GCN Circular 23042

M. L. Cherry (LSU), 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), S. Ricciarini (U of Florence),
A. V. Penacchioni, P. S. Marrocchesi (U of Siena)
and the CALET collaboration:

The extremely bright, long GRB 180720B (Swift-BAT trigger #848890:
Siegel et al., GCN Circ. 22973
Not found
 
 
, Barthelmy et al. GCN Circ. 22998
Not found
 
 
;
Fermi-LAT detection: Bissaldi and Racusin, GCN Circ. 22980
Not found
 
 
;
Fermi-GBM observation: Roberts and Meegan, GCN Circ. 22981
Not found
 
 
;
MAXI/GGS detection: Negoro et al., GCN Circ. 22993
Not found
 
 
;
Konus-Wind observation: Frederiks et al., GCN Circ. 23011
Not found
 
 
)
triggered the CALET Gamma-ray Burst Monitor (CGBM)
at 14:21:40.948 UTC on 20 July 2018.  Because of a problem in
one of the ground alert processing script, the GCN notice was not
distributed automatically for this event.  The burst signal was seen
by all CGBM detectors.

The CGBM data cover the time interval from T-243 sec to T+1449 sec
(when the HV was on and the source was in the CGBM FoV).

The burst light curve shows the main emission episode comprised
of several bright overlapped pulses which starts
at T-2.9 sec, peaks at 15.3 sec, and ends at T+54.0 sec,
followed by the weak tail seen at least up to T+120.1 sec.
The T90 and the T50 durations measured by the SGM data are
51.1 +- 3.0 sec and 15.4 +- 1.4 sec (40-1000 keV), respectively.

No any precursor is seen in the time interval from T-243 sec to T-2.9 sec.

The time-averaged spectrum of the main episode (measured by the SGM
from T+0.8 sec to T+52.8 sec) is best fit in the 30 keV - 20 MeV range
by a GRB (Band) model with alpha = -1.29 +- 0.04,
Epeak =  686(-77, +88) keV, and beta = -2.23(-0.12, +0.09)
(chi2 = 254.4/236 dof).  The emission is seen up to ~20 MeV.
The resulting fluence in the 30 keV - 10 MeV range is
5.79(-0.19, +0.20)x10^-4 erg/cm2 .

Assuming a redshift of z=0.654 (Vreeswijk et al., GCN 22996
Not found
 
 
)
and a FlatLambdaCDM cosmology with H0=68 km/s/Mpc and Omega_M=0.308
(Planck Collaboration 2016, A&A, 594, A13 (Paper XIII)),
the isotropic energy release, Eiso, is 6.82(-0.22, 0.24)x10^53 erg.

The quoted errors are at the 90% CL.

The ground processed light curve is available at

http://cgbm.calet.jp/cgbm_trigger/ground/1216131585/

All the quoted values are preliminary.

The CALET data used in this analysis are provided by the Waseda CALET
Operation Center located at the Waseda University.