TITLE: GCN CIRCULAR NUMBER: 20166 SUBJECT: GRB 161104A: Chandra X-ray observations DATE: 16/11/10 17:27:14 GMT FROM: Wen-fai Fong at U of Arizona R. Margutti (Northwestern U.), E. Berger (Harvard U.), and W. Fong (U. Arizona) report: "We observed the location of the short GRB161104A with the Chandra X-ray Observatory (CXO) on 2016 Nov 8th, 14:41:45 UT, 364 ks after trigger (Mingo et al., GCN 20123) under a pre-approved guest observer program (PI Berger). No X-ray source is detected within the enhanced XRT position (Goad et al., GCN 20124). In particular, we do not find evidence for statistically significant X-ray emission at the location of the optical source reported by Troja et al., GCN 20137 at RA, Dec = 05:11:34.5, -51:27:33.90. Using 19.8 ks of CXO observations we infer a 3 sigma count-rate limit of 3.0E-4 c/s in the 0.5-8 keV energy range. The neutral hydrogen absorption in the direction of the transient is 1.4E+20 cm-2 (Kalberla et al., 2005). Assuming the spectral parameters that best fit the Swift-XRT observations (power-law photon index Gamma=1.36 and no evidence for intrinsic neutral hydrogen absorption, Burrows et al., GCN 20128) the inferred absorbed (unabsorbed) flux limit is Fx< 4.4E-15 erg/s/cm2 (Fx<4.5E-15 erg/s/cm2) in the 0.3-10 keV energy range. Our observations indicate a steepening of the X-ray afterglow temporal decay. Starting from 20 ks after trigger, the X-ray afterglow decays as t^-alpha with alpha>1.3. We thank the CXO team for rapid scheduling of our observations."