LIGO/Virgo/KAGRA S241125n: Upper limits from Fermi-GBM Observations

For LIGO/Virgo/Kagra (LVK) S241125n (GCN 38305) and the Swift/BAT-GUANO candidate counterpart (Tohuvavohu, A. et al. 2024, GCN 38308), we used the LVK/BAT combined skymap (combined-ext.multiorder.fits,2; LVK and Swift/BAT, GCN 38315). Fermi-GBM was observing 97.1% of the localization probability at event time. There was no Fermi-GBM onboard trigger around the event time of the LVK detection of GW trigger S241125n. 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-BAT localization region is behind the Earth for Fermi, located at an RA=335.8, Dec=12.1 with a radius of 67.7 degrees. We therefore set upper limits on impulsive gamma-ray emission for the LVK-BAT 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: 1.7 2.9 5.5 1.024 s: 0.6 1.0 1.7 8.192 s: 0.2 0.3 0.6 Assuming the median luminosity distance of 3213.8 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: 3.3 4.9 15.8 1.024s: 1.3 1.6 4.9 8.192s: 0.4 0.5 1.6