Ultraviolet Signatures of Jet-Ejecta Interaction in Early Kilonovae: Prediction from Realistic Atomic Opacities

TL;DR

This work presents the first radiative-transfer predictions for jet-interacted binary neutron star merger ejecta using detailed, early-time opacities up to ion XI. It shows that jet propagation creates a thin, low-density outer layer that elevates opacity and yields cooler, outer-layer–driven emission, shifting the spectral peak toward longer wavelengths and dimming early polar light curves. The results emphasize the critical role of wavelength-dependent opacities in predicting UV signatures, and they demonstrate that UV bands such as UVEX NUV, Swift UVW2, and UVM2 offer the strongest observational leverage, with detectability out to viewing angles of about $60^{\circ}$ within $t\le 1$ day. The study also contrasts with prior gray-opacity approaches, underscoring the need for detailed opacities to accurately capture early kilonova behavior and guiding follow-up strategies with upcoming UV facilities like ULTRASAT and UVEX.

Abstract

We investigate the signature of the jet-ejecta interaction in early kilonova (t < 1day) using detailed atomic opacities developed in Banerjee et al. (2020, 2024), appropriate for early times (t~1hour after merger). We explore jets with different powers and opening angles. We find that the presence of the jet shifts the spectral peak to longer wavelengths, with the strongest effect near the polar viewing angle. This occurs because the jet creates a thin, low-density outer layer ahead of the bulk ejecta. The opacity of this layer can be as high as kappa ~200 cm2/g, causing photons to escape from cooler, faster-moving outer layer rather than from the hot inner ejecta. The bolometric light curves likewise exhibit a clear imprint of the jet-ejecta interaction, showing suppressed early-time luminosity near polar viewing angles compared to the equatorial one, as the photosphere resides in this thin layer where radioactive heating is lower than in the bulk ejecta. These signatures are also evident in multi-color light curves, particularly in the ultraviolet and u-bands. In the Swift-UVW2 band at t~= 0.15 days for a source at 100 Mpc, the ultraviolet luminosities can reach ~ 19.5 mag if no jet is present, while the presence of the jet can make it fainter by ~ 2.5 mag. The strongest observational signature occurs in the UVEX-NUV, Swift-UVW2, and UVM2 bands, which remains detectable out to viewing angles of ~ 60 deg for t <=1 days. Rapid follow-up with future ultraviolet facilities, such as ULTRASAT and UVEX, will provide powerful probes of jet-ejecta interaction through early-time kilonova observations.

Figures (13)

• Figure 1: Two-dimensional ejecta structures for different models at $t = 0.03$ days, which is the starting point of our radiative-transfer simulations. The hydrodynamic simulation results are post-processed by mapping them onto an axisymmetric velocity grid with the resolution adopted for the radiative-transfer calculations. Moreover, we exclude all material with $v > 0.9c$. Left: ejecta after interaction with the narrow jet (NJ); Right: ejecta after interaction with the wide jet (WJ). Only the high-luminosity jet models (HE) are shown here. The jet–ejecta interaction produces a thin layer at the leading edge of the ejecta, particularly along the polar direction, while the equatorial regions, being far from the jet core, remain largely unaffected. The black central region denotes material with $v < 0.025c$. This component is omitted from our simulations, as its low mass and extreme optical depth make its contribution to the early-time light curves negligible.
• Figure 2: One-dimensional ejecta structures for all models shown in \ref{['tab:model']} at $t = 0.03$ days, marking the starting point of our radiative-transfer simulations, shown along the polar ($\theta_{v} = 0-14^{\circ}$, left) and equatorial ($\theta_{v} = 88-90^{\circ}$, right) directions. Different colors indicate different models, including the PL model without jet interaction. Both the narrow- and wide-jet cases (NJ and WJ) exhibit significant modifications to the density structure relative to the PL model, most prominently along the polar direction. In both cases, a thin layer develops at the leading edge of the polar ejecta, while the equatorial ejecta remains largely unchanged. For the narrow-jet models (HE-NJ and LE-NJ), this layer follows a steeply declining power-law–like profile, whereas in the wide-jet models, the density distribution differs owing to the broader interaction induced by the larger opening angle.
• Figure 3: Opacity evolution with temperature for different densities at $t = 0.1$ days taken from Smaranikab20Smaranikab24. The black line shows the gray opacity adopted by most earlier works (e.g., Klion21).
• Figure 4: Kilonova spectra for different models at viewing angles: polar ($\theta_{\rm v}=0$–$14^\circ$, top left), GW170817-like ($\theta_{\rm v}=25$–$29^\circ$, top right), and equatorial ($\theta_{\rm v}=88$–$90^\circ$, bottom right). The spectral peak shifts towards longer wavelengths for the jet-interacted ejecta, especially for near-polar viewing angles. For the equatorial viewing angle, the differences almost vanish.
• Figure 5: Temperature profiles for the different models at $t = 0.1$ days are shown along the polar ($\theta_{\rm v}=0$–$14^\circ$, left) and equatorial ($\theta_{\rm v}=88$–$90^\circ$, right) directions. The jet-interacted ejecta exhibit modified temperature structures, reflecting the modified densities caused by the jet interaction, particularly along the polar direction. At high velocities in the polar region, these deviations are the strongest in the wide-jet models, consistent with their more significant modification of the thin outer layer. In contrast, along the equatorial direction, temperature differences are small, except in the innermost ejecta, where minor changes arise from weak density perturbations induced by the jet.