TITLE: GCN CIRCULAR NUMBER: 20288 SUBJECT: GRB 161218B: POLAR observation DATE: 16/12/18 19:04:42 GMT FROM: Yuanhao Wang at IHEP/CAS Y. H. Wang (IHEP), Z. H. Li (IHEP), S. L. Xiong (IHEP) report for the POLAR collaboration: At 2016-12-18T08:32:41.341 UT (T0), during a routine on-ground  search of data, POLAR detected the GRB 161218B, which was also detected by the Fermi/GBM (trigger 503742764/161218356). The POLAR light curve consists of multiple peaks, with a duration (T90) of 26.28 s measured from T0+1.0 s . The 1s peak rate measured from T0+1.25 s is 9849 cnts/s, The total counts is about 29340 cnts.The above measurements are in the energy range of about 80-500 keV. The preliminary estimation of minimum detectable polarization(MDP) is 36.8%[3-sigma, statistical only]. Follow-up observations are strongly encouraged. LC_URL: http://polar.ihep.ac.cn/grb/2016/GRB161218B/lc/POLAR_lc_grb161218356.png Using the best location from the Fermi/GBM, which is (J2000): RA: 358.640 [deg] Dec: -16.950 [deg] Err: 1.00 [deg] the incident angle in POLAR coordinate at T0 is: theta:  77.7612  [deg] phi:   252.239  [deg] The analysis results presented above are preliminary. POLAR is a dedicated Gamma-Ray Burst polarimeter (50-500 keV) on-board the Chinese space laboratory Tiangong-2 launched on Sep 15, 2016. More information about POLAR can be found at http://polar.ihep.ac.cn/en/, http://isdc.unige.ch/polar/ and http://polar.psi.ch/html/