[vsnet-grb-info 28469] GRB 210731A: Swift/UVOT Detection of an optical afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30572
SUBJECT: GRB 210731A: Swift/UVOT Detection of an optical afterglow
DATE:    21/08/01 14:44:28 GMT
FROM:    Paul Kuin at MSSL  <npkuin at gmail.com>

N. P. M. Kuin (UCL-MSSL) and E. Troja (NASA/GSFC/UMCP) report on
behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210731A
210 s after the BAT trigger (Troja et al., GCN Circ. 30568).
An optical afterglow consistent with the XRT position (D'Ai et al.
GCN Circ. 30569) and discovered by Meerlicht (de Wet ert al, GCN Circ.
30570) is also detected in the initial UVOT exposures. Interestingly, the
initial rise to peak is slow.

Preliminary 3-sigma magnitudes using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial exposures
are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white              210          273           62         20.2  +/- 0.38
b                17547        18204          587         19.02 +/- 0.08
u                29863        30504          624         18.66 +/- 0.09
v                11787        12145          348         18.60 +/- 0.17
uvw1             28956        29857          886         19.33 +/- 0.14
uvw2             34767        35667          886         20.41 +/- 0.27
uvm2             23308        23846          530         19.40 +/- 0.21

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.092 in the direction of the burst
(Schlegel et al. 1998).