Self-consistent 3D radiative transfer for kilonovae: directional spectra from merger simulations

We present three-dimensional 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 $\alpha$ and $\beta^-$ decay reactions. In contrast to expansion opacities and other wavelength-binned treatments, a line-by-line treatment enables us 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 near-polar inclination show a feature at around 8000 A, similar to AT2017gfo. However, they evolve on a more rapid timescale, likely due to the low ejecta mass (0.005 M$_\odot$) 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.Comment: 12 pages, 5 figures. Accepted by ApJ

Towards inferring the geometry of kilonovae

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).Comment: 11 pages, submitted to MNRA

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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

Towards inferring the geometry of kilonovae

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).</p

The Arctic Summer Cloud-Ocean Study (ASCOS): Overview and experimental design

The climate in the Arctic is changing faster than anywhere else on Earth. Poorly un-derstood feedback processes relating to Arctic clouds and aerosol-cloud interactionscontribute to a poor understanding of the present changes in the Arctic climate system,and also to a large spread in projections of future climate in the Arctic. The problem is exacerbated by the paucity of research-quality observations in the central Arctic. Im-proved formulations in climate models require such observations, which can only comefrom measurements in-situ in this difficult to reach region with logistically demandingenvironmental conditions.The Arctic Summer Cloud-Ocean Study (ASCOS) was the most extensive central Arctic Ocean expedition with an atmospheric focus during the International Polar Year(IPY) 2007–2008. ASCOS focused on the study of the formation and life cycle of low-level Arctic clouds. ASCOS departed from Longyearbyen on Svalbard on 2 August andreturned on 9 September 2008. In transit into and out of the pack ice, four short re-search stations were undertaken in the Fram Strait; two in open water and two in the marginal ice zone. After traversing the pack-ice northward an ice camp was set up on12 August at 87◦21′N 01◦29′W and remained in operation through 1 September, drift-ing with the ice. During this time extensive measurements were taken of atmosphericgas and particle chemistry and physics, mesoscale and boundary-layer meteorology,marine biology and chemistry, and upper ocean physics. ASCOS provides a unique interdisciplinary data set for development and testing ofnew hypotheses on cloud processes, their interactions with the sea ice and ocean andassociated physical, chemical, and biological processes and interactions. For exam-ple, the first ever quantitative observation of bubbles in Arctic leads, combined withthe unique discovery of marine organic material, polymer gels with an origin in the ocean, inside cloud droplets suggest the possibility of primary marine organically de-rived cloud condensation nuclei in Arctic stratocumulus clouds. Direct observations ofsurface fluxes of aerosols could, however, not explain observed variability in aerosol concentrations and the balance between local and remote aerosols sources remainsopen. Lack of CCN was at times a controlling factor in low-level cloud formation, andhence for the impact of clouds on the surface energy budget. ASCOS provided de-tailed measurements of the surface energy balance from late summer melt into theinitial autumn freeze-up, and documented the effects of clouds and storms on the surface energy balance during this transition. In addition to such process-level studies, theunique, independent ASCOS data set can and is being used for validation of satelliteretrievals, operational models, and reanalysis data sets.ISSN:1680-7375ISSN:1680-736