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

TL;DR

This paper introduces the Ep-Liso correlation as a new diagnostic tool for kilonova transients, based on observations of AT2017gfo and supported by radiative transfer simulations, enhancing understanding of kilonova physics.

Contribution

The study discovers a novel correlation between peak energy and luminosity in kilonovae, supported by simulations, offering a new method for analyzing these events.

Findings

Ep-Liso correlation observed in AT2017gfo data

Correlation predicted by radiative transfer models

Potential for improved kilonova diagnostics

Abstract

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, providing evidence of r-process nucleosynthesis. The kilonova signal was extensively studied in the spectral and time domains, giving 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 EF_E 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/-0.02) + 0.62 (+0.02/-0.02) * log10[Liso/(1e41 erg/s)] (68% C.L.) while in subsequent 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, 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.