GCN Circular 34095

S. Dalessi (UAH) reports on behalf of the Fermi-GBM Team:


For S230627c (GCN 34086
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 and GCN 34087
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) and using the updated Bilby skymap, Fermi-GBM was observing 99.8% 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 S230627c . 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 [1,2,3], the most sensitive, coherent search for GRB-like signals, was run from +/-30 s around merger time, and also identified no counterpart candidates.

For Fermi, part of the LVK localization region is behind the Earth, located at an RA=84.8, Dec=-18.5 with a radius of 67.6 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
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, 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:   1.9     3.3      5.4
1.024 s:   0.7     0.9      1.5
8.192 s:   0.2     0.3      0.5

Assuming the median luminosity distance of 291.0 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:   0.047    0.066    0.215
1.024s:   0.015    0.019    0.060
8.192s:   0.004    0.006    0.018

[1] Blackburn et al. 2015, ApJS 217, 8
[2] Goldstein et al. arXiv:1612.02395
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[3] Goldstein et al. arXiv:1903.12597
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