Spectral modelling of kilonovae (KNe)requireslarge amounts of collisional excitation and photoionization atomic data for lowly ionized (neutral,singly, and doubly ionized)species of heavy elements. Much of the data currently used is calculated using approximate hydrogenic results or adoptssemi-empirical formulae. We present atomic data forions of tellurium (Te) computed using the well-known R-matrix method. Results will also be presented for radiative and thermal collisions of Te iv and v, for which the required atomic data are also typically limited in the literature. The Multi-Configuration-Dirac- Hartree-Fock method is used to produce model atomic structures and radiative rates. These modelstructures are then used to calculate electron-impact-excitation and photoionization cross-sections. The resulting excitation and radiative rates are further used in a collisional radiative model to produce synthetic spectra, which are compared with observations. We also investigate the possibility of Te iv contributing to the 1.08 mu m emission feature in the mid-epochs of AT2017gfo alongside the established P-Cygni feature of Sr ii.

Mergers of neutron stars are believed to be one of the primary sites for the synthesis of the universe's heavy elements via the rapid neutron capture process. AT2017gfo, the kilonova following GW170817 provided the first direct spectroscopic evidence of the r-process happening in the universe. A prominent line feature near 1 μm in its spectrum was attributed to strontium – a claim that has been independently recovered by several teams. However, in recent years it has been debated whether the feature arises instead from helium. Here, we present non–local thermodynamic equilibrium (NLTE) radiative transfer modelling of the observed kilonova spectra, including detailed radiation-matter interaction physics for both strontium and helium. We make use of freshly calculated strontium atomic data for e^- impact collisions, photoionization, and recombination processes. Our strontium model self-consistently reproduces the temporal evolution of the 1 μm feature at early times, with its absence at 0.92days to its clear emergence at 1.17days. This transition mimics LTE, because at early epochs (t≲ 1.5days) the radiation field dominates the ionization state of the ejecta over thermal and non-thermal electron collisions. We further test if helium can form the feature under the same plasma conditions. The helium mass required at 1.17days is comparable to the total ejecta mass, while a few percent by mass of helium suffices at 4.4 days. On the other hand, the strength of the strontium lines decrease with time, and may require a radially stratified abundance to consistently produce the feature. We conclude that strontium is required to explain the onset of the feature at early times, but helium can contribute to, or even dominate the feature at later epochs.

Little red dots (LRDs) are candidate high-redshift supermassive black holes accreting in dense gas. They remain undetected in X-rays. In previous work, we provided the first quantitative models that reproduce the optical and near-infrared spectra of LRDs with the radiative transfer code, thereby constraining the properties of the surrounding gas. Here, we use these constraints to predict the X-ray attenuation produced by dense gas cocoons, and explore its dependence on Balmer-break strength, metallicity, intrinsic X-ray spectral energy distribution, and observed bandpass as a function of redshift. The X-ray constraints are very tight, requiring extinction by a Compton-thick gas column (N_ H∼10^25 cm^-2) with moderate metallicity 0.05-0.1 Z_⊙ and intrinsically weak X-ray emission (bolometric to X-ray luminosity ratio, k_ bol,X≳ 30), as observed in narrow-line active galactic nuclei with high accretion rates, to make LRDs sufficiently faint to evade detection. Intrinsically bright X-ray emitters as seen in typical broad-line active galactic nuclei would be detected even behind the typical Compton-thick gas columns with modest metallicity that were inferred from the optical spectra. Very low metallicity objects might be detected in X-rays even with low intrinsic X-ray luminosities, suggesting that LRDs are not (currently) chemically pristine.

Context. Thermonuclear electron-capture supernovae (tECSNe) are a potential fate of certain intermediate mass stars forming ONe cores at the end of their evolution. While simulations suggest that these explosions are a viable alternative to their collapsing counterpart, no synthetic observables exist that would allow for their identification among observed transients. Aims. In this work, we present first of their kind synthetic observables of a tECSN explosion simulation, aiming to establish whether these explosions can occur in nature. Additionally, we investigate potential observational signatures that could be used to separate these explosions from other similar astronomical transients such as pure deflagrations in CO white dwarfs. Methods. We carry out 3D photospheric phase and 1D late phase simulations using the Monte Carlo radiative transfer code ARTIS. As input, we use a tECSN explosion simulation and a CO deflagration simulation with comparable 56Ni production, both of which have been computed with the LEAFS code. Results. We find that both models have similar observational characteristics, akin to SNe Iax-like events. The ejecta of the tECSN model are characterized by a M(56Ni)/Mej ratio 25% lower than that of comparable CO deflagration models. At early times, the tECSN model is characterized by a slower decline in the red colors compared to the CO deflagration due the greater amount of Ti and Cr synthesized in the tECSN explosion, leading to an increased absorption from these elements at blue wavelengths and subsequent fluorescence to red wavelengths. At late times, the tECSN model exhibits an exceptionally strong 12.8 μm Ne II emission line, which strengthens substantially over time, whereas its strength remains largely unchanged in the CO deflagration model. Conclusions. Our results suggest that tECSNe could potentially result in SN Iax-like transients. Importantly, we did not find any features that are in tension with existing observables. So far, there are no indicators that unambiguously and robustly separate tECSNe from deflagrations in CO white dwarfs. Nonetheless, our work highlights the potential importance of the mid-infrared wavelength range for distinguishing possible explosion mechanisms.

