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

We investigate how the gamma-ray burst (GRB) jet–ejecta interaction in neutron star mergers affects the early r -process-powered kilonova emission ( t ≤ 1 day), using the detailed atomic opacities developed in S. Banerjee et al. (2020, 2024), appropriate for epochs as early as t ∼ 1 hr after the merger. Exploring jets with different powers and opening angles, we find that the interaction shifts the spectral peak to longer wavelengths, most strongly for lines of sight near the polar (jet) axis. This is caused by the jet’s cocoon, which, after breaking out of the ejecta, forms a low-density outer layer. In this layer, the opacity can reach as high as κ exp ∼ 200 cm 2 g − 1 , forcing photons to escape from cooler, faster-moving material rather than from the hot inner ejecta. Similarly, the bolometric light curves are also modified, showing suppressed early time luminosity toward polar viewing angles compared to the equatorial direction, since the photosphere resides in this outer layer where radioactive heating is weaker than in the bulk ejecta. These signatures are also evident in multicolor light curves, particularly in the ultraviolet and u bands. The strongest observational signatures are expected in the UVEX NUV and Swift UVW2 and UVM2 bands, and remain detectable out to viewing angles of ∼60° for t ≲ 1 day. For example, in the UVEX NUV band at 100 Mpc, the jet can suppress the emission by up to ∼2.7 mag in the polar direction. A rapid follow-up with future ultraviolet facilities such as ULTRASAT and UVEX will provide valuable constraints on GRB jets through early time kilonova observations.