Emergence hour-by-hour of $r$-process features in the kilonova AT2017gfo

TL;DR

This study analyzes the spectral evolution of kilonova AT2017gfo over the first 9.4 days, revealing key features about ejecta velocities, ionization states, and temperature, providing insights into neutron star merger physics.

Contribution

It presents a detailed spectral time series analysis of AT2017gfo, linking spectral feature emergence to ejecta properties and ionization conditions, with new measurements of ejecta velocities and temperature.

Findings

Detection of the earliest 1μm P Cygni line at 1.17 days.

Evidence for the fastest ejecta component at 0.40-0.45c.

Consistent ionization temperature with blackbody radiation temperature.

Abstract

The spectral features in the optical/near-infrared counterparts of neutron star mergers (kilonovae, KNe), evolve dramatically on hour timescales. To examine the spectral evolution we compile a temporal series complete at all observed epochs from 0.5 to 9.4 days of the best optical/near-infrared (NIR) spectra of the gravitational-wave detected kilonova AT2017gfo. Using our analysis of this spectral series, we show that the emergence times of spectral features place strong constraints on line identifications and ejecta properties, while their subsequent evolution probes the structure of the ejecta. We find that the most prominent spectral feature, the 1$\mathrm{\mu}$m P Cygni line, appears suddenly, with the earliest detection at 1.17 days. We find evidence in this earliest feature for the fastest kilonova ejecta component yet discovered, at 0.40-0.45$c$; while across the observed epochs and wavelengths, the velocities of the line-forming regions span nearly an order of magnitude, down to as low as 0.04-0.07$c$. The time of emergence closely follows the predictions for Sr II, due to the rapid recombination of Sr III under local thermal equilibrium (LTE) conditions. The time of transition between the doubly and singly ionised states provides the first direct measurement of the ionisation temperature, This temperature is highly consistent, at the level of a few percent, with the temperature of the emitted blackbody radiation field. Further, we find the KN to be isotropic in temperature, i.e. the polar and equatorial ejecta differ by less than a few hundred Kelvin or within 5%, in the first few days post-merger, based on measurements of the reverberation time-delay effect. This suggests that a model with very simple assumptions, with single-temperature LTE conditions, reproduces the early kilonova properties surprisingly well.