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"Sim, Stuart"
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Self-consistent 3D Radiative Transfer for Kilonovae: Directional Spectra from Merger Simulations
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.
Journal Article
Post-truth, scepticism & power
This book examines the concept of post-truth and the impact it is having on contemporary life, bringing out both its philosophical and political dimensions. Post-truth is contextualised within the philosophical discourse of truth, with particular reference to theories of scepticism and relativism, to explore whether it can take advantage of these to claim any intellectual credibility. Sim argues that post-truth cannot be defended on either sceptical or relativistic grounds ? even those provided by recent iconoclastic philosophical movements such as poststructuralism and postmodernism. The affinity between post-truth and conspiracy theory is emphasised, and the extent to which post-truth plays a role in religious doctrine is also considered. Post-truth is seen to constitute a threat to liberal democratic ideals and our Enlightenment heritage, raising the question of whether we are moving into a post-liberal age where the far right would hold power. To prevent this, post-truth urgently needs to be countered.
Book
Spherical symmetry in the kilonova AT2017gfo/GW170817
The mergers of neutron stars expel a heavy-element enriched fireball that can be observed as a kilonova
1
–
4
. The kilonova’s geometry is a key diagnostic of the merger and is dictated by the properties of ultra-dense matter and the energetics of the collapse to a black hole. Current hydrodynamical merger models typically show aspherical ejecta
5
–
7
. Previously, Sr
+
was identified in the spectrum
8
of the only well-studied kilonova
9
–
11
AT2017gfo
12
, associated with the gravitational wave event GW170817. Here we combine the strong Sr
+
P Cygni absorption-emission spectral feature and the blackbody nature of kilonova spectrum to determine that the kilonova is highly spherical at early epochs. Line shape analysis combined with the known inclination angle of the source
13
also show the same sphericity independently. We conclude that energy injection by radioactive decay is insufficient to make the ejecta spherical. A magnetar wind or jet from the black-hole disk could inject enough energy to induce a more spherical distribution in the overall ejecta; however, an additional process seems necessary to make the element distribution uniform.
Spectra taken after the kilonova associated with GW170817 show a high degree of spherical symmetry and a line shape is found that is consistent with a completely spherical expansion to within a few per cent.
Journal Article
Introducing critical theory
The last few decades have seen an explosion in the production of critical theories, with deconstructionists, poststructuralists, postmodernists, second-wave feminists, new historicists, cultural materialists, postcolonialists, black critics and queer theorists, among a host of others, all vying for our attention. The world around us can look very different on the critical theory applied to it. This vast range of interpretations can leave one feeling confused and frustrated. This book provides a route through the tangled jungle of competing theories.
Book
Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with mass ∼0.9M
Supernovae true to type
Type Ia supernovae are potentially invaluable as cosmological distance indicators, but if they are to provide a reliable measure of the expansion history of the Universe and nature of dark energy, more evidence that they are a largely homogeneous population will be required. A subclass of type Ia supernova, the 'sub-luminous 1991bg-like' objects, has proved problematic, as current models fail to predict their formation from the presumed supernova precursors, white dwarf stars. It had long been speculated that mergers of two white dwarfs should trigger such events, and now a new set of numerical simulations adds support to this idea. A merger of two equal-mass white dwarfs leads to a sub-luminous explosion if a single common-envelope phase is involved, and if the component stars are each about 0.9 solar masses in size.
Existing models of type Ia supernovae generally explain their observed properties, with the exception of the sub-luminous 1991bg-like supernovae. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event, but simulations so far have failed to produce an explosion. Here, a simulation of the merger of two equal-mass white dwarfs is presented that leads to a sub-luminous explosion; it requires a single common-envelope phase and component masses of about 0.9 solar masses.
Type Ia supernovae are thought to result from thermonuclear explosions of carbon–oxygen white dwarf stars
1
. Existing models
2
generally explain the observed properties, with the exception of the sub-luminous 1991bg-like supernovae
3
. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event
4
,
5
, but hitherto simulations have failed to produce an explosion
6
,
7
. Here we report a simulation of the merger of two equal-mass white dwarfs that leads to a sub-luminous explosion, although at the expense of requiring a single common-envelope phase, and component masses of ∼0.9
M
⊙
. The light curve is too broad, but the synthesized spectra, red colour and low expansion velocities are all close to what is observed for sub-luminous 1991bg-like events. Although the mass ratios can be slightly less than one and still produce a sub-luminous event, the masses have to be in the range 0.83
M
⊙
to 0.9
M
⊙
.
Journal Article
Effect of Positronium on the γ-Ray Spectra and Energy Deposition in Type Ia Supernovae
Type Ia supernovae (SNe Ia) are powered by the radioactive decay of isotopes such as 56Ni and 56Co, 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.
Journal Article
Models for Type Ia Supernovae and Related Astrophysical Transients
We give an overview of recent efforts to model Type Ia supernovae and related astrophysical transients resulting from thermonuclear explosions in white dwarfs. In particular we point out the challenges resulting from the multi-physics multi-scale nature of the problem and discuss possible numerical approaches to meet them in hydrodynamical explosion simulations and radiative transfer modeling. We give examples of how these methods are applied to several explosion scenarios that have been proposed to explain distinct subsets or, in some cases, the majority of the observed events. In case we comment on some of the successes and shortcoming of these scenarios and highlight important outstanding issues.
Journal Article