TITLE:   GCN CIRCULAR
NUMBER:  38775
SUBJECT: GRB 250101A: Swift/UVOT Detection
DATE:    25/01/02 07:11:25 GMT
FROM:    Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>

M. H. Siegel (PSU) and K. L. Page (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250101A
133s after the BAT trigger (Page et al., GCN Circ. 38752).
A fading source consistent with the XRT position (Goad et al., GCN Circ. 37761)
and the previously reported optical source, since confirmed by
multiple observations (Li et al., GCN Circ. 38753; Mohan et al., GCN
Circ. 38754; Zhu et al., GCN Circ., 38755; Wu et al., GCN Circ 38578;
Moskvitin & Spiridonova, GCN Circ. 38762; Odeh et al., GCN Circ 38763;
Hu et al., GCN Circ. 38764; D'Avino & L. Izzo, GCN Circ. 38768;
Escudero-Coca, GCN Circ. 38769; Leonini et al., GCN Circ. 38771)
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  02:28:16.47 =  37.06864 (deg.)
    Dec (J2000) = +19:11:39.6  =  19.19433 (deg.)
with an estimated uncertainty of 0.43 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              133          283          147         18.03 +/- 0.05
v                  620          813           39         18.10 +/- 0.28
b                  546          738           39         18.66 +/- 0.21
u                  291          540          246         17.93 +/- 0.08
w1                 670         5019          235        >19.8
m2                4614         4814          197        >19.1
w2                 596         4404          216        >18.9

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.164 in the direction of the burst
(Schlegel et al. 1998).

We note that the lack of detection in the NUV bands for this bright burst would
be consistent with the redshift of z=2.49 reported by Zhu et al. (GCN Circ. 38759).