The Ep−Liso correlation: A new diagnostic tool for kilonova transients

The AT2017gfo kilonova transient remains a unique multi-messenger event thanks to its proximity (𝑧 = 0.00987) and the possibility to investigate time-resolved spectra, thus providing evidence of r-process nucleosynthesis. The kilonova signal was extensively studied in the spectral and time domains, providing key insights into the chemical composition and physical properties of the ejecta. Here, we report the discovery of a novel correlation between two fundamental observables: the peak energy of the 𝐸 𝐹𝐸 spectrum, 𝐸p , and the isotropic-equivalent luminosity, 𝐿iso . In particular, we show that up to about 2.5 days after the merger, the AT2017gfo spectrum evolves according to: log10 [𝐸p ∕eV] = −0.13+0.02 + 0.62+0.02 log10 [𝐿iso ∕(1041 erg s−1 )] (68 % C.L.) while in the sub-sequent epochs, 𝐸p remains almost constant with 𝐿iso , flattening around 1 eV. Exploiting simulations from a state-of-the-art radiative transfer code, we demonstrate that our kilonova model inherently predicts this peculiar correlation, hence suggesting a new diagnostic tool for comparing observables against simulations. Future kilonova observations will provide additional insight into the physics behind the 𝐸p − 𝐿iso correlation.