TITLE:   GCN CIRCULAR
NUMBER:  25770
SUBJECT: LIGO/Virgo S190915ak: No transient candidates in CALET observations
DATE:    19/09/17 04:41:17 GMT
FROM:    Yuta Kawakubo at Louisiana State U./CALET  <kawakubo1@lsu.edu>

P. S. Marrocchesi (U of Siena), A. Yoshida, T. Sakamoto, V. Pal'shin,
S. Sugita (AGU), Y. Kawakubo (LSU), K. Yamaoka (Nagoya U), 
S. Nakahira (RIKEN), Y. Asaoka, S. Ozawa, S. Torii (Waseda U),
Y. Shimizu, T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC), 
M. L. Cherry (LSU), S. Ricciarini (U of Florence),
and the CALET collaboration:

The CALET Gamma-ray Burst Monitor (CGBM) was operating at the trigger 
time of S190915ak T0 = 2019-09-15 23:57:02.691 UT (The LIGO Scientific
Collaboration and Virgo Collaboration, GCN Circ. 25753).

No CGBM on-board trigger occurred around the event time.  Based on the
LIGO-Virgo high probability localization region, the summed LIGO 
probabilities inside the CGBM HXM (7 - 3000 keV) and SGM 
(40 keV - 28 MeV) fields of view are 1 % and 2 %, respectively (and 100 % 
credible region of the initial localization map was above the horizon).
The HXM and SGM fields of view were centered at RA = 107.0 deg, 
Dec = -4.2 deg and RA = 99.7 deg, Dec = -11.1 deg at T0, respectively.

Based on the analysis of the light curve data with 0.125 sec time 
resolution from T0-60 sec to T0+60 sec, we found no significant excess
(signal-to-noise ratio >= 7) around the trigger time in either the HXM or
the SGM data.

The CALET Calorimeter (CAL) was operating in the high energy trigger
mode at the trigger time of S190915ak. Using the CAL data, we have
searched for gamma-ray events in the 10-100 GeV band from -60 sec
to +60 sec from the GW trigger time and found no candidates. There is
no significant overlap with the LIGO-Virgo high probability localization 
region. The CAL FOV was centered at RA=99.7 deg, Dec=-11.1 deg at T0.