S. Ricciarini (U of Florence), A. Yoshida, T. Sakamoto,
V. Pal'shin, S. Sugita, Y. Kawakubo, A. Tezuka, S. Matsukawa,
H. Onozawa, T. Ito, H. Morita, Y. Sone (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (RIKEN), I. Takahashi (IPMU),
Y. Asaoka, S. Ozawa, S. Torii (Waseda U),
Y. Shimizu, T. Tamura (Kanagawa U),
W. Ishizaki (ICRR), M. L. Cherry (LSU),
A. V. Penacchioni, P. S. Marrocchesi (U of Siena)
and the CALET collaboration:
We have performed a search of hard X-ray/gamma-ray counterpart
to the IceCube neutrino event IceCube-180908A (The IceCube Collaboration,
GCN Circ. 23214) in the CALET Gamma-ray Burst Monitor (CGBM) data
around the event time at T0=19:59:31.84 UT on 2018 September 8.
The event position (RA, Dec (J2000) = 144.58 [-1.45,+1.55] deg,
-2.13 [-1.2,+0.9] deg) at that time was in the CGBM FOV at
favorable angles (the off-axis angles 19 deg and 16 deg for HXM
and SGM detector respectively).
No CGBM on-board trigger occurred around T0 (the nearest GRB triggered
CGBM at 20:28:41.478 UTC on 9 August 2018: Cherry et al., GCN 23116;
all subsequent CGBM triggers were due to local particles).
By using the CGBM time-history data (0.125 sec time resolution),
we found no statistically significant signal within T0 +- 800 sec interval
in either the HXM (7-3000 keV) or the SGM (40 keV - 28 MeV) data.
We estimate 7-sigma upper limit in the 40-1000 keV band as
6.7 x 10^-7 erg/cm2/s assuming a simple power-law model with
a photon index of -2 and 7.5 x 10^-7 erg/cm2/s assuming a GRB (Band) model
with the typical parameters for long GRBs: alpha = -1.0, Ep = 300 keV,
and beta = -2.5 (both in 1 sec exposure).
The CALET data used in this analysis are provided by the Waseda
CALET Operation Center located at the Waseda University.