TITLE: GCN CIRCULAR NUMBER: 22608 SUBJECT: Chandra observations of GRB161219B 400 days after the explosion DATE: 18/04/06 02:38:08 GMT FROM: Aprajita Hajela at Northwestern U A. Hajela, R. Margutti (Northwestern University), C. Guidorzi (U Ferrara), A. Kamble (Harvard), A. MacFadyen (NYU), D. Milisavljevic (Purdue), J. Parrent (Harvard), A. Zauderer (NSF) report: "We started deep X-ray follow-up observations of GRB161219B with the Chandra X-ray Observatory on 2017, August 8 UT (dt ~ 232 days after explosion, exposure time of 15 ks), PI Margutti, program 18500396. An X-ray source is detected at the location of GRB161219B with count-rate of ~ 0.0024 cts/s (0.5 - 8 keV) and significance of 16 - sigma. The spectrum is well modeled with an absorbed simple power law with best-fitting photon index, Gamma = 1.95 +/- 0.38 and Galactic absorption, N_h = 2.8e+20 cm-2. We do not find any evidence of intrinsic absorption. For these parameters, the unabsorbed flux is (3.41 +/- 0.76) x 10^-14 ergs cm-2 s-1 (0.3 - 10 keV). A second Chandra observation was acquired between 2018, January 16th and January 18th (dt ~ 393-395 days since explosion, exposure time of 32 ks). GRB161219B is detected with the count-rate of ~ 0.0013 cts/s (0.5 - 8 keV) and a significance of 9 -sigma. The best-fitting photon-index is Gamma = 2.05 +/- 0.34 . The corresponding unabsorbed flux is (1.72 +/- 0.28) x 10^-14 ergs cm-2 s-1 (0.3 - 10 keV). We find that GRB161219B continues to decay following a power-law with index, alpha = 1.22 +/- 0.04 , which is consistent with the decay inferred from XRT data at t < 115 days. We thank the entire Chandra team for making these observations possible." [GCN OPS NOTE(06apr18): Per author's request, the last two paragraphs were added.]