GCN Circular 12222
Subject
GRB110731A: Swift/UVOT Observations
Event
Date
2011-08-01T12:17:25Z (15 years ago)
From
Samantha Oates at MSSL <sro@mssl.ucl.ac.uk>
S. R. Oates (MSSL-UCL) reports on behalf of the Swift-UVOT team
The Swift/UVOT began settled observations of the field of GRB 110731A 75s
after the trigger (Oates et al., GCN Circ. 12215). We detect a fading
source in the white, v, b and u filters at a refined position of
RA(J2000), DEC(J2000)= 280.50413 deg, -28.537167 deg. This is equivalent to:
RA (J2000) 18:42:00.99
Dec (J2000) -28:32:13.8
with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence).
This is consistent with the UVOT position reported in Oates et al.
(GCN Circ. 12215) and with the enhanced position of the X-ray afterglow
(Beardmore et al., GCN Circ. 12219). The non-detection in the uv filters is
consistent with a redshift of between 2 and 3.
Preliminary magnitudes using the UVOT photometric system (Poole et al. 2008,
MNRAS, 383, 627) for the finding chart (FC) and summed exposures at the
location of the optical afterglow are:
Filter T_start(s) T_stop(s) Exp(s) Mag
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wh (FC) 75 225 147 15.85+/-0.02
wh 567 587 19 17.64+/-0.11
v 790 810 19 16.90+/-0.23
b 542 562 19 17.87+/-0.23
u (FC) 287 537 246 17.71+/-0.08
u 1118 1138 19 18.37+/-0.44
uvw1 666 1286 78 >18.88
uvm2 642 1261 78 >18.77
uvw2 592 1385 97 >19.24
#####################################################
We note that photometry is difficult due to the field being crowded and the presence of a faint
contaminating source in the source region. The values quoted above are not
corrected for the non-negliable Galactic extinction due to the reddening
of E(B-V) = 0.18 in the direction of the burst (Schlegel et al. 1998).