Kilonova evolution -- the rapid emergence of spectral features

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]

A&A, in press

References in corpus (22)

• Origin of the heavy elements in binary neutron-star mergers from a gravitational wave event
• Swope Supernova Survey 2017a (SSS17a), the Optical Counterpart to a Gravitational Wave Source
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. II. UV, Optical, and Near-IR Light Curves and Comparison to Kilonova Models
• Spectroscopic identification of r-process nucleosynthesis in a double neutron star merger
• A kilonova as the electromagnetic counterpart to a gravitational-wave source
• Light Curves of the Neutron Star Merger GW170817/SSS17a: Implications for R-Process Nucleosynthesis
• The Emergence of a Lanthanide-Rich Kilonova Following the Merger of Two Neutron Stars
• Molecfit: A general tool for telluric absorption correction. I. Method and application to ESO instruments
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. IV. Detection of Near-infrared Signatures of r-process Nucleosynthesis with Gemini-South
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. III. Optical and UV Spectra of a Blue Kilonova From Fast Polar Ejecta
• Molecfit: A general tool for telluric absorption correction II. Quantitative evaluation on ESO-VLT X-Shooter spectra
• Identification of strontium in the merger of two neutron stars
• Galactic Chemical Evolution and solar s-process abundances: dependence on the 13C-pocket structure
• JWST detection of heavy neutron capture elements in a compact object merger
• Follow up of GW170817 and its electromagnetic counterpart by Australian-led observing programs
• The Rapid Reddening and Featureless Optical Spectra of the optical counterpart of GW170817, AT 2017gfo, During the First Four Days
• Modelling the spectra of the kilonova AT2017gfo -- I: The photospheric epochs
• Spherical symmetry in the kilonova AT2017gfo/GW170817
• Self-consistent 3D radiative transfer for kilonovae: directional spectra from merger simulations
• Discovery of a 760 nm P Cygni line in AT2017gfo: Identification of yttrium in the kilonova photosphere
• Measuring the Hubble constant with kilonovae using the Expanding Photosphere Method
• Emergence hour-by-hour of $r$-process features in the kilonova AT2017gfo

Cited by in corpus (5)

• Emergence hour-by-hour of $r$-process features in the kilonova AT2017gfo
• Helium features are inconsistent with the spectral evolution of the kilonova AT2017gfo
• Light Travel Time Effects in Kilonova Models
• Electron impact excitation of Te IV and V and Level Resolved R-matrix Photoionization of Te I - IV with application to modelling of AT2017gfo
• Inferring kilonova ejecta photospheric properties from early blackbody spectra