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GCN Circular 36793

LIGO/Virgo/KAGRA S240629by: Upper limits from Fermi-GBM Observations
2024-06-30T02:18:52Z (19 days ago)
Vidushi Sharma at NASA GSFC/UMBC <>
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V. Sharma (NASA GSFC/UMBC) reports on behalf of the Fermi-GBM Team:

For S240629by (GCN 36784) and using the initial bayestar skymap, Fermi-GBM was observing 17.8% 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 S240629by. 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=30.0, Dec=-25.6 with a radius of 67.4 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:  2.1 	 3.5 	7.2
1.024 s:  0.7 	 1.1 	2.1
8.192 s:  0.3 	 0.5 	0.9

Assuming the median luminosity distance of 1205 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:  0.6	0.8	  2.9 
1.024s:  0.2	0.3	  0.8 
8.192s:  0.1	0.1       0.4
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