GCN Circular 33826

F. Verrecchia (SSDC, and INAF/OAR), M. Tavani (INAF/IAPS, and Univ.
Roma Tor Vergata), A. Ursi (ASI), C. Pittori (SSDC, and INAF/OAR),
M. Cardillo, C. Casentini, L. Foffano, G. Piano (INAF/IAPS), F.
Lucarelli (SSDC, and INAF/OAR), L. Baroncelli, A. Bulgarelli, A.
Ciabattoni, A. Di Piano, V. Fioretti, G. Panebianco, N. Parmiggiani
(INAF/OAS-Bologna), M. Pilia (INAF/OA-Cagliari), F. Longo (Univ.
Trieste, and INFN Trieste), report on behalf of the AGILE Team:

In response to the LIGO/Virgo GW event S230518h at T0 = 2023-05-18
12:59:08.167 (UT), a preliminary analysis of the AGILE minicalorimeter
(MCAL) triggered data found no significant event candidates within
a time interval covering -/+ 2 sec from the LIGO/Virgo T0.

At the T0, about 25% of the S230518h 90% c.l. localization region
(LR) was accessible to the AGILE/MCAL.

Three-sigma upper limits (ULs) are obtained in the 0.4-1 MeV energy
range, for a 1 s integration time at different celestial positions
within the accessible S230518h LR, from a minimum of 1.4E-06 erg
cm^-2 to a maximum of 6.6E-06 erg cm^-2 (assuming as spectral model
a single power-law with photon index 1.5).

An independent procedure based on photon counting statistics
provides UL fluences in the range 0.4-1 MeV, for a 300 microseconds
integration time, from a minimum of 1.1E-08 erg cm^-2 to a maximum
of 4.4E-08 erg cm^-2.

We notice the detection of a short pulse in the energy range 0.4-1.4
MeV with S/N 5.7 at time T0+10.77 s.

The AGILE/MCAL detector is a CsI detector with a 4 pi FoV, sensitive
in the energy range 0.4-100 MeV. Additional analysis of AGILE data
is in progress.