TITLE:   GCN CIRCULAR
NUMBER:  41886
SUBJECT: GRB 250919A/EP250919A: Swift/UVOT afterglow detection
DATE:    25/09/19 16:03:30 GMT
FROM:    Samantha Oates at University of Birmingham <samantha.oates@alumni.ucl.ac.uk>

S. R. Oates (Lancaster U.) and R. Gupta (GSFC) report on behalf of the Swift/UVOT team:

Swift/UVOT has performed follow-up observations of GRB 250919A/EP250919a (Fermi GBM team GCN 41874;
Liang et al., GCN 41879, Wang et al., GCN 41882). Swift/UVOT began observations in the
u band 21.4ks after the detection by Fermi/GBM (Fermi GBM team GCN 41874).

A source is detected consistent with the optical counterpart reported by Li et al., (GCN 41877).
The preliminary detection magnitude reported below is calculated using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373).

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

 u             21418         23150        1705     17.14+\-0.03

The magnitude in the table is not corrected for the Galactic extinction due to the
reddening of E(B-V) = 0.057 in the direction of the burst (Schlegel et al. 1998).