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GCN Circular 25764

Subject
LIGO/Virgo S190915ak: Upper limits from Fermi-GBM Observations
Date
2019-09-16T18:36:03Z (5 years ago)
From
Colleen A. Wilson at NASA/MSFC/NSSTC <colleen.wilson@nasa.gov>
E. Burns (NASA/GSFC) and C.A. Wilson-Hodge (NASA/MSFC) reports on behalf of the Fermi-GBM Team and the GBM-LIGO/Virgo group

For S190915ak and using the initial bayestar skymap, Fermi-GBM was observing 98.0% 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 S190915ak (GCN 25753). 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 LVC localization region is behind the Earth for Fermi, located at RA=129.5, Dec=-19.0 with a radius of 67.5 degrees. 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:   11      13      23
1.024 s:   2.5     3.2      6.9
8.192 s:   1.1     1.3     1.6

Assuming the median luminosity distance of 1557 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
------------------------------------
0.128 s:    4.7      5.0      15
1.024 s:    1.1      1.2      4.4
8.192 s:    0.47     0.50     1.0
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