TITLE: GCN CIRCULAR NUMBER: 20418 SUBJECT: GRB 170109A: POLAR Observation DATE: 17/01/10 15:30:24 GMT FROM: Radek Marcinkowski at PSI/POLAR R. Marcinkowski (PSI), H. Xiao (PSI) and W. Hajdas (PSI) report on behalf of the POLAR collaboration: At 2017-01-09 03:17:35 UT(T0), during a routine on-ground search of data, POLAR detected the GRB 170109A, which was also observed by Fermi GBM (trigger # 505624660). Significant detection was recorded in 5 of 25 POLAR detection modules. The POLAR light curve consists of multipeak structure compatible with Fermi GBM with duration (T90) of 9.0 +/- 1.0 s measured from T0. The 1.0 s peak flux at T0 + 4.5 s is equal to 530 +/- 60 counts/sec. POLAR (5 modules) recorded 3140 +/- 140 events from the burst. Above measurements are in the energy range of about 80 - 500 keV. LC_URL: http://polar.psi.ch/triggers/GRB_170109A_raw_5of25.png or http://polar.psi.ch/pub/lc.php?event=GRB+170109A Using the best location from the Fermi GBM, which is (J2000): RA : 114.4 [deg] Dec: -6.8 [deg] Err: 2.85 [deg] the incident angle in the POLAR coordinate at T0 is: Theta: 81.0 [deg] Phi: 114.3 [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.