TITLE: GCN CIRCULAR NUMBER: 39183 SUBJECT: LIGO/Virgo/KAGRA S250206dm: Upper limits from Fermi-GBM Observations DATE: 25/02/07 02:04:13 GMT FROM: oindabimukherjee@gmail.com

O. Mukherjee (USRA) and R. Hamburg (USRA) report on behalf of the Fermi-GBM Team:

For S250206dm (GCN 39175; GCN 39178) and using the updated Bilby skymap, Fermi-GBM was observing 35.7% 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 S250206dm. 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=24.7, Dec=-4.2 with a radius of 67.9 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 2.4 5.1 1.024 s: 0.44 0.72 1.3 8.192 s: 0.08 0.17 0.31

Assuming the median luminosity distance of 358.7 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: 0.71 1.02 2.95 1.024s: 0.09 0.14 0.47 8.192s: 0.02 0.02 0.04

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