GRB 180120A

Y. Shimizu (Kanagawa U), A. Yoshida, T. Sakamoto, V. Pal'shin, Y. Kawakubo,
M. Moriyama, Y. Yamada, A. Tezuka, S. Matsukawa (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (RIKEN), I. Takahashi (IPMU),
Y. Asaoka, S. Ozawa, S. Torii (Waseda U),
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 long-duration GRB 180120A (Fermi-GBM trigger #538117098;
Konus-Wind trigger time on 04:58:18.23) triggered the CALET
Gamma-ray Burst Monitor (CGBM) at 04:58:09.54 on 20 January 2018.
The burst signal was seen by the all CGBM instruments.

The light curve shows two bright overlapping pulses.  The first pulse starts
at T-2 sec, peaks at T+6 sec and ends at T+20 sec.  There is a possible
additional pulse around T+15 sec.  The second pulse starts at T+15 sec,
peaks at T+25 sec and ends at T+40 sec.  The T90 and the T50 durations
measured by the SGM data are 24.9 +- 4.1 sec and 16.3 +- 0.4 sec
(40-1000 keV), respectively.

The ground processed light curve is available at

http://cgbm.calet.jp/cgbm_trigger/ground/1200459271/

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

M. Stanbro and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 04:58:13.00 UT on 20 January 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 180120A (trigger 538117098 / 180120207)
which was also detected by the CALET Gamma-ray Burst Monitor
(Y. Shimizu et al. 2018, GCN 22366
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 ).

The on-ground calculated location, using the GBM trigger
data, is RA = 151.77, DEC = 27.79, with an uncertainty
of 1 degree (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which we have characterized as a core-plus-tail model, with 90% of
GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg
systematic error. [Connaughton et al. 2015, ApJS, 216, 32].

The angle from the Fermi LAT boresight at the GBM trigger time is 85
degrees.

The GBM light curve consists of several episodes
with a duration (T90) of about 29 s (50-300 keV).
The time-averaged spectrum from T0+0.90 s to T0+30.59 s is
best fit by a Band function with Epeak = 130 +/- 2 keV,
alpha = -1.05 +/- 0.01, and beta = -2.58 +/- 0.05.

The event fluence (10-1000 keV) in this time interval is
(6.27 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+17.66 s in the 10-1000 keV band
is 36.70 +/- 0.50 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

V. Sharma (IUCAA), V. Bhalerao (IIT-B), D. Bhattacharya (IUCAA),  A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the Astrosat CZTI collaboration:

Analysis of Astrosat CZTI data showed the detection of a long GRB 180120A, which was also detected by Fermi-GBM (Stanbro M. et al., GCN 22367
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 ) and CALET (Shimizu Y. et al., GCN 22366
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 ).

The source was clearly detected in the 40-200 keV energy range. The light curve shows multiple peaks of emission with strongest peak at 04:58:15.5 UT, 2.5 s after the Fermi-GBM trigger. The measured peak count rate is 695.2 cts/s above the background in combined data of four quadrants, with a total of 7955 cts. The local mean background count rate was 557 cts/s. Using cumulative rates, we measure a T90 of 26.8 s.

It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range.

CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.