TITLE: GCN CIRCULAR NUMBER: 16746 SUBJECT: GRB 140824B: Swift observations DATE: 14/08/26 14:33:02 GMT FROM: Claudio Pagani at U of Leicester C. Pagani (U. Leicester), N. P. M. Kuin (UCL-MSSL) and J. R. Cummings (GSFC/UMBC) report on behalf of the Swift team: We report on Swift follow-up observations of the Fermi/GBM GRB 140824B (Zhang, GCN Circ. 16743) centred at the position of the candidate optical counterpart reported by the MASTER II robotic telescope (Tyurina et al., GCNs 16740, 16741). In 4.9 ks of XRT Photon Counting (PC) mode data, from 90.6 ks to 103.4 ks after the Fermi/GBM trigger, no X-ray source is detected at the MASTER II position. The 3-sigma upper limit on the count rate is 1.8E-03 cts/s, which is 9.2E-14 erg cm^-2 s^-1 using a typical flux conversion of 5E-11 erg cm^-2 count^-1. The UVOT detected the reported candidate OT. The preliminary UVOT position is: RA (J2000) = 01:11:01.14 = 17.75473 (deg.) Dec (J2000) = +60:33:38.6 = 60.56071 (deg.) with an estimated uncertainty of 0.43 arc sec. (radius, 90% confidence). Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 91250 91850 590 17.97 ± 0.05 white 96997 97610 603 17.94 ± 0.05 white 102774 103372 590 18.09 ± 0.05 v 91857 92457 590 17.47 ± 0.07 v 97617 98230 603 17.61 ± 0.07 u 90644 91244 590 17.65 ± 0.06 u 96991 96991 603 17.62 ± 0.06 u 102158 102768 600 17.91 ± 0.07 The magnitudes in the table are not corrected for the large, but uncertain Galactic extinction due to the reddening of E(B-V) = 0.79 in the direction of the burst (Schlegel et al. 1998). The UVOT magnitudes when compared to the reported unfiltered magnitudes of 17.0 and 17.5 by MASTER, suggest a slowly varying source. Based on the Swift observations, the optical transient detected by MASTER II is therefore unlikely to be the GRB afterglow.