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

TL;DR

This paper predicts ultraviolet signatures of jet-ejecta interactions in early kilonovae using detailed atomic opacities, highlighting observable differences in light curves and spectra depending on jet properties and viewing angles.

Contribution

It introduces realistic atomic opacity calculations to model early kilonova signatures of jet-ejecta interactions, providing new predictions for ultraviolet observational features.

Findings

Jet presence shifts spectral peak to longer wavelengths.

Ultraviolet luminosity can be suppressed by ~2.5 mag due to jets.

UV signatures remain detectable up to 60 degrees viewing angle.

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.