TITLE: GCN CIRCULAR NUMBER: 25823 SUBJECT: LIGO/Virgo S190923y: Upper limits from Fermi-GBM Observations DATE: 19/09/23 19:16:28 GMT FROM: Bagrat Mailyan at UAH B. Mailyan (UAH) and J. Wood (NASA) report on behalf of the Fermi-GBM Team and the GBM-LIGO/Virgo group For S190923y and using the initial bayestar skymap, Fermi-GBM was observing 42.3% of the localization probability at event time. There was no Fermi-GBM onboard trigger around the event time of the LIGO/Virgo detection of GW trigger S190923y (GCN 25814). 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. We therefore set upper limits on impulsive gamma-ray emission for the LVC 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 norm hard -------------------------------------- 0.128 s: 2.2 4.1 8.8 1.024 s: 0.74 1.3 2.4 8.192 s: 0.26 0.4 0.79 Assuming the median luminosity distance of 438 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^48 erg/s): Timescale Soft Normal Hard ------------------------------------ 0.128s: 9.4 16.2 57.4 1.024s: 3.2 5.1 15.6 8.192s: 1.1 1.6 5.2