TITLE: GCN CIRCULAR NUMBER: 20839 SUBJECT: GRB 170306B: POLAR observation DATE: 17/03/08 14:02:46 GMT FROM: Haulin Xiao at PSI/POLAR H.L. Xiao (PSI), R. Marcinkowski (PSI) and W. Hajdas (PSI) report on behalf of the POLAR collaboration: At 2017-03-06 14:07:20.0 UT(T0), during a routine on-ground search of data, POLAR detected GRB 170306B, which was also observed by Fermi-GBM (trigger # 510502047), Fermi-LAT (trigger #170306588) and SPI ACS at 2017-03-06 14:07:25. The POLAR light curve consists of multiple peaks with duration (T90) of 25.0 +/- 1.0 s measured from T0. The 1.00 s peak flux at T0 + 11.50 s is equal to 2380 +/- 100 counts/sec. POLAR recorded 31640 events from the burst. Above measurements are in the energy range of about 15 - 300 keV. LC_URL: http://polar.psi.ch/triggers/GRB_170306B_raw.png or http://polar.psi.ch/pub/lc.php?event=GRB+170306B Using the best location from Fermi-LAT, which is (J2000): RA : 154.650 [deg] Dec: 51.630 [deg] Err: 39.00 [arcmin] the incident angle in the POLAR coordinate at T0 is: Theta: 87.0 [deg] Phi: 98.4 [deg] The analysis results presented above are preliminary. 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. 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.