Emergence hour-by-hour of r-process features in the kilonova AT2017gfo
: Sneppen, Albert; Watson, Darach; Damgaard, Rasmus; Heintz, Kasper E.; Vieira, Nicholas; Väisänen, Petri; Mahoro, Antoine
Publisher: EDP Sciences
: LES ULIS CEDEX A
: 2024
: Astronomy and Astrophysics
: ASTRONOMY & ASTROPHYSICS
: ASTRON ASTROPHYS
: A398
: 690
: 15
: 0004-6361
: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202450317
: https://doi.org/10.1051/0004-6361/202450317
: https://research.utu.fi/converis/portal/detail/Publication/459294932
The spectral features in the optical/near-infrared counterparts of neutron star mergers (kilonovae, KNe) evolve dramatically on hourly timescales. To examine the spectral evolution, we compiled a temporal series that was complete at all observed epochs from 0.5 to 9.4 days of the best optical/near-infrared (NIR) spectra of the gravitational-wave detected kilonova AT2017gfo. Using our analysis of this spectral series, we show that the emergence times of spectral features place strong constraints on line identifications and ejecta properties, while their subsequent evolution probes the structure of the ejecta. We find that the most prominent spectral feature, the 1 μm P Cygni line, appears suddenly, with the earliest detection at 1.17 days. We find evidence in this earliest feature for the fastest yet discovered kilonova ejecta component at 0.40-0.45c. Across the observed epochs and wavelengths, the velocities of the line-forming regions span nearly an order of magnitude, down to as low as 0.04-0.07c. The time of emergence closely follows the predictions for Sr II because Sr III combines rapidly under local thermal equilibrium (LTE) conditions. The transition time between the doubly and singly ionised states provides the first direct measurement of the ionisation temperature. This temperature is highly consistent with the temperature of the emitted blackbody radiation field at a level of a few percent. Furthermore, we find the KN to be isotropic in temperature, that is, the polar and equatorial ejecta differ by less than a few hundred Kelvin or ≲5%, in the first few days post-merger based on measurements of the reverberation time-delay effect. This suggests that a model with very simple assumptions, with single-temperature LTE conditions, reproduces the early kilonova properties surprisingly well.
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The authors would like to thank Igor Andreoni, Steven Crawford, Jonathan Selsing, and Jesse Palmerio for sharing, introducing and clarifying the various datasets and data-reductions. We would further like to express our gratitude to Stuart Sim, Christine Collins, Luke Shingles, and Fiona McNeill for discussions on the spectral modelling. The Cosmic Dawn Center (DAWN) is funded by the Danish National Research Foundation under grant DNRF140. AS, DW, RD, and KEH are co-funded by the European Union (ERC, HEAVYMETAL, 101071865). Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. PV and AM acknowledge support from the National Research Foundation of South Africa, and AM financial support from the Swedish International Development Cooperation Agency (SIDA) through the International Science Programme (ISP) – Uppsala University to the University of Rwanda through the Rwanda Astrophysics, Space and Climate Science Research Group (RASCSRG). Data and code: As discussed, the spectral series presented in this paper is composed of a series of different observing programmes at various telescopes. We request that any use of the data, whether compiled or re-reduced for this analysis, includes appropriate citation to the original papers. Spectra from the 6.5 m Magellan Telescopes (0.49 and 0.53 days) located at Las Campanas Observatory, Chile, can be found at: https://arxiv.org/abs/1710.05432. The 0.92 day spectra were obtained with the Australian National University (ANU) the 2.3 m telescope located at Siding Spring Observatory. The 1.17 day data obtained with the Southern African Large Telescope (SALT) under the Director’s Discretionary Time programme 2017–1-DDT-009, are available at https://ssda.saao.ac.za with the newly reduced spectra (e.g. with improved flux-calibration, see Appendix A) now available at https://github.com/Sneppen/Kilonova-analysis. X-shooter data from European Space Observatory (ESO) telescopes at the Paranal Observatory under programmes 099.D-0382 (principal investigator [PI]: E. Pian), 099.D-0622 (PI: P. D’Avanzo), 099.D-0376 (PI: S. J. Smartt), which are available at http://archive.eso.org and WISeREP (https://wiserep.weizmann.ac.il/). The re-reduced X-shooter spectra examining evolution in sub-epoch exposures at 1.4 days (derived in Sneppen et al. (2023a)) are available from https://github.com/Sneppen/Kilonova-analysis. Gemini-south data were obtained at the Gemini Observatory (Program IDs GS-2017B-Q-8 and GS-2017B-DD-4; PI: Chornock) and are together with SOAR spectra available at https://kilonova.org. HST observations were obtained using programs GO 14771 (PI: N. Tanvir) and GO 14804 (PI: A. Levan). We used the implementation of the P Cygni profile in the elementary supernova from https://github.com/unoebauer/public-astro-tools with generalisations to account for reverberation, time-delay effects, and special relativistic corrections (see https://github.com/Sneppen/Kilonova-analysis).
