GCN Circular 33818

S. Dalessi (UAH) and C. Fletcher (USRA) report on behalf of the Fermi-GBM Team and the GBM-LIGO/Virgo/KAGRA group For S230518h and using the updated Bilby skymap (GCN 33816), Fermi-GBM was observing 48.9% of the localization probability at event time. There was no Fermi-GBM onboard trigger around the event time of the LIGO/Virgo/KAGRA (LVK) detection of GW trigger S230518h (GCN 33813). An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM also identified no counterpart candidates. The GBM targeted search, the most sensitive, coherent search for GRB-like signals, was run from +/-30 s around merger time, and also identified no counterpart candidates. Part of the LVK localization region is behind the Earth for Fermi, located at an RA=32.3, Dec=-3.6 with a radius of 67.6 degrees. We therefore set upper limits on impulsive gamma-ray emission for the LVK Consortia localization region visible to Fermi at merger time. Using the representative soft, normal, and hard GRB-like templates described in arXiv:1612.02395, we set the following 3 sigma flux upper limits over 10-1000 keV, weighted by GW localization probability (in units of 10^-7 erg/s/cm^2): Timescale Soft Normal Hard ------------------------------------ 0.128 s: 1.4 2.7 6.1 1.024 s: 0.4 0.8 2.0 8.192 s: 0.1 0.3 0.8 Assuming the median luminosity distance of 277.9 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^50 erg/s): Timescale Soft Normal Hard ------------------------------------ 0.128s: 0.020 0.035 0.132 1.024s: 0.006 0.010 0.043 8.192s: 0.002 0.004 0.018