TITLE:   GCN CIRCULAR
NUMBER:  30247
SUBJECT: GRB 210610B: Swift/UVOT Detection 
DATE:    21/06/17 13:37:18 GMT
FROM:    Mike Siegel at PSU/Swift MOC  <mhs18@psu.edu>

M. H. Siegel (PSU), M. Baer (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 210610B
92 s after the BAT trigger (Page et al., GCN Circ. 30170). A fading source consistent
with the XRT position (Osborne et al. GCN Circ. 30189) and the previously reported
optical counterpart (Kumar et al., GCN Circ. 30174; Rumyantsev et al. GCN Circ.
30175; Hu et al., GCN Circ. 30177; de Wet et al., GCN Circ. 30180; Romanov, GCN Circ.
30181; Fynbo et al., GCN Circ. 30182; Moskvitin, GCN Circ. 30187; Fu et al., GCN Circ.
30188) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
   RA  (J2000) =  16:15:40.40 = 243.91833 (deg.)
   Dec (J2000) = +14:23:56.9  =  14.39915 (deg.)
with an estimated uncertainty of 0.42 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 (fc)          92          242          147      13.63+/-1.10
white              584         1355          225      15.24+/-0.02
white           172034       172423          377      20.22+/-0.14
white           286441       292869          328      20.71+/-0.23
white           452008       515231         4066      21.69+/-0.16
v                  634         1405           97      15.63+/-0.05
v                 6631        51903          659      18.26+/-0.09
b                  560         1331           77      15.78+/-0.04
b                81150       133027          865      20.08+/-0.14
u (fc)             304          554          245      13.86+/-0.03
u                  707         1306           58      14.93+/-0.04
u                 4850         5040          186      16.68+/-0.05
u                45786        46567          761      18.06+/-0.05
u               131743       132650          885      20.00+/-0.18
u               365977       418267         3970      21.42+/-0.29
uvw1               683         1282           58      14.51+/-0.06
uvw1              4645         4845          196      16.26+/-0.07
uvw1              7041        45779          989      17.54+/-0.06
uvw1            125974       137480          881      19.53+/-0.20
uvw1            200561       247267         1111      19.95+/-0.24
uvm2              6836         7036          196      16.61+/-0.10
uvw2               783         1034           38      15.05+/-0.09
uvw2             50521        51421          885      18.44+/-0.10

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.044 in the direction of the burst
(Schlegel et al. 1998).