GCN Circular 20501
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.