Radiation pressure on spectral lines is a promising mechanism for powering disc winds from accreting white dwarfs (AWDs) and active galactic nuclei (AGNs). However, in radiation-hydrodynamic simulations, overionization reduces line opacity and quenches the line force, which suppresses outflows. Here, we show that small-scale clumping can resolve this problem. Adopting the microclumping approximation, our new simulations demonstrate that even modest volume filling factors (fV similar to 0 . 1-0 . 01) can dramatically increase the wind mass-loss rate by lowering its ionization state-raising <(M)over dot(wind) and yielding <(M)over dot(wind)/<(M)over dot(acc) greater than or similar to 10(-4)for such modest filling factors. Clumpy wind models produce the UV resonance lines that are absent from smooth wind models. They can also reprocess a significant fraction of the disc luminosity and thus dramatically modify the broad-band optical/UV SED. Given that theory and observations indicate that disc winds are intrinsically inhomogeneous, clumping offers a physically motivated solution. Together, these results provide the first robust, self-consistent demonstration that clumping can reconcile line-driven wind theory with observations across AWDs and AGNs.

”Little Red Dots” (LRDs) are broad-line sources at high redshift, initially identified by their compact morphologies, red colours and prominent Balmer breaks. The origin of their optical-to-near-infrared continua is debated, with proposed explanations ranging from direct recombination emission to thermalised blackbodies from stellar-like atmospheres. Here we report evidence for Paschen jumps in a subset of LRDs, consistent with free-bound recombination to hydrogen n=3. The Paschen and Brackett continuum shapes across the sample are consistent with minimally reddened emission from low-temperature gas with T_e≲10 000 K, while the presence of Paschen jump signatures limits scenarios in which the emission is thermalised. Further, the extreme Hα equivalent widths and the tight observed correlation between Hα and the continuum follow naturally if both originate in recombination emission. This provides an observational upper limit on the contribution of any direct AGN accretion component and any stellar-atmosphere-like component, as well as on the fraction of line emission that can be thermalised as it traverses the cocoon. Ultimately, nebular radiative-transfer models provide a self-consistent explanation of the continuum, line strengths and line profiles without requiring multiple separately fitted components.

Context. Thermonuclear electron-capture supernovae (tECSNe) are a potential fate of certain intermediate mass stars forming ONe cores at the end of their evolution. While simulations suggest that these explosions are a viable alternative to their collapsing counterpart, no synthetic observables exist that would allow for their identification among observed transients. Aims. In this work, we present first of their kind synthetic observables of a tECSN explosion simulation, aiming to establish whether these explosions can occur in nature. Additionally, we investigate potential observational signatures that could be used to separate these explosions from other similar astronomical transients such as pure deflagrations in CO white dwarfs. Methods. We carry out 3D photospheric phase and 1D late phase simulations using the Monte Carlo radiative transfer code ARTIS. As input, we use a tECSN explosion simulation and a CO deflagration simulation with comparable Ni-56 production, both of which have been computed with the LEAFS code. Results. We find that both models have similar observational characteristics, akin to SNe Iax-like events. The ejecta of the tECSN model are characterized by a M(Ni-56)/M-ej ratio 25% lower than that of comparable CO deflagration models. At early times, the tECSN model is characterized by a slower decline in the red colors compared to the CO deflagration due the greater amount of Ti and Cr synthesized in the tECSN explosion, leading to an increased absorption from these elements at blue wavelengths and subsequent fluorescence to red wavelengths. At late times, the tECSN model exhibits an exceptionally strong 12.8 mu m Ne II emission line, which strengthens substantially over time, whereas its strength remains largely unchanged in the CO deflagration model. Conclusions. Our results suggest that tECSNe could potentially result in SN Iax-like transients. Importantly, we did not find any features that are in tension with existing observables. So far, there are no indicators that unambiguously and robustly separate tECSNe from deflagrations in CO white dwarfs. Nonetheless, our work highlights the potential importance of the mid-infrared wavelength range for distinguishing possible explosion mechanisms.

