TITLE: GCN CIRCULAR NUMBER: 17431 SUBJECT: GRB 150101B/ Swift J123205.1-105602: Second epoch Chandra observations DATE: 15/02/10 18:37:00 GMT FROM: Andrew Levan at U.of Leicester A.J. Levan (U. Warwick), J. Hjorth (DARK/NBI), N.R. Tanvir (U. Leicester), A.J. van der Horst (GWU) report on behalf of a larger collaboration: "We obtained a second epoch of observations of the very short GRB 150101B/ Swift J123205.1-105602 (Cummings et al. GCN 17267) with Chandra. Observations began on 10 Feb 2015, 39 days after the burst, and 32 days after the first epoch of observations (Troja et al. GCN 17289). As in the first epoch of observations a total of ~15 ks of observations were obtained with the target placed on the ACIS-S3 chip. In these observations we clearly identify the two sources identified in previous observations. In a 1.5" aperture to avoid contamination of the sources with light from each other, we finds count rates for source SRCX1 and SRCX2 as identified by Troja et al. (GCN 17289) to be SRCX1=(35.8 +/- 1.6)e-3 and SRCX2=(1.5 +/- 0.3)e-3 respectively. This compares to SRCX1= (38.3 +/- 1.6)e-3 cps and SRCX2=(9.4 +/- 0.9)e-3 cps in the first epoch. This implies that the AGN has remained constant, and that the second source, coincident with the optical afterglow (Fong et al. GCN 17333, Levan et al. 17321) has faded by a factor of ~6. The X-ray variability observed from source SRCX2 confirms it as the afterglow of GRB 150101B/Swift J123205.1-105602. The corresponding decay index is approximately t^-1.1, this is relatively slow for a short GRB afterglow so late after burst, and at z=0.134 (Levan et al. GCN 17281) places it in a region of parameter space much more luminous than most short bursts of comparable prompt fluence at similar times. We thank Belinda Wilkes, Scott Wolk at the team at the Chandra X-ray Observatory for their help with these observations."