Scientists decode the light fingerprints of colliding neutron stars
Researchers have created the first detailed models of how light escapes from the violent aftermath of neutron star collisions, identifying specific chemical signatures that astronomers can now use to detect and study these rare cosmic events. The work could help calibrate observations from next-generation telescopes and improve estimates of how much gold and other heavy elements these collisions produce.
Originaltitel: NLTE spectra of kilonovae
<p>The electromagnetic transient following a binary neutron star merger is known as a kilonova (KN). Owing to rapid expansion velocities and small ejecta masses, KNe rapidly transition into the non-local thermodynamic equilibrium (NLTE) regime. In this study, we present synthetic NLTE spectra of KNe from 5 to 20 d after merger using the sumo spectral synthesis code. We study three homogeneous composition, 1D multizone models with characteristic electron fractions of <em>Y</em><sub>e</sub> ∼ 0.35, 0.25, and 0.15. We find that emission features in the spectra tend to emerge in windows of reduced line blocking, as the ejecta are still only partially transparent even at 20 d. For the <em>Y</em><sub>e</sub> ∼ 0.35 (lanthanide-free) ejecta, we find that the neutral and singly ionized species of Rb, Sr, Y, and Zr dominate the spectra, all with good potential for identification. We directly test and confirm an impact of Sr on the 10 000 Å spectral region in lanthanide-free ejecta, but also see that its signatures may be complex. We suggest the Rb i5p<sup>1</sup>–5s<sup>1</sup>7900 Å transition as a candidate for the λ<sub>0</sub> ∼ 7500–7900 Å P-Cygni feature in AT2017gfo. For the <em>Y</em><sub>e</sub> ∼ 0.25 and 0.15 compositions, lanthanides are dominant in the spectral formation, in particular Nd, Sm, and Dy. We identify key processes in KN spectral formation, notably that scattering and fluorescence play important roles even up to 20 d after merger, implying that the KN ejecta are not yet optically thin at this time.</p>