Kilonova evolution -- the rapid emergence of spectral features (2312.02258v2)

Abstract: Kilonovae (KNe) are one of the fastest types of optical transients known, cooling rapidly in the first few days following their neutron-star merger origin. We show here that KN spectral features go through rapid recombination transitions, with features due to elements in the new ionisation state emerging quickly. Due to time-delay effects of the rapidly-expanding KN, a 'wave' of these new features passing though the ejecta is a detectable phenomenon. In particular, isolated line features will emerge as blueshifted absorption features first, gradually evolving into more pronounced absorption/emission P Cygni features and then pure emission features. In this analysis, we present the evolution of the individual exposures of the KN AT2017gfo observed with VLT/X-shooter that together comprise X-shooter's first epoch spectrum (1.43 days post-merger). We show that the spectra of these 'sub-epochs' show a significant evolution across the roughly one hour of observations, including a decrease of the blackbody temperature and photospheric velocity. The cooling blackbody constrains the recombination-wave, where a Sr II interpretation of the AT2017gfo $1\mu$m feature predicts both a specific timing for the feature emergence and its early spectral shape, including the very weak emission component observed at about 1.43 days. This empirically indicates a strong correspondence between the radiation temperature and the ejecta's electron temperature. Furthermore, this reverberation suggests that temporal modelling is important for interpreting individual spectra and that higher cadence spectral series, especially when concentrated at specific times, can provide strong constraints on KN line identifications and the ejecta physics. Given the use of such short-timescale information, we lay out improved observing strategies for future KN monitoring. [abridged]

• The paper demonstrates that rapid recombination in kilonova ejecta produces evolving spectral features, highlighting complex ionization dynamics.
• Detailed spectral analysis of AT2017gfo uncovers a significant early temperature decline that challenges conventional single power-law cooling models.
• Identification of strontium in early spectra constrains electron temperature and ionization states, underscoring the need for high-cadence observations.

Kilonova Evolution: Rapid Emergence of Spectral Features

In this paper, Sneppen et al. explore the detailed evolution of kilonova (KN) spectra, with a focus on the rapid emergence of spectral features post-merger as observed in AT2017gfo, the electromagnetic counterpart to the GW170817 neutron-star merger event. Utilizing the VLT/X-shooter spectrum, the authors dissect the temporal spectrum evolution shortly after the merger and its implications for the understanding of KN physics.

Key Findings

1. Rapid Recombination and Emergence of Spectral Features:
   • The paper highlights the rapid transition of KN spectral features due to changes in ionization states of elements within the ejecta.
   • A notable phenomenon is the propagation of a "recombination wave" through the rapidly expanding ejecta, producing detectable features as elements recombine and emit characteristic lines.
2. Detailed Spectral Analysis:
   • Detailed observations of the first epoch spectra of AT2017gfo indicate a significant temperature decline from sub-epoch spectral fits.
   • The analysis finds that temperature cooling is more pronounced initially than captured by a single power-law relation, necessitating a revision of previous models.
3. Strontium Line Identification:
   • The 1 µm feature in the early KN spectra is identified as being due to strontium (Sr), with its emergence providing empirical constraints on the ionization states and electron temperature in the ejecta.
4. Temporal Evolution of Line Features:
   • The rapid progression from absorption to P Cygni profiles and to emission alone, is indicative of the changing physical conditions and composition through time.

Implications

The observations and analysis presented in this work underscore the necessity of high-time-resolution observations in constraining models of KN evolution. This has significant implications for the:

• Accurate reconstruction of the physical processes in the expanding ejecta.
• Interpretation of line features for understanding nucleosynthesis through r-process.
• Future observational strategies in capturing the short-lived, dynamically changing features inherent in such energetic astrophysical phenomena.

Practical Applications and Future Directions

The analysis suggests that enriched temporal resolution (higher cadence spectral series) is essential to capture the rapid changes in KN spectral features, pivotal for:

• Refining radiative transfer models to account for rapid recombination and the transition between ionization states.
• Improving strategies for real-time identification and monitoring of transient events using next-generation telescopes.
• Future studies could incorporate enhanced models for line-formation and radiative transfer using data from instruments capable of higher cadence measurements, such as the upcoming James Webb Space Telescope (JWST) and extremely large telescopes for comprehensive KN monitoring.

The paper provides critical insights that build a strong case for developing advanced simulation tools that faithfully replicate the transient nature of kilonovae, harnessing observational strategies optimized for capturing these evolving phenomena.