TITLE: GCN CIRCULAR NUMBER: 23582 SUBJECT: GRB 181222B: Swift follow-up observations of MASTER optical transient J204928.79+245744.0 DATE: 18/12/28 17:36:50 GMT FROM: Antonino D'Ai at IASF-PA A. Tohuvavohu (PSU), A. D'Ai (INAF-IASFPA), M. Siegel (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT and Swift-UVOT teams: Swift has performed follow-up observations of the MASTER optical transient J204928.79+245744.0 (V. Lipunov et al. GCN Circ. 23588), within the IPN localization region of Fermi/GBM and Konus detected GRB 181222B (P. Veres GCN Circ. 23551, D. Svinkin et al. GCN Circ. 23556). Swift/XRT collected 3.9 ks of Photon Counting (PC) mode data between T0+414.9 ks and T0+427.0 ks. No X-ray source was detected at the MASTER position with an upper limit of 2.4 e-3 ct s^-1. Two uncatalogued X-ray sources have been detected in the field during this observation. Both sources are below the RASS limit and show no definitive signs of fading. Given the large localization region of this burst and the amount of time elapsed between the GRB and these observations, we believe it unlikely that these X-ray sources are related to the GRB. Details of the sources are given below: Source 1: RA (J2000.0): 312.2708 = 20:49:52.88 Dec (J2000.0): +25.0419= +25:02:30.9 Error: 2.5 arcsec (radius, 90% conf.) Count-rate (9.9 [+2.0, -1.9])e-3 ct s^-1 Distance: 7.2 arcminutes from MASTER position. Flux: (2.3 [+0.5, -0.4])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV) Source 2: RA (J2000.0): 312.3700 = 20:49:28.81 Dec (J2000.0): +24.9769 = +24:58:37.0 Error: 7.7 arcsec (radius, 90% conf.) Count-rate: (3.03 [+1.21, -0.97])e-3 ct s^-1 Distance: 53 arcsec from MASTER position. Flux: (1.22 [+0.49, -0.39])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV). Swift/UVOT began observing the field in the White filter 414 ks after the GRB trigger. We do not find any optical source at the MASTER position, with an upper limit of 21.48 +/- 0.22 magnitude (AB) in a 3094 second exposure. This magnitude is not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.24 in the direction of the burst (Schlegel et al. 1998). The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at http://www.swift.ac.uk/ToO_GRBs/00020850/. This circular is an official product of the Swift team.