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GCN Circular 21641

LIGO/Virgo G298048: CALET Observations
2017-08-22T09:36:51Z (7 years ago)
Takanori Sakamoto at Aoyama Gakuin U <>
S. Nakahira (RIKEN), A. Yoshida, T. Sakamoto, Y. Kawakubo, M. Moriyama,
Y. Yamada, A. Tezuka, S. Matsukawa (AGU), K. Yamaoka (Nagoya U),
I. Takahashi (IPMU), Y. Asaoka, S. Ozawa, S. Torii (Waseda U),
Y. Shimizu, T. Tamura (Kanagawa U), W. Ishizaki (ICRR), M. L. Cherry (LSU),
S. Ricciarini (U of Florence), A. V. Penacchioni, P. S. Marrocchesi (U of Siena)
and the CALET collaboration:

The CALET Gamma-ray Burst Monitor (CGBM) was operating at the trigger time
of G298048 (The LIGO Scientific Collaboration and the Virgo Collaboration,
GCN Circ. 21509).  No CGBM on-board trigger occurred at the time
of the event.  Based on the updated LVC localization sky map
(preliminary-LALInference.fits.gz), none of the probability area was in the
field-of-view of the CGBM/HXM.  However, 99% of the summed probability was
inside the field-of-view of the CGBM/SGM.  The CGBM/SGM boresight position
(R.A., Dec. = 223.8 deg, 43.4 deg) was 73 deg off-axis from the maximum LVC
probability location (The LIGO Scientific Collaboration and the Virgo Collaboration,
GCN Circ. 21527).

Based on the analysis of the light curve data with 0.125 sec time resolution
from -60 sec to 60 sec from the trigger time, we found no significant excess
around the trigger time in either the HXM (7-3000 keV) or the SGM (40 keV -28 MeV)
data.  The incident angle of SGM to the position of SSS17a (Coulter et al.,
GCN Circ. 21529) was 71 deg.  Assuming the association of GRB 170817A
(Connaughton et al., GCN Circ. 21506) and SSS17a, we estimated the upper
limit of the SGM.  The 7-sigma upper limit of SGM using alpha=-0.9 and Epeak=130 keV
(Goldstein et al., GCN Circ. 21528) is 5.5e-7 erg/cm2/s in the 10-1000 keV band
at 1 s exposure assuming no shielding by ISS structures.  This estimated upper
limit is at the similar level to the reported peak flux, 7.3e-7 erg/cm2/s, of
GRB 170817A by Fermi-GBM.  We further noticed that the position of SSS17a
was covered by the large structure of ISS at the time of the trigger.  Because
of this, the SGM sensitivity should be worse than what we reported above.
Therefore, it is consistent that no signal of GRB 170817A is seen by the SGM data.

The CALET Calorimeter (CAL) was operating in the high energy trigger mode at the
trigger time of G298048.  However, no LVC high probability region was included
in the CAL's field of view at the time of the trigger.
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