{
  "submittedHow": "web",
  "circularId": 41441,
  "bibcode": "2025GCN.41441....1D",
  "eventId": "LIGO/Virgo/KAGRA S250818k",
  "format": "text/plain",
  "createdOn": 1755687855959,
  "submitter": "Cuán de Barra at UCD <cuan.debarra@ucdconnect.ie>",
  "body": "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.\n\nThere 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.\n\nPart 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):\n\nTimescale  Soft   Normal   Hard \n------------------------------------\n0.128 s: 1.5 \t2.5 \t4.3\n1.024 s: 0.66   0.87    1.5\n8.192 s: 0.22   0.29    0.45\n\nAssuming 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):\n\nTimescale  Soft     Normal   Hard\n------------------------------------\n0.128s: 0.018 0.029 0.081\n1.024s: 0.008 0.010 0.028\n8.192s: 0.003 0.003 0.008",
  "subject": "LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations"
}