TITLE:   GCN CIRCULAR
NUMBER:  41915
SUBJECT: GRB 250920B: Swift/UVOT Detection
DATE:    25/09/20 19:22:10 GMT
FROM:    Paul Kuin at MSSL <npkuin@gmail.com>

Paul Kuin (MSSL) and Rahul Gupta (NASA GSFC) report on behalf of the
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250920B 99 s
after the BAT trigger (Gupta et al., GCN Circ. 41898).  A source consistent
with the XRT position (Evans et al.  GCN. Circ. 41910) is detected in the
initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  12:29:16.07 = 187.31697 (deg.)
    Dec (J2000) = +47:50:14.4  =  47.83732 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).

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               99          249          147         13.89 +/- 0.02
v                 4433         4633          197         16.53 +/- 0.05
b                 3818         4018          197         16.62 +/- 0.04
u                  311          452          139         13.26 +/- 0.02
w1                4843         5043          197         17.02 +/- 0.08
m2                4638         4838          197        >19.2
w2                4228         4428          197         19.21 +/- 0.25

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.011 in the direction of the burst
(Schlegel et al. 1998).

The observation also included a 49s long UV grism exposure. Inspection of
the noisy grism spectrum suggests a redshift of 2.2 based on identifying an
broad absorption with the the Ly-alpha line.