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TITLE:   GCN CIRCULAR
NUMBER:  20501
SUBJECT: GRB 170121A: POLAR Observation
DATE:    17/01/22 22:11:04 GMT
FROM:    Haulin Xiao at PSI/POLAR  <hualin.xiao@psi.ch>

H.L. Xiao (PSI) , W. Hajdas (PSI) and R. Marcinkowski (PSI) report on behalf of the POLAR collaboration:

At 2017-01-21 01:36:55.20 UT(T0), during a routine on-ground search of data, POLAR detected the GRB 170121A,
which was also observed by Fermi GBM (trigger #506655418) .

The POLAR light curve consists of one peak with duration (T90) of 3.25 +/- 0.25 s measured from T0.
The 0.25 s peak flux at T0 + 0.75 s  is equal to 3000 +/- 100 counts/sec.
POLAR recorded 4290 events from the burst.
Above measurements are in the energy range of about 80 - 500 keV.

LC_URL: http://polar.psi.ch/triggers/GRB_170121A_raw.png
     or raw LC: http://polar.psi.ch/pub/lc.php?event=GRB+170121A

Using the best location from Fermi GBM, which is (J2000):
RA : 3.0 [deg]
Dec: -75.62 [deg]
the incident angle in POLAR coordinate at T0 is:
Theta: 37.5 [deg]
Phi: 290.8 [deg]

The analysis results presented above are preliminary. Calibration of the instrument is ongoing.
POLAR is a dedicated Gamma-Ray Burst polarimeter which was launched on-board the Chinese space
laboratory Tiangong-2 (TG-2) on Sep 15, 2016. The energy detection range of POLAR is ~ 50-500 keV.
More information about POLAR can be found at http://polar.psi.ch/pub ,
 http://polar.ihep.ac.cn/en/ and http://isdc.unige.ch/polar/ .

This message is quotable in publications.