58}$Ni in the $\alpha$-rich freeze-out regime. The isotopes of the metallicity implementation further serve as seed nuclei for additional reactions in the shell detonation. Most significantly, the production of $^{55}$Mn increases with metallicity confirming the results of previous work. A comparison of elemental ratios relative to iron shows a relatively good match to solar values for some models. Super-solar values are reached for Mn at 3$Z_\odot$ and solar values in some models at $Z_\odot$. This indicates that the required contribution of SNe Ia originating from Chandrasekhar mass WDs can be lower than estimated in orevious work to reach solar values of [Mn/Fe] at [Fe/H]$=0$. Our galactic chemical evolution models suggest that SNe Ia from sub-Chandrasekhar mass white dwarfs, along with core-collapse supernovae, could account for more than 80% of the solar Mn abundance. Using metallicity-dependent SN Ia yields helps to reproduce the upward trend of [Mn/Fe] as a function of metallicity for the solar neighborhood. These chemical evolution predictions, however, depend on the massive star yields adopted in the calculations.

Sub-Chandrasekhar mass carbon-oxygen white dwarfs with a surface helium shell have been proposed as progenitors of Type Ia supernovae (SNe Ia). If true, the resulting thermonuclear explosions should be able to account for at least some of the range of SNe Ia observables. To study this, we conducted a parameter study based on three-dimensional simulations of double detonations in carbon-oxygen white dwarfs with a helium shell, assuming different core and shell masses. An admixture of carbon to the shell and solar metallicity are included in the models. The hydrodynamic simulations were carried out using the A REPO code. This allowed us to follow the helium shell detonation with high numerical resolution, and this improves the reliability of predicted nucleosynthetic shell detonation yields. The addition of carbon to the shell leads to a lower production of 56 Ni, while including solar metallicity increases the production of intermediate mass elements. The production of higher mass elements is further shifted to stable isotopes at solar metallicity. Moreover, we find different core detonation ignition mechanisms depending on the core and shell mass configuration. This has an influence on the ejecta structure. We present the bolometric light curves predicted from our explosion simulations using the Monte Carlo radiative transfer code A RTIS and make comparisons with bolometric SNe Ia data. The bolometric light curves of our models show a range of brightnesses, which is able to account for subluminous to normal brightness SNe Ia. We show the model bolometric width-luminosity relation compared to data for a range of model viewing angles. We find that, on average, our brighter models lie within the observed data. The ejecta asymmetries produce a wide distribution of observables, which might account for outliers in the data. However, the models overestimate the extent of this compared to data. We also find that the bolometric decline rate over 40 days, Δm 40 (bol), appears systematically faster than data.