Swift-BAT GUANO follow-up of Gravitational Wave triggers in the third advanced LIGO/Virgo Observing Run

In recent years, the implementation of the Gamma-ray Urgent Archiver for Novel Opportunities (GUANO) system has significantly enhanced the sensitivity of Swift-BAT in detecting and localizing astrophysical transients. In this presentation, I will share the results of our search for electromagnetic (EM) counterparts of subthreshold gravitational wave (GW) candidates as well as cataloged GW events, utilizing data obtained by GUANO during the LIGO-Virgo O3b observing run. As part of this work, we carry out targeted searches on the sample using the Non-Imaging Transient Reconstruction And TEmporal Search (NITRATES) pipeline and derive the most sensitive upper limits to the EM emission from these candidates. We construct skymaps that report flux upper limits in the 15-350 keV band as a function of sky position for all the catalog events. The derived Swift-BAT upper limits are further used to infer constraints about the putative EM emission associated with binary black hole mergers. This work highlights the significance of carrying out systematic subthreshold searches with instruments like Swift-BAT in the multi-messenger era, to achieve the most sensitive monitoring of the transient sky and to derive the most stringent constraints on the EM counterparts to GWs.