Identification of strontium in the merger of two neutron stars

Abstract

Half of all the elements in the universe heavier than iron were created by rapid neutron capture. The theory for this astrophysical 'r-process' was worked out six decades ago and requires an enormous neutron flux to make the bulk of these elements. 1 Where this happens is still debated. 2 A key piece of missing evidence is the identification of freshly-synthesised r-process elements in an astrophysical site. Current models 3-5 and circumstantial evidence 6 point to neutron star mergers as a probable r-process site, with the optical/infrared 'kilonova' emerging in the days after the merger a likely place to detect the spectral signatures of newly-created neutron-capture elements. 7-9 The kilonova, AT2017gfo, emerging from the gravitational-wave-discovered neutron star merger, GW170817, 10 was the first kilonova where detailed spectra were recorded. When these spectra were first reported 11, 12 it was argued that they were broadly consonant with an outflow of radioactive heavy elements, however, there was no robust identification of any element. Here we report the identification of the neutron-capture element strontium in a re-analysis of these spectra. The detection of a neutron-capture element associated with the collision of two extreme-density stars establishes the origin of r-process elements in neutron star mergers, and demonstrates that neutron stars comprise neutron-rich matter 13 .The most detailed information available for a kilonova comes from a series of spectra of AT2017gfo taken over several weeks with the medium resolution, ultraviolet (320 nm) to near-infrared (2,480 nm) spectrograph, X-shooter, mounted at the Very Large Telescope at the European Southern Observatory. These spectra 11, 12 , allow us to track the evolution of the kilonova's primary electromagnetic output from 1.5 days until 10 days after the event. Detailed modelling of these spectra has yet to be done owing to the limited understanding of the phenomenon and the expectation that a very large number of moderate to weak lanthanide lines with unknown oscillator strengths would dominate the spectra 14,15 . Despite the expected complexity, we sought to identify individual elements in the early spectra because these spectra are well-reproduced by relatively simple models 11 .The first epoch spectrum can be reproduced over the entire observed spectral range with a single-temperature blackbody with an observed temperature 4, 800 K. The two major deviations short of 1 µm from a pure blackbody are due to two very broad (∼ 0.2c) absorption components. These components are observed centred at about 350 nm and 810 nm (Fig. 1). The shape of the ultraviolet absorption component is not well constrained because it lies close to the edge of our sensitivity limit and may simply be cut off below about 350 nm. The presence of the absorption feature at 810 nm at this epoch has been noted in earlier publications 11,12 .The fact that the spectrum is very well reproduced by a single temperature blackbody in the first epoch suggests a population of states 0.3...

Evidence for ≳4 Gyr timescales of neutron star mergers from Galactic archaeology

“…The delayed r-process source is most likely neutron star mergers (NSMs) since they are confirmed r-process sites (e.g. Watson et al 2019). The estimated time delay of the event GW170817, which is currently the only directly observed NSM with a confirmed host galaxy (Pan et al 2017;Blanchard et al 2017), is in good agreement with our timescale of t delayed 4 Gyr.…”

First Exploration of Neutron Shell Structure below Lead and beyond N=126

“…J π n s S 0 4 9/2 + 1g 9/2 0.82 (13) 1195 (20) (2) (5/2 + ) (2d 5/2 ) a 0.47 (9) 1600 (45) (2) (5/2 + ) (2d 5/2 ) a 0.13 (2) 1810 (20) (2) (5/2 + ) (2d 5/2 ) a 0.42 (7) 1960(30) (0) (1/2 + ) (3s 1/2 ) ≡ 1.00 2335 (20) (2) (3/2 + ) (2d 3/2 ) 1.00 (17) 2530 (20) (4) (7/2 + ) (1g 7/2 ) 0.62 (12) a The centroid of the 2d 5/2 strength lies at 1500(50) keV, with C 2 S = 1.02 (17).…”

“…The robustness of the N = 126 neutron shell closure plays a crucial role in the nucleosynthesis of the actinides [3][4][5][6][7]. The recent observation of a neutron star merger has provided a new focus of interest [8,9], suggesting a possible astrophysical environment for r-process nucleosynthesis [10-13].Approaching the r-process path along the N = 126 isotonic chain from Pb, the binding energies (the degree to which neutrons are bound by the mean-field potential created by the decreasing number of all other nucleons) decrease, eventually crossing zero binding and becoming unbound. Near closed shells, the level density is low, so the usual statistical assumptions of many resonances participating in neutron capture is not valid, and specific nuclear-structure properties become important.…”

“…The robustness of the N = 126 neutron shell closure plays a crucial role in the nucleosynthesis of the actinides [3][4][5][6][7]. The recent observation of a neutron star merger has provided a new focus of interest [8,9], suggesting a possible astrophysical environment for r-process nucleosynthesis [10][11][12][13].…”

Science case for the Einstein telescope

“…Many remarkable results in astrophysics and in fundamental physics have already been obtained thanks to these first detections. To mention only a few highlights, the observation of the NS-NS binary coalescence GW170817 solved the long-standing problem of the origin of (at least some) short gamma ray bursts [2,10,11]; the multi-band observations of the associated kilonova revealed that NS-NS mergers are a site for the formation of some of the heaviest elements through r-process nucleosynthesis [12][13][14][15]; the observation of tens of BH-BH coalescences has revealed a previously unknown population of stellar-mass BHs, much heavier than those detected through the observation of X-ray binaries [16], and has shown that BH-BH binaries exist, and coalesce within a Hubble time at a detectable rate. Concerning fundamental physics, cosmology and General Relativity (GR), the observation of the GWs and the gamma-ray burst from the NS-NS binary GW170817 proved that the speed of GWs is the same as the speed of light to about a part in 10 15 [5]; the GW signal, together with the electromagnetic determination of the redshift of the source, provided the first measurement of the Hubble constant with GWs [17]; the tail of the waveform of the first observed event, GW150914, showed oscillations consistent with the prediction from General Relativity for the quasi-normal modes of the final BH [18]; several possible deviations from GR (graviton mass, post-Newtonian coefficients, modified dispersion relations, etc.)…”