We present 3D kilonova radiative transfer simulations for a series of binary neutron star merger models. The masses of the neutron stars are varied as well as the total mass of the system and two different equations of state were used (SFHO and DD2), producing a range in dynamical ejecta masses and elemental abundance patterns. In this paper, we focus on the bolometric light curves and spectra in the polar direction for comparison with observations of the kilonova AT2017gfo. We calculate line-by-line opacities and include new calibrated lanthanide atomic data. All of the simulated spectra show strong features from Sr II, La III, Gd III and Ce III, which appear to correspond to features identified in AT2017gfo, although the simulated features are generally more blueshifted. The models with the lowest lanthanide fraction in the polar direction also show a Y II feature. Ce III, Ce II, Nd III and Nd II play an important role in shaping the spectral continuum. While the bolometric luminosities in the polar direction vary with the ejecta mass of each model, we find only little sensitivity of the spectral properties to the merger configuration. Our study demonstrates that dynamical ejecta alone can reproduce (although at earlier times) many spectral properties of AT2017gfo, suggesting dynamical ejecta may have a strong impact on the early spectral evolution. However, future simulations are needed to also elucidate the role of other ejecta components for shaping the kilonova spectrum.

Disc winds from active galactic nuclei (AGNs) can be launched by radiation pressure acting on spectral lines. However, launching a line-driven wind in the X-ray-rich environment of AGNs is challenging, as the wind easily gets overionized. Previous simulations suggested that X-ray self-shielding could enable line-driving, though it remained unclear whether this relied on simplified treatments of radiation and ionization. Here, we revisit the X-ray shielding scenario using the first multifrequency multidirectional Monte Carlo radiative photoionization hydrodynamical simulations of AGN linedriven winds. We find that sustaining a steady wind with mass-loss rates of approximate to 20 per cent of the accretion rate requires an unrealistically weak X-ray flux (alpha(OX) < -3). For stronger X-ray emission (-3 < alpha(OX) < -1), self-shielding is only transient, leading to episodic ejections with mass-loss rates approaching the accretion rate. Our steady winds naturally produce FeLoBAL, HiBAL, and broad emission-line signatures, depending on the disc spectral energy distribution and the observer's inclination. At moderate X-ray luminosities (alpha(OX )similar to -3), transient winds can generate short-lived BAL and ultra-fast outflow (UFO) features. At the highest X-ray luminosities (alpha(OX) similar to -1), the winds are too ionized to form BALs, but still produce UFOs. These results imply that additional physics is required to explain BAL outflows at realistic X-ray levels and to drive winds strong enough for AGN feedback. None the less, our simulations provide a new framework for interpreting the observed diversity of AGN outflow signatures with fully coupled radiation and dynamics.

Context. Recent Type Ia supernova (SN Ia) simulations featuring a double detonation scenario have managed to reproduce the overall trend of the Phillips relation reasonably well. However, most, if not all, multidimensional simulations struggle to reproduce the scatter of observed SNe around this relation, exceeding it substantially.Aims. In this study, we investigate whether the excessive scatter around the Phillips relation can be caused by an off-center ignition of the carbon-oxygen (CO) core in the double detonation scenario and if this can help constrain possible SN Ia explosion channels. Methods. We simulated the detonation of three different initial CO white dwarfs of 0.9, 1.0, and 1.1 M-circle dot, artificially ignited at systematically offset locations using the AREPO code. After nucleosynthetic postprocessing, we generated synthetic observables using the ARTIS code and compared these results against observational data and models of other works. Results. We find that our simulations produce synthetic observables well within the range of the observed data in terms of viewing angle scatter. The majority of the viewing angle variability seems to be caused by line blanketing in the blue wavelengths of intermediate-mass elements and lighter iron-group elements, which are asymmetrically distributed in the outer layers of the ashes. Conclusions. Our results suggest that although the off-center ignition of the CO introduces substantial line of sight effects, it is not responsible for the excessive viewing angle scatter observed in other models. Instead, this effect seems to be caused by the detonation ashes from the rather massive helium (He) shells in current state-of-the-art models. Further reducing the He-shell masses of double detonation progenitors may be able to alleviate this issue and yield observables that reproduce the Phillips relation.

