TITLE:   GCN CIRCULAR
NUMBER:  33710
SUBJECT: GRB 230427A: Swift/UVOT Detection
DATE:    23/04/28 18:03:41 GMT
FROM:    N. Klingler at NASA-GSFC/UMBC/CRESST II <noelklingler@gmail.com>

N. J. Klingler (GSFC/UMBC/CRESTII) and E. Troja (UToV) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 230427A 98 s after the BAT trigger (Troja et al., GCN Circ. 33687).  A source consistent with the XRT position (Evans et al. 2023, GCN Circ. 33688) is detected in the initial UVOT exposures.

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               98          248          147         20.19 +/- 0.15
v                  586         4999          255        >19.3
b                  512         1158           58        >19.6
u                  256          854          265        >20.5
w1                 810         5404          231        >20.0
m2                5004         5204          197        >19.6

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.007 in the direction of the burst (Schlegel et al. 1998).