WINTER on S250206dm: A Near-infrared Search for an Electromagnetic Counterpart to a Gravitational-wave Event

We present near-infrared follow-up observations of the International Gravitational Wave Network event S250206dm with the Wide-Field Infrared Transient Explorer (WINTER). Near-infrared observations are a critical component of electromagnetic follow-up to gravitational-wave events, as kilonovae are expected to exhibit long-lived emission at these wavelengths, especially from lanthanide-rich ejecta. WINTER is a near-infrared time-domain survey facility designed for EM follow-up of gravitational-wave sources, featuring a wide field of view (1.2 deg²), a dedicated 1 m robotic telescope, and coverage spanning 0.9–1.7 μm. S250206dm is the only neutron star merger in the fourth observing run, to date, localized to ≤300 deg² with a False Alarm Rate below one per year, making it a particularly valuable target for follow-up. It has a 55% probability of being a neutron star-black hole merger and a 37% probability of being a binary neutron star merger. The event’s estimated distance is 373 Mpc, with a 50% credible region spanning 38 deg². WINTER covered 43% of the probability area at least once and 35% at least three times. Through automated and human candidate vetting, all transients were rejected as kilonova candidates. Given the large distance of the event, the WINTER upper limits do not place meaningful constraints on kilonova models. However, similar observations of future events-or in combination with optical surveys-can begin to exclude portions of the kilonova model space. This study highlights the promise of systematic infrared searches and the need for future wider and deeper infrared surveys.