AT2025ulz and S250818k: Deep X-Ray and Radio Limits on Off-axis Afterglow Emission and Prospects for Future Discovery

Abstract The first joint electromagentic (EM) and gravitational-wave (GW) detection, known as GW170817, marked a critical juncture in our collective understanding of compact object mergers. However, it has now been 8 yr since this discovery, and the search for a second EM-GW detection has yielded no robust discoveries. Recently, on 2025 August 18, the LIGO–Virgo–KAGRA Collaboration reported a low-significance (high-false-alarm rate) binary neutron star merger candidate S250818k. Rapid optical follow-up revealed a single optical candidate AT2025ulz ( z = 0.08484) that initially appeared consistent with kilonova emission. We quickly initiated a set of observations with Swift, XMM-Newton, Chandra, and the Very Large Array to search for nonthermal afterglow emission. Our deep X-ray and radio search rules out that the optical rebrightening of AT2025ulz is related to the afterglow onset, reinforcing its classification as a stripped-envelope supernova (SN 2025ulz). We derive constraints on the afterglow parameters for a hypothetical binary neutron star merger at the distance of AT2025ulz (≈400 Mpc) based on our X-ray and radio limits. We conclude that our observational campaign could exclude a GW170817-like afterglow out to viewing angles of θ v ≈ 12 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 5. We briefly discuss the prospects for the future discovery of off-axis afterglows.