GRB 221009A/Swift J1913.1+1946: AGILE-GRID detection

ATel #15662; G. Piano (INAF/IAPS), F. Verrecchia (SSDC and INAF/OAR), A. Bulgarelli (INAF/OAS-Bo), A. Ursi (INAF/IAPS), G. Panebianco (Univ. Bologna - INAF/OAS-Bo), C. Pittori (SSDC and INAF/OAR), M. Tavani (INAF/IAPS, and Univ. Roma Tor Vergata), F. Longo (Univ. Trieste and INFN Trieste), M. Cardillo, C. Casentini (INAF/IAPS), V. Fioretti, N. Parmiggiani, A. Addis, L. Baroncelli, A. Di Piano (INAF/OAS-Bo), F. Lucarelli (SSDC and INAF/OAR), S. Vercellone (INAF/OA-Brera), I. Donnarumma (ASI), F. Gianotti, M. Trifoglio (INAF/OAS-Bo), A. Giuliani, S. Mereghetti, P. Caraveo, F. Perotti (INAF/IASF-Mi), A. Chen (Wits University), A. Argan, E. Costa, E. Del Monte, Y. Evangelista, M. Feroci, L. Foffano, I. Lapshov, E. Menegoni, L. Pacciani, P. Soffitta, V. Vittorini (INAF/IAPS), F. Lazzarotto (INAF/OAPD), G. Di Cocco, F. Fuschino, M. Galli, C. Labanti (INAF/OAS-Bo), M. Marisaldi (University of Bergen), A. Pellizzoni, M. Pilia, A. Trois (INAF/OA-Cagliari), G. Barbiellini (INFN Trieste), E. Vallazza (INFN Milano Bicocca), A. Morselli, P. Picozza (INFN and Univ. Roma Tor Vergata), M. Prest (Univ. dell'Insubria), P. Lipari, D. Zanello (INFN and Univ. Roma Sapienza), P. W. Cattaneo, A. Rappoldi (INFN Pavia), A. Ferrari (Univ. Torino and CIFS), F. Paoletti (East Windsor RSD Hightstown and INAF/IAPS), L. A. Antonelli (INAF/OAR), P. Giommi, L. Salotti, G. Valentini, and F. D'Amico (ASI)
on 11 Oct 2022; 14:59 UT
Credential Certification: Giovanni Piano (giovanni.piano@inaf.it)

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The GRID detector onboard the AGILE satellite detected the bright emission classified as GRB 221009A or Swift J1913.1+1946 (z = 0.151, GCN #32648) , reported by Swift/BAT (GCN #32635) at T0 = 2022-10-09 13:16:59 (UTC), and detected by several facilities up to TeV energies (ATel #15653, #15655, #15656, #15660, #15661, GCN #32636, #32641, #32658, #32661, #32668, #32677).

Integrating from 2022-10-09 UT 13:16:59 (T0) to 2022-10-09 UT 14:16:59 (T0 + 1h), a preliminary multi-source likelihood analysis yields a detection with a significance of 42 sigma and a gamma-ray flux F(>100 MeV) = (1.11 +/- 0.08) x 10^-3 photons/cm^2/s, by adopting a power-law model with a (standard reference) photon index value of -2.1 (GCN #32657). The angular position of the gamma-ray event, as detected by the AGILE/GRID, is: (l, b) = (52.99, 4.26) +/- 0.17 deg (stat.) +/- 0.10 deg (syst.)

The AGILE-GRID count map in Galactic coordinates, related to this time-integration, can be found at http://www.agilescienceapp.it/notices/Swift_J1913_09oct_T0_1h_cts.png , with the AGILE-GRID contour level (95% C.L., stat. only, green) and the nominal position of Swift J1913.1+1946 (black cross) as reported by Swift/BAT (ATel #15650).

Interestingly, AGILE-GRID found significant detections both below and above 400 MeV in the time interval between T0 and T0 + 1h. For 100 MeV < E < 400 MeV and E > 400 MeV, preliminary multi-source likelihood analyses found a detection with a significance of 32 sigma with a gamma-ray flux F(<400 MeV) = (7.8 +/- 0.7) x 10^-4 photons/cm^2/s, and a detection with a significance of 27 sigma with a gamma-ray flux F(>400 MeV) = (3.5 +/- 0.4) x 10^-4 photons/cm^2/s, respectively.

After the burst, integrating from T0 + 1h to T0 + 2h, we found a faint detection with a significance of 3.4 sigma and a gamma-ray flux F(>100 MeV) = (2 +/- 1) x 10^-5 photons/cm^2/s. Before the burst, integrating from T0 - 2h to T0, no gamma-ray signal was detected with a 2-sigma UL(>100 MeV) = 8.2 x 10^-6 photons/cm^2/s. Additional analysis of the AGILE data is in progress.

The AGILE-GRID detection was obtained while AGILE is observing in spinning mode, surveying a large fraction of the sky each day. The source can be monitored with the public mobile application "AGILEScience" developed by the AGILE Team, available for both Android and iOS devices.