GCN Circular 21263

Adam Goldstein (USRA), Rachel Hamburg (UAH), and Colleen Wilson-Hodge
(NASA/MSFC) report on behalf of the GBM-LIGO Group:
Lindy Blackburn (CfA), Michael S. Briggs (UAH), Jacob Broida (Carleton
College), Eric Burns (UAH), Jordan Camp (NASA/GSFC), Tito Dal Canton
(NASA/GSFC), Nelson Christensen (Carleton College), Valerie Connaughton
(USRA), C. Michelle Hui (NASA/MSFC), Pete Jenke (UAH), Dan Kocevski
(NASA/MSFC), Nicolas Leroy (LAL), Tyson Littenberg (NASA/MSFC), Julie
McEnery (NASA/GSFC), Rob Preece (UAH), Judith Racusin (NASA/GSFC), Peter
Shawhan (UMD), Karelle Siellez (GA Tech), Leo Singer (NASA/GSFC), John
Veitch (Birmingham), Peter Veres (UAH)

Fermi GBM observed 89% of the Bayestar sky map at the time of the LIGO
trigger, and we set the following flux upper limits for the entire visible
sky map (excluded region is a circle with radius of 68 degrees centered on
RA, Dec = 197.9, 19.5).

Using a hard Band function with (Epeak, alpha, beta) = (500 keV, -0.5,
-2.5), we set a 3 sigma, 1-second-averaged flux upper limit for any
transient within 30 s of the LIGO trigger time in the 10-1000 keV band
ranging from 4.6e-7 to 9.2e-7 erg/s-cm^2.  Using an exponentially cut-off
power law parametrized with (Epeak, index) = (566 keV, -0.42), which
represents the average GBM-triggered short GRB, the upper limit ranges from
5.1e-7 to 9.8e-7 erg/s-cm^2.

Using the Earth Occultation technique [1] to estimate the amount of
persistent emission during a 48-hour period centered on the LIGO trigger
time, we place the following range of 3-sigma day-averaged flux upper
limits based on observed sources over the entire LIGO sky map:

Energy       min  max  median
--------------------------------
 12- 27 keV: 0.07 0.50 0.09 Crab
 27- 50 keV: 0.12 0.71 0.16 Crab
 50-100 keV: 0.18 0.97 0.23 Crab
100-300 keV: 0.33 1.88 0.41 Crab
300-500 keV: 2.25 14.9 2.84 Crab

These limits are based on the minimum requirement that each source in the
Earth Occultation catalog was Earth-occulted at least 6 times in each of
the 24 hour periods preceding and following the LIGO trigger and that the
occultations were well separated from nearby bright sources.

The location of the Fermi/LAT candidate (Omodei et al., LVC GCN #21227
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 ) was
behind the Earth at the time of the LIGO trigger. The location came out
from behind the Earth at ~85 s after the LIGO trigger and was visible until
Fermi entered the South Atlantic Anomaly (SAA) ~1 hour later and was then
behind the Earth again after Fermi exited the SAA. During this one hour
window, the targeted search ([2], [3]) found three long and spectrally soft
candidates (False Alarm Rates of 1 per 2.2 hours, 1 per 1.1 hours, and 1
per 0.7 hours), all of which localize to the Galactic plane and far from
the LAT candidate location.  At ~48 minutes into this one hour window
(518583038.924 MET / 2017-06-08 02:50:33.924 UTC) another, spectrally
harder, candidate was found with a duration of ~0.5 s with a False Alarm
Rate of ~1 per 1.5 hours. An initial localization of this candidate
indicates that the LAT candidate location is within the 2-sigma
statistical-only confidence region.  Follow-up analysis and classification
of this candidate is inconclusive at this time, as is any potential
association with the LAT candidate.  Absent of any GBM counterpart to the
LAT candidate, we set a 3-sigma, 1-second-averaged flux upper limit that
ranges from 4.7e-7 to 5.6e-7 erg/s-cm^2 during this time window.

[1] C. Wilson-Hodge et al. 2012, ApJS, 201, 33
[2] L. Blackburn et al. 2015, ApjS 217, 8
[3] A. Goldstein et al. arXiv:1612.02395
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