Disc winds play a crucial role in many accreting astrophysical systems across all scales. In accreting white dwarfs (AWDs) and active galactic nuclei (AGNs), radiation pressure on spectral lines is a promising wind-driving mechanism. However, the efficiency of line driving is extremely sensitive to the ionization state of the flow, making it difficult to construct a reliable physical picture of these winds. Recently, we presented the first radiation-hydrodynamics simulations for AWDs that incorporated detailed, multidimensional ionization calculations via fully frequency-dependent radiative transfer, using the sirocco code coupled to pluto. These simulations produced much weaker line-driven winds (M-wind(center dot)/M-acc(center dot)<10(-5) for our adopted parameters) than earlier studies using more approximate treatments of ionization and radiative transfer (M-wind(center dot)/M-acc (center dot)similar or equal to 10(-5) which yielded ). One remaining limitation of our work was the assumption of an isothermal outflow. Here, we relax this by adopting an ideal gas equation of state and explicitly solving for the multidimensional temperature structure of the flow. In the AWD setting, accounting for the thermal state of the wind does not change the overall conclusions drawn from the isothermal approximation. Our new simulations confirm the line-driving efficiency problem: the predicted outflows are too highly ionized, meaning they neither create optimal driving conditions nor reproduce the observed ultraviolet wind signatures. Possible solutions include wind clumping on subgrid scales, a softer-than-expected spectral energy distribution or additional driving mechanisms. With the physics now built into our simulations, we are well equipped to also explore line-driven disc winds in AGN.

Blueshifted absorption is the classic spectroscopic signature of an accretion disc wind in X-ray binaries and cataclysmic variables (CVs). However, outflows can also create pure emission lines, especially at optical wavelengths. Therefore, developing other outflow diagnostics for these types of lines is worthwhile. With this in mind, we construct a systematic grid of 3645 synthetic wind-formed H alpha line profiles for CVs with the radiative transfer code sirocco. Our grid yields a variety of line shapes: symmetric, asymmetric, single- to quadruple-peaked, and even P-Cygni profiles. About 20 per cent of these lines - our 'Gold' sample - have strengths and widths consistent with observations. We use this grid to test a recently proposed method for identifying wind-formed emission lines based on deviations in the wing profile shape: the 'excess equivalent width diagnostic diagram'. We find that our Gold sample can preferentially populate the suggested 'wind regions' of this diagram. However, the method is highly sensitive to the adopted definition of the line profile 'wing'. Hence, we propose a refined definition based on the full width at half-maximum to improve the interpretability of the diagnostic diagram. Furthermore, we define an approximate scaling relation for the strengths of wind-formed CV emission lines in terms of the outflow parameters. This relation provides a fast way to assess whether - and what kind of - outflow can produce an observed emission line. All our wind-based models are open-source and we provide an easy-to-use web-based tool to browse our full set of H alpha spectral profiles.

Type Ia supernovae (SNe Ia) are powered by the radioactive decay of isotopes such as ^56 Ni and ^56 Co, making their γ -ray spectra useful probes of the explosion mechanism and ejecta structure. Accurate interpretation of γ -ray observables, including line ratios and continuum fluxes, requires a detailed understanding of the microphysical processes that shape the spectra. One such process is positronium formation during electron–positron annihilation, which can redistribute flux from the 511 keV line into the surrounding continuum. To assess the impact of positronium on the emergent spectra, we developed a new open-source module, tardis-He , for time-dependent three-dimensional γ -ray transport, integrated into the radiative transfer code tardis . The code simulates γ -ray spectra and light curves from one-dimensional supernova ejecta models and allows for flexible incorporation of decay chains and opacity treatments. Using tardis-He , we explore the effect of positronium formation by varying the positronium fraction from 0% to 100%, and assuming an extreme case where 75% of positronium decays result in three-photon emission. We find that full positronium formation can reduce the 511 keV line flux by ≈70% and modestly enhance energy deposition by up to 2% at around 100 days postexplosion, compared to models without positronium. These results demonstrate that, while the effect is not dominant, positronium formation introduces measurable changes to γ -ray observables. Future observations with missions such as the Compton Spectrometer and Imager may offer constraints on positronium formation in SNe Ia and help refine models of their radioactive energy transport.

