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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 <inline-formula> <mml:math><mml:msub><mml:mrow><mml:mi>κ</mml:mi></mml:mrow><mml:mrow><mml:mi>exp</mml:mi></mml:mrow></mml:msub><mml:mo>∼</mml:mo><mml:mn>200</mml:mn><mml:mspace></mml:mspace><mml:msup><mml:mrow><mml:mi>cm</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msup><mml:mspace></mml:mspace><mml:msup><mml:mrow><mml:mi>g</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> </inline-formula>, 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.

We compute deviations from observed gravitational-wave signals, where the amplitude of the signal is unchanged. As an example, we consider the detectability of low-lying dynamical tides in binary neutron star or neutron star black hole mergers. Tidal forces can excite oscillatory modes of one or both of the stars in the binary when the orbital frequency of the binary system sweeps through the resonant mode frequency dissipating energy into the vibrational mode. The orbital energy loss to the vibrational mode extracts energy from the orbital motion, advancing the time to merger. The inspiral then continues with an excess phase and a time advance. Both will cause a mismatch when fitting to a system that has not gone through the resonance. To resolve this effect, we compute the mismatch for current and planned detectors using both a quasianalytical approach that relies on the computation of moment integrals and an optimized version of the standard numerical match function. We conclude that detectability can occur for time advances of the order of 1 ms with advanced LIGO-Virgo-KAGRA detectors for an excess energy flux that is a few percent of the gravitational-wave emission. Our results contrast with previous work, which model this effect solely as a phase shift of the waveform or by using the difference in the number of cycles induced by the resonant behavior. We show that tidal resonance effects primarily cause a time advance of the merger, rather than a phase difference, and that the single-frequency approximation commonly used in the literature significantly overestimates the detectability of this effect.