TITLE:   GCN CIRCULAR
NUMBER:  8846
SUBJECT: GRB 090123: Swift/UVOT Observations
DATE:    09/01/23 17:29:12 GMT
FROM:    Frank Marshall at GSFC  <marshall@milkyway.gsfc.nasa.gov>

F.E. Marshall (NASA/GSFC) and D. Grupe (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT observed the field of GRB 090123 starting 107 s after
the BAT trigger (Grupe et al., GCN Circ. 8842). We detect the optical
afterglow (Grupe et al.) in the white, v, b, u, uvw1, uvm2, and
uvw2 filters at the position:

  RA(J2000)  =  00:27:08.73 =   6.78639
  DEC(J2000) = -23:30:04.1  = -23.50113

with an estimated uncertainty of 0.5 arc sec. (radius, 90% confidence).
There is a broad peak in the light curve ~600 sec. after the trigger.
The 3.4-sigma detection in the uvw2 filter indicates that the redshift 
is less than ~1.4.

The magnitudes in the UVOT photometric system 
(Poole et al. 2008, MNRAS, 383, 627) in the first finding chart (FC) 
exposure and subsequent exposures are:

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

white (FC)         107          257          147         17.8 +/- 0.1
v                  649          669           20         15.4 +/- 0.1
b                  575          595           20         15.8 +/- 0.1
u                  320          570          250         16.4 +/- 0.1
uvw1               698          718           20         17.3 +/- 0.23
uvm2              1251         8112          294         19.6 +/- 0.25
uvw2              1547         7843          413         20.5 +/- 0.29

The values quoted above are not corrected for the Galactic extinction due
to the reddening of E(B-V) = 0.02 in the direction of the burst 
(Schlegel et al. 1998).