LIGO/Virgo/KAGRA S241109bn: Upper limits from Fermi-GBM Observations

For S241109bn (GCN 38142) and using the update bayestar skymap, Fermi-GBM was observing 71.8% 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 S241109bn. 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=229.0, Dec=-22.5 with a radius of 67.7 degrees. We therefore set upper limits on impulsive gamma-ray emission for the GW 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: 1.2 2.0 4.6 1.024 s: 0.34 0.59 1.2 8.192 s: 0.13 0.24 0.53 Assuming the median luminosity distance of 602.6 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: 0.080 0.122 0.466 1.024s: 0.023 0.036 0.122 8.192s: 0.009 0.015 0.054