GCN Circular 23242

GRB 180914B: AGILE/MCAL refined analysis

A. Ursi (INAF/IAPS), F. Verrecchia (SSDC, and INAF/OAR), M. Tavani
(INAF/IAPS, and Univ. Roma Tor Vergata), F. Longo (Univ. Trieste and INFN
Trieste), A. Argan, M. Cardillo, Y. Evangelista, G. Piano (INAF/IAPS), F.
Lucarelli, C. Pittori (SSDC, and INAF/OAR), A. Bulgarelli, V. Fioretti, F.
Fuschino, N. Parmiggiani (INAF/OAS), M. Marisaldi (INAF/OAS, and Bergen
University), M. Pilia, A. Trois (INAF/OA-Cagliari), I. Donnarumma (ASI), A.
Giuliani (INAF/IASF-Mi), report on behalf of the AGILE Team:

We present a refined analysis of the long GRB 180914B detected by
AGILE/MCAL (Ursi et al., GCN #23226), AGILE/GRID (Verrecchia et al., GCN
#23231), Fermi/LAT (Bissaldi et al., GCN #23232), and Konus-Wind (Frederiks
et al., GCN #23240), whose afterglow has been so far detected by Swift/XRT
(D'Ai et al., GCN #23236) and Swift/UVOT (Kuin et al., GCN #23241), KAIT
(Zheng & Filippenko, GCN #23237), RATIR (Troja et al., GCN #23238), and OAJ
(de Ugarte Postigo et al., GCN #23239).

The AGILE Mini-CALorimeter (MCAL) detector high time resolution data
acquisition was triggered twice by the GRB on sub-millisecond and 16 ms
timescales.

A first GRB peak triggered complete MCAL data acquisition, starting at T1 =
2018-09-14 18:23:02.22 +/- 0.01 s (UTC) and ending at about T1+6.4 s, and
lasted the whole duration of the acquisition. The time-integrated spectrum
measured between T1 and T1+6.4 s can be fitted in the energy range 0.5-40
MeV with a power law with photon index 1.74 -0.13/+0.06 and a cut-off
energy of 2.35 -2.2/+1.54 MeV, with a reduced chi-square of 1.13 (66
d.o.f.). The burst fluence in the same energy range and time interval is
2.4e-04 erg/cm^2 (90% confidence level).

After ~85 s, a second GRB peak triggered complete MCAL data acquisition,
starting at T2 = 2018-09-14 18:24:27.84 +/- 0.01 s and ending at about
T2+6.4, and lasted about 2.7 s. The time-integrated spectrum measured
between T2 and T2+2.7 s can be fitted in the 0.5-40 MeV energy range with a
simple power law with ph.ind. = 2.13 -0.07/+0.08 and a reduced chi-square
of 1.23 (68 d.o.f.). The corresponding fluence is 1.94e-05 erg/cm^2 (90%
confidence level).

The AGILE-MCAL detector has a full solid angle acceptance, and is
operational in the range 0.4 - 100 MeV. This observation was done with the
MCAL trigger capability set to a reduced mode with substantial TM
constraints, and the Super-AGILE imaging being turned-off. A substantially
enhanced AGILE instrument configuration is foreseen to be implemented
shortly.