[vsnet-grb-info 5902] GRB 080330 Swift-UVOT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7552
SUBJECT: GRB 080330 Swift-UVOT refined analysis
DATE:    08/03/30 23:28:41 GMT
FROM:    Paul Kuin at MSSL  <npkuin at googlemail.com>

N.P.M. Kuin (MSSL/UCL) and J.Mao (INAF) report on behalf of the
Swift-UVOT team:

The Swift/UVOT started observations of  GRB 080330 (Mao et al,
GCN Circ. 7537) with a settling exposure in the UVOT v-filter
starting 2008-03-30 at 03:42:19 U, 63 seconds after the BAT
trigger, and a finding chart exposure in the white filter starting
83 seconds after the trigger. During the subsequent exposure in the
V filter, starting at 189 seconds after the trigger, the spacecraft
lost lock and drifted. In the V image, a clear trail is visible
that all the stars in the field follow. In other filters, there is
no clear evidence of lost counts, except in one image in b. Those
exposures are short, or have much background (white), which suggest
that only a fraction of the counts is lost.

The extraction of the magnitudes below relies on a 5¨ aperture
around the source. No counts are visible due to the source outside
the aperture in the images. However, these magnitudes should be
regarded as upper limits until a more detailed reduction can be
done. Based on some tests, the magnitude may be underestimated as
much as 0.7 magnitudes. The v-filter data were extracted using
an aperture closely matching the shape of the trail and were
measured from the event data. There is an uncertainty in this
approach as well, since it is not clear what part of the PSF is
sampled. Therefore the V magnitudes can be over-or underestimated
by 0.25 magnitudes.

The observations start before a peak is reached. In the white filter,
a peak is apparent at T+128 +/- 5 seconds. In the v filter data, two
peaks may be present at T+ 438 and 1038 +/- 25s. It is unlikely that
the satellite drift caused these variations because the aperture
included the whole trail. The uvw1 magnitude has upper limits until quite
late in the burst, and must rise between T+791 and T+1416s. This
is an indication of achromatic evolution in this burst.

Only an upper limit is found in the uvw2 and uvm2 filters,
which is consistent with the reported redshift of 1.51 (Malesani,
et al. GCN 7544, and Cucchiara et al. GCN 7547).

Filter Tstart(s) Tstop(s) Exp(s) Mag     Mag-error [systematic error]

wh         83     183      98    18.2       0.06 [+0.7]
v         189     588     399.8  18.5-17.2  0.3  [+/-0.25] (*)
uvm2      594    1390      58.4 >18.55           (3 sigma UL)
uvw1      619     638      19.5 >18.02           (3 sigma UL)
u         644     664      19.4  17.14      0.16 [+0.7]
b         669     678       9.6  18.9       0.6  [+0.7]
wh        683     693       9.8  17.76      0.14 [+0.7]
uvw2      698     870      38.9 >18.77           (3 sigma UL)
uvw1      772     791      19.8 >18.04           (3 sigma UL)
u         796     815      19.4  16.66      0.17 [+0.7]
b         822     832       9.6  17.66      0.22 [+0.7]
white     836     979     107.8  17.55      0.03 [+0.7]
v         985    1384     400    18.0-17.2  0.3  [+/-0.25] (*)
uvw1     1416    1436      19.4  17.7       0.3  [+0.7]
b        1464    1483      19.8  18.14      0.22 [+0.7]
white    1490    1499       9.6  17.91      0.15 [+0.7]

(*) For the V magnitudes the range is given as seen in the event data.

The values quoted above are not corrected for the expected Galactic
extinction corresponding to a reddening of E_{B-V} = 0.02 mag in the
direction of the burst (Schlegel et al. 1998).