TITLE: GCN CIRCULAR NUMBER: 12087 SUBJECT: Swift J185003.2-005627: Swift-XRT/UVOT refined analysis DATE: 11/06/25 16:51:02 GMT FROM: Andy Beardmore at U Leicester A. P. Beardmore (U. Leicester), S. Campana (INAF-OAB), J. A. Kennea (PSU) and C. A. Swenson (PSU) report on behalf of the Swift/XRT and UVOT teams: We have analysed 2.8 ks of Swift/XRT data from the transient Swift J185003.2-005627 (Beardmore, et al., GCN Circ. 12083), from 88 s to 17.5 ks after the BAT trigger. The data comprise 421 s in Windowed Timing (WT) mode (the first 5 s were taken while Swift was slewing), with the remainder in Photon Counting (PC) mode. The astrometrically corrected X-ray position for this source (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue), derived from 1660 s of XRT Photon Counting mode data and 2 UVOT images, is RA, Dec = 282.51390, -0.93980 which is equivalent to: RA (J2000): 18 50 3.33 Dec (J2000): -00 56 23.3 with an uncertainty of 1.7 arcsec (radius, 90% confidence). This position may be improved as more data are received. The position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). The X-ray light curve initially decays like a powerlaw, with an index of 1.158 (+0.102, -0.125), before flattening to a slope of 0.161 (+0.035, -0.072) at T +365 (+109, -40) s after the trigger. A spectrum formed from the WT mode data from T+101 to T+429 s after the trigger is best fit by an absorbed blackbody, with a temperature of 0.83 +/- 0.03 keV and column density of 1.71 +/- 0.19 x 10^22 cm^-2, consistent with the Galactic column in the direction of the source (Kalberla et al. 2005). The observed (unabsorbed) 0.3-10 keV flux for this spectrum is 4.85e-10 (6.76e-10) ergs cm^-2 s^-1. The XRT spectrum, together with the soft BAT spectrum reported by Markwardt et al. (GCN Circ. 12086) and the location of the source in the Galactic plane, is suggestive of emission from a type I burst. Swift/UVOT observations of the field were also performed but no optical counterpart is detected at the XRT position. Preliminary 3-sigma upper limits, using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627), are: Filter T_start(s) T_stop(s) Exp(s) Mag --------------------------------------------------------------------------- white 104 5305 628 >18.9 v 593 1763 136 >17.2 b 519 1863 136 >18.9 u 263 1837 382 >19.9 w1 642 1813 130 >19.7 w2 742 5340 125 >20.3 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 16.09 in the direction of the transient (Schlegel et al. 1998).