Constraining Possible Γ-Ray Burst Emission from GW230529 Using Swift-BAT and Fermi-GBM

GW230529 is the first compact binary coalescence detected by theLIGO-Virgo-KAGRA collaboration with at least one component mass confidently inthe lower mass-gap, corresponding to the range 3-5M_⊙. If interpretedas a neutron star-black hole merger, this event has the most symmetric massratio detected so far and therefore has a relatively high probability ofproducing electromagnetic (EM) emission. However, no EM counterpart has beenreported. At the merger time t_0, Swift-BAT and Fermi-GBM together covered100% of the sky. Performing a targeted search in a time window [t_0-20s,t_0+20 s], we report no detection by the Swift-BAT and theFermi-GBM instruments. Combining the position-dependent γ-ray flux upperlimits and the gravitational-wave posterior distribution of luminositydistance, sky localization and inclination angle of the binary, we deriveconstraints on the characteristic luminosity and structure of the jet possiblylaunched during the merger. Assuming a top-hat jet structure, we exclude at90% credibility the presence of a jet which has at the same time an on-axisisotropic luminosity ≳ 10^48 erg s^-1, in the bolometric band 1keV-10 MeV, and a jet opening angle ≳ 15 deg. Similar constraints arederived testing other assumptions about the jet structure profile. ExcludingGRB 170817A, the luminosity upper limits derived here are below the luminosityof any GRB observed so far.