| 1. |
- Tanvir, N. R., et al.
(author)
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The Emergence of a Lanthanide-rich Kilonova Following the Merger of Two Neutron Stars
- 2017
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In: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 848:2
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Journal article (peer-reviewed)abstract
- We report the discovery and monitoring of the near-infrared counterpart (AT2017gfo) of a binary neutron-star merger event detected as a gravitational wave source by Advanced Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo (GW170817) and as a short gamma-ray burst by Fermi Gamma-ray Burst Monitor (GBM) and Integral SPI-ACS (GRB 170817A). The evolution of the transient light is consistent with predictions for the behavior of a kilonova/ macronova powered by the radioactive decay of massive neutron-rich nuclides created via r-process nucleosynthesis in the neutron-star ejecta. In particular, evidence for this scenario is found from broad features seen in Hubble Space Telescope infrared spectroscopy, similar to those predicted for lanthanide-dominated ejecta, and the much slower evolution in the near-infrared K-s-band compared to the optical. This indicates that the late-time light is dominated by high-opacity lanthanide-rich ejecta, suggesting nucleosynthesis to the third r-process peak (atomic masses A approximate to 195). This discovery confirms that neutron-star mergers produce kilo-/macronovae and that they are at least a major-if not the dominant-site of rapid neutron capture nucleosynthesis in the universe.
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| 2. |
- Evans, P. A., et al.
(author)
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Swift and NuSTAR observations of GW170817 : Detection of a blue kilonova
- 2017
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In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 358:6370, s. 1565-1569
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Journal article (peer-reviewed)abstract
- With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and x-ray observations by Swift and the Nuclear Spectroscopic Telescope Array of the EM counter part of the binary neutron star merger GW170817. The bright, rapidly fading UV emission indicates a high mass (approximate to 0.03 solar masses) wind-driven outflow with moderate electron fraction (Y-e approximate to 0.27). Combined with the x-ray limits, we favor an observer viewing angle of approximate to 30 degrees away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultrarelativistic, highly collimated ejecta (a gamma-ray burst afterglow).
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| 3. |
- Korobkin, Oleg, et al.
(author)
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Axisymmetric Radiative Transfer Models of Kilonovae
- 2021
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 910:2
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Journal article (peer-reviewed)abstract
- The detailed observations of GW170817 proved for the first time directly that neutron star mergers are a major production site of heavy elements. The observations could be fit by a number of simulations that qualitatively agree, but can quantitatively differ (e.g., in total r-process mass) by an order of magnitude. We categorize kilonova ejecta into several typical morphologies motivated by numerical simulations, and apply a radiative transfer Monte Carlo code to study how the geometric distribution of the ejecta shapes the emitted radiation. We find major impacts on both spectra and light curves. The peak bolometric luminosity can vary by two orders of magnitude and the timing of its peak by a factor of five. These findings provide the crucial implication that the ejecta masses inferred from observations around the peak brightness are uncertain by at least an order of magnitude. Mixed two-component models with lanthanide-rich ejecta are particularly sensitive to geometric distribution. A subset of mixed models shows very strong viewing angle dependence due to lanthanide curtaining, which persists even if the relative mass of lanthanide-rich component is small. The angular dependence is weak in the rest of our models, but different geometric combinations of the two components lead to a highly diverse set of light curves. We identify geometry-dependent P Cygni features in late spectra that directly map out strong lines in the simulated opacity of neodymium, which can help to constrain the ejecta geometry and to directly probe the r-process abundances.
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| 4. |
- Wollaeger, Ryan T., et al.
(author)
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Impact of ejecta morphology and composition on the electromagnetic signatures of neutron star mergers
- 2018
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 478:3, s. 3298-3334
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Journal article (peer-reviewed)abstract
- The electromagnetic transients accompanying compact binary mergers (gamma-ray bursts, after-glows and 'macronovae') are crucial to pinpoint the sky location of gravitational wave sources. Macronovae are caused by the radioactivity from freshly synthesized heavy elements, e.g. from dynamic ejecta and various types of winds. We study macronova signatures by using multidimensional radiative transfer calculations. We employ the radiative transfer code SUPERNU and state-of-the-art LTE opacities for a few representative elements from the wind and dynamical ejecta (Cr, Pd, Se, Te, Br, Zr, Sm, Ce, Nd, U) to calculate synthetic light curves and spectra for a range of ejecta morphologies. The radioactive power of the resulting macronova is calculated with the detailed input of decay products. We assess the detection prospects for our most complex models, based on the portion of viewing angles that are sufficiently bright, at different cosmological redshifts (z). The brighter emission from the wind is unobscured by the lanthanides (or actinides) in some of the models, permitting non-zero detection probabilities for redshifts up to z = 0.07. We also find that the nuclear mass model and the resulting radioactive heating rate are crucial for the detectability. While for the most pessimistic heating rate (from the finite range droplet model) no reasonable increase in the ejecta mass or velocity, or wind mass or velocity, can possibly make the light curves agree with the observed near-infrared excess after GRB130603B, a more optimistic heating rate (from the Duflo-Zuker model) leads to good agreement. We conclude that future reliable macronova observations would constrain nuclear heating rates, and consequently help constrain nuclear mass models.
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