GCN Circular 30572

Subject

GRB 210731A: Swift/UVOT Detection of an optical afterglow

Event

Date

2021-08-01T14:44:28Z (5 years ago)

From

Paul Kuin at MSSL <npkuin@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. 30568Loading...
 
 ).
An optical afterglow consistent with the XRT position (D'Ai et al.
GCN Circ. 30569Loading...
 
 ) and discovered by Meerlicht (de Wet ert al, GCN Circ.
30570Loading...
 
 ) 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).