The Axelrod approximation is widely used in astrophysical modelling codes to evaluate electron-impact excitation effective collision strengths for forbidden transitions. Approximate methods such as this are a necessity for many heavy elements with open shells where collisional data is either non existent or sparse as the use of more robust methods prove prohibitively expensive. Atomic data for such forbidden transitions are essential for producing full collisional radiative models that do not assume Local-Thermodynamic-Equilibrium (LTE). In this short work we present the re-optimization the simple Axelrod formula for a large number of R-matrix data sets, ranging from Fe and Ni to the first r-process peak elements of Sr, Y and Zr, to higher Z systems Te, W, Pt and Au. We show that the approximate treatment of forbidden transitions can be a significant source of inaccuracy in such collisional radiative models. We find a large variance of the optimized coefficients for differing systems and charge states, although some general trends can be seen based on the orbital structure of the ground-state-configurations. These trends could potentially inform better estimates for future calculations for elements where R-matrix data is not available.

Type Ia supernovae play a fundamental role as cosmological probes of dark energy and produce more than half of the iron in our Galaxy. Despite their central importance, a comprehensive understanding of their progenitor systems and triggering mechanism is still a long-standing fundamental problem. Here we present high-resolution integral field spectroscopic observations of the young supernova remnant SNR 0509-67.5 in the Large Magellanic Cloud. We uncover a double-shell morphology of highly ionized calcium [Ca XV] and a single shell of sulphur [S XII], observed in the reverse shocked ejecta. Our analysis reveals that the outer calcium shell originates from the helium detonation at the base of the outer envelope, while the inner shell is associated with the carbon-oxygen core detonation. This morphological distribution of intermediate-mass elements agrees qualitatively with the predicted signature of the double detonation of a sub-Chandrasekhar-mass white dwarf from a hydrodynamical explosion simulation. Our observations reveal two distinct, spatially separated peaks in surface brightness of [Ca XV] from the supernova remnant phase, providing substantial evidence that sub-Chandrasekhar-mass explosions through the double-detonation mechanism could occur in nature. They also highlight the importance of remnant tomography in understanding explosion mechanisms from the remnant phase.

Disc winds are a common feature in accreting astrophysical systems on all scales. In active galactic nuclei (AGN) and accreting white dwarfs (AWDs), specifically, radiation pressure mediated by spectral lines is a promising mechanism for driving these outflows. Previous hydrodynamical simulations have largely supported this idea, but relied on highly approximate treatments of ionization and radiative transfer. Given the sensitivity of line driving to the ionization state and radiation field in the outflow, here we present a new method for carrying out 2.5D radiation-hydrodynamic simulations that takes full account of the frequency-dependent radiative transfer through the wind, the corresponding ionization state and the resulting radiative accelerations. Applying our method to AWDs, we find that it is much harder to drive a powerful line-driven outflow when the interaction between matter and radiation is treated self-consistently. This conclusion is robust to changes in the adopted system parameters. The fundamental difficulty is that discs luminous enough to drive such a wind are also hot enough to over-ionize it. As a result, the mass-loss rates in our simulations are much lower than those found in earlier, more approximate calculations. We also show that the ultraviolet spectra produced by our simulations do not match those observed in AWDs. We conclude that, unless the over-ionization problem can be mitigated (e.g. by sub-grid clumping or a softer-than-expected radiation field), line driving may not be a promising mechanism for powering the outflows from AWDs. These conclusions are likely to have significant implications for disc winds in AGN also.

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.

Spectroscopy is an important tool for providing insights into the structure of core-collapse supernova explosions. We use the Monte Carlo radiative transfer code ARTIS to compute synthetic spectra and light curves based on a two-dimensional explosion model of an ultra-stripped supernova. These calculations are designed both to identify observable fingerprints of ultra-stripped supernovae and as a proof-of-principle for using synthetic spectroscopy to constrain the nature of stripped-envelope supernovae more broadly. We predict very characteristic spectral and photometric features for our ultra-stripped explosion model, but find that these do not match observed ultra-stripped supernova candidates like SN 2005ek. With a peak bolometric luminosity of $6.8\times10^{41}\,\mathrm{erg}\,\mathrm{s}^{-1}$, a peak magnitude of $-15.9\,\mathrm{mag}$ in R-band, and $\Delta m_{15,\mathrm{R}}=3.50$, the model is even fainter and evolves even faster than SN 2005ek as the closest possible analogue in photometric properties. The predicted spectra are extremely unusual. The most prominent features are Mg II lines at 2,800 Angstrom and 4,500 Angstrom and the infrared Ca triplet at late times. The Mg lines are sensitive to the multi-dimensional structure of the model and are viewing-angle dependent. They disappear due to line blanketing by Fe group elements in a spherically averaged model with additional microscopic mixing. In future studies, multi-D radiative transfer calculations need to be applied to a broader range of models to elucidate the nature of observed Type Ib/c supernovae.

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).