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Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger

Pian, E. and Kasliwal, M. M. (2017) Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger. Nature, 551 (7678). pp. 67-70. ISSN 0028-0836.

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The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical–near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). The transients, named ‘macronovae’ or ‘kilonovae’, are believed to be cradles of production of rare elements such as gold and platinum8. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst at redshift z = 0.356, although suggestive findings of bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source GW170817 (ref. 12) and γ-ray burst GRB 170817A (refs 13, 14) associated with a galaxy at a distance of 40 megaparsecs. Using a series of spectra from ground-based observatories covering the ultraviolet through the near-infrared wavelength range, we find that the kilonova can be characterized by rapidly expanding ejecta with line features similar to those predicted in current models. The ejecta are optically thick early on, with a velocity of about 0.2 times light speed, reaching a radius of about 50 astronomical units in only 1.5 days. As the ejecta expand, atomic species imprint broad absorption-like lines on the spectral continuum, the products of nucleosynthesis occurring in a post-merger fast-moving (0.2 times light speed) dynamical ejecta and two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03–0.05 solar masses, including high-opacity lanthanides.

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Kasliwal, M. M.0000-0002-5619-4938
Alternate Title:Spectroscopic identification of a kilonova associated with a gravitational wave signal and a short gamma-ray burst
Additional Information:© 2017 Macmillan Publishers Limited, part of Springer Nature. Received 12 September; accepted 20 September 2017. Accelerated Article Preview Published online 16 October 2017. Data availability: The data presented in this paper and supporting the findings of this study are available from the corresponding author upon reasonable request. This work is based on observations made with the ESO telescopes at the Paranal Observatory under programmes ID 099.D-0382 (Principal Investigator (PI): E. Pian), 099.D-0622 (PI: P.D’A.), 099.D-0191 (PI: A. Grado) and with the REM telescope at the ESO La Silla Observatory under programme ID 35020 (PI: S. Campana). Gemini observatory data were obtained under programme GS-2017B-DD-1 (PI: L. P. Singer). We thank the Gemini Observatory for performing these observations, the ESO Director General for allocating discretionary time and the ESO operation staff for support. We thank D. Fugazza for technical support with operating the REM telescope remotely and REM telescope director E. Molinari. We acknowledge INAF for supporting the project ‘Gravitational Wave Astronomy with the first detections of adLIGO and adVirgo experiments—GRAWITA’ (PI: E.B.) and support from ASI grant I/004/11/3. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). M.M.K. acknowledges support from the GROWTH (Global Relay of Observatories Watching Transients Happen) project funded by the National Science Foundation under PIRE grant number 1545949. Author Contributions: E. Pian and P. D’Avanzo are Principal Investigators of the two active ESO VLT programmes and coordinated the work. J. Selsing reduced all the X-shooter spectra presented in Fig. 2 and wrote the relevant sections. M. Tanaka developed the kilonova spectral models. E. Cappellaro assisted with the spectral analysis. P. Mazzali provided the liaison between spectral observations and kilonova theory: he coordinated the theoretical interpretation, developed the match between the synthetic and observed spectra (Fig. 3), and wrote the part on their description and discussion. S. Campana coordinated the REM observations. S. Covino, A. Grado and A. Melandri reduced and analysed the optical photometry (Fig. 1). M. M. Kasliwal provided the Gemini spectrum. D. Malesani assisted with early observation planning. G. Ghirlanda, G. Ghisellini and O. S. Salafia wrote the section on the off-beam jet with contributions from L. Amati, Y. Z. Fan, Z. P. Jin and T. Piran. D. Watson assisted with the analysis of spectra in light of thermal models and assisted with paper writing. E. Brocato was the Principal Investigator of the GRAvitational Wave Inaf TeAm (GRAWITA) for GW electromagnetic follow-up. M. Branchesi liaised GRAWITA with LIGOVIRGO collaborations activities. A. Grado coordinated the ESO-VST observations. L. Limatola and F. Getman developed the pipeline to reduce the VST data. N. Tanvir and A. Levan assisted with NIR data calibration issues. J. P. U. Fynbo, J. Hjorth and C. Kouveliotou assisted with paper writing and short GRB expertise. L. Nicastro supervised the data flow and handling. S. Piranomonte and V. D’Elia contributed to the data reduction and analysis of the X-shooter spectra. E. Palazzi, A. Rossi, G. Stratta and G. Greco participated in the organization of the observations and image analysis and provided specific input for photometry calibration. L. Tomasella, S. Yang, and S. Benetti contributed to the data analysis, with particular reference to ISM spectral features. P. Møller assisted with issues related to ESO policies and observation planning. This effort was led by GRAWITA, that includes most co-authors, and is based on GW electromagnetic follow-up programs at ESO and at many telescopes both in Italy and at the Canary Islands. All GRAWITA members contributed to the work development at many phases from preparation of proposals, coordination with the LIGO-VIRGO collaborations, activation of approved programs at many facilities, data acquisition, reduction, analysis, interpretation and presentation. Reviewer Information: Nature thanks R. Chevalier, C. Miller and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Group:Astronomy Department
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Istituto Nazionale di Astrofisica (INAF)UNSPECIFIED
Agenzia Spaziale Italiana (ASI)I/004/11/3
Issue or Number:7678
Record Number:CaltechAUTHORS:20170917-210708930
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:81509
Deposited By: George Porter
Deposited On:17 Oct 2017 23:08
Last Modified:15 Apr 2020 00:30

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