TITLE:   GCN CIRCULAR
NUMBER:  40778
SUBJECT: GRB 250617B: Swift/UVOT Detection
DATE:    25/06/18 13:09:34 GMT
FROM:    Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>

M. H. Siegel (PSU) and K. L. Page (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250617B 797 s after the BAT trigger (Page et al., GCN Circ. 40760). A source consistent with the XRT position (Osborne et al., GCN Circ
40767) is detected in the initial UVOT exposures. The lack of detection in the uvw2 and uvm2 filters would be consistent with the redshift reported by Corcoran et al. (GCN Circ. 40776).

The preliminary UVOT position is:
    RA  (J2000) =  22:11:35.25 = 332.89686 (deg.)
    Dec (J2000) = +32:43:57.1  =  32.73253 (deg.)
with an estimated uncertainty of 0.44 arc sec. (radius, 90% confidence).

Preliminary detections and 3-sigma upper limits 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              797          946          147         18.18 +/- 0.05
v                 1130         2528          172         18.72 +/- 0.19
b                 1229         1596           58         18.91 +/- 0.18
u                 1204         1397           39         18.10 +/- 0.19
w1                1179         2408          156         19.32 +/- 0.28
m2                1155         2384          117        >18.3
w2                1106         2507          175        >19.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.087 in the direction of the burst
(Schlegel et al. 1998).