[vsnet-grb-info 337] GRB 020813 multiband optical observations

[GCN Circulars <gcncirc@lheawww.gsfc.nasa.gov>]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1474
SUBJECT: GRB 020813 multiband optical observations
DATE:    02/08/13 09:02:29 GMT
FROM:    James Rhoads at STScI  <rhoads@stsci.edu>

James Rhoads (STScI), Matthew Holman (CfA), JJ Kavelaars (McMaster),
Andrew Fruchter (STScI), and Andrew Levan (STScI) report:

We observed the error circle of GRB 020813 (HETE Trigger #2262) with
the CTIO 4m Blanco telescope and Mosaic II imager starting at 020813
04:34 UT (110 minutes after the burst) and continuing for an hour.  We
obtained a total of ten 300 second images, four in B band and three
each in R and I bands.  The weather was good with subarcsecond seeing.

Preliminary analysis of the first image (a B band frame) confirms the
transient reported by Price et al (GCNC 1470).  Astrometric calibration
using 759 USNO-A2 stars brighter than R=16 places this image on a world
coordinate system.  We find coordinates of
 19:46:41.865 -19:36:04.76  (J2000.0) for the optical transient, with
an estimated error of 0.3" on each axis based on the 0.28" RMS residuals
of the USNO star positions.

Crude photometric calibration using the blue USNO-A2 magnitudes
of three nearby stars gives B=18.6 at 04:36 UT.
This B band detection immediately implies an upper limit z < 3.8
to the GRB redshift.
(The positions and adopted magnitudes of the reference stars were
 19:46:43.477 -19:36:05.66  B=17.10
 19:46:42.729 -19:36:06.65  B=17.20
 19:46:42.187 -19:35:37.00  B=15.90 .)
The third B band image, taken 14 minutes later, shows a decline of 0.07
magnitudes, corresponding to a rough decay slope of -0.55, consistent
with that reported by Li et al (GCNC 1473).  Analysis of the full data
set
is underway and will give both refined decay slopes and spectral slopes.

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