M. Kerr, J. E. Grove, C.C. Cheung, R. Woolf (Naval Research Laboratory) and A. Goldstein (USRA) report:
The compact binary merger candidate S230518h (event time, T0 = 2023-05-18 12:59:08.167 UTC) was identified by the LIGO Observatory (H1, L1) during its engineering run prior to the start of the O4 observing run (GCN 33813).
The Glowbug gamma-ray instrument [1,2], launched in March 2023, observed much of the sky to which a high probability for the candidate origin was assigned. Glowbug has an all-sky field-of-view limited only by earth occultation and large-scale structures on the International Space Station (ISS). At T0, the Glowbug boresight was pointed towards R.A., Dec. = 158, 59 deg. The probability peak for the candidate origin (R.A., Dec. = 96, -11 deg.) was thus 78 deg. below the Glowbug boresight and more than 30 deg. above the earth limb. In total, 81% of the 90% confidence counterpart region was unocculted by the earth.
Using 50-2000 keV data and representative three representative (soft, medium, and hard spectrum) GRB templates from , we searched for transient gamma-ray signals using maximum likelihood methods and found no plausible counterpart up to 30s before or after T0. We determined 3-sigma upper limits on the flux for a GRB originating at the probability peak (R.A., Dec. = 96, -11 deg.) by selecting data centered on T0 and integrating the posterior probability to determine the flux beyond which the tail probability is 0.27%. For various timescales and the three GRB spectral templates, these limits in units of 1e-7 erg/cm2/s are:
Timescale Soft Normal Hard
0.128 s: 0.9 1.8 9.1
1.024 s: 0.2 0.5 2.5
8.192 s: 0.2 0.3 1.2
These results do not account for scattering or occultation by structures on the ISS.
Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS. The detector comprises 12 large-area (15 cm x 15 cm) CsI:Tl panels covering the surface of a half cube, and two hexagonal (5-cm diameter, 10-cm length) CLLB scintillators, giving it a large field of view (instantaneous FoV ~2/3 sky) over a wide energy band of 50 keV to >2 MeV.
 Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
 Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
 Goldstein, A. et al. 2020, ApJ 895, 40, arXiv :1909.03006
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