TITLE: GCN CIRCULAR NUMBER: 41441 SUBJECT: LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations DATE: 25/08/20 11:04:15 GMT FROM: Cuán de Barra at UCD cuan.debarra@ucdconnect.ie

For S250818k (GCN 41437, LVK Collaboration) and using the updated bayestar skymap, Fermi-GBM was observing 51.0% 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 S250818k . 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=66.9, Dec=-25.2 with a radius of 68.0 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: 1.5 2.5 4.3 1.024 s: 0.66 0.87 1.5 8.192 s: 0.22 0.29 0.45

Assuming the median luminosity distance of 259.4 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: 0.018 0.029 0.081 1.024s: 0.008 0.010 0.028 8.192s: 0.003 0.003 0.008

View this GCN Circular online at https://gcn.nasa.gov/circulars/41441. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...