[vsnet-grb-info 1897] GRB050801: Optical photometry and decay index

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3736
SUBJECT: GRB050801: Optical photometry and decay index
DATE:    05/08/02 15:01:05 GMT
FROM:    Brian Lindgren Jensen at U.of Copenhagen  <brian_j at astro.ku.dk>

GRB050801.769: Optical photometry and decay index

J. P. U. Fynbo, B. L. Jensen, J. Hjorth, K. G. Woller, D. Watson
(Niels Bohr Institute), P. Fouque (Obs-MiP, Toulouse), M. I. Andersen
(Potsdam, AIP) report on behalf of the DARK Cosmology Centre:

"We obtained BVR-band imaging of the XRT error circle of
GRB050801.769 (Band et al. GCN#3725, Perri et al. GCN#3731) with
the Danish 1.5m (La Silla) on Aug. 2.0 UT (approx. 6hr after the
burst). We detect the optical afterglow candidate source reported
by Rykoff et al. (GCN#3726), with position (RA, Dec)(J2000.0)

   13:36:35.363, -21:55:42.03

Based on a preliminary std. star calibration, we obtain the
following photometry of the afterglow candidate:

Date (UT):  Filter: Exptime: Mag.
Aug 1.997   B       600s     22.36+-0.05
Aug 2.012   V       600s     21.03+-0.05
Aug 1.989   R       600s     20.75+-0.04
Aug 2.005   R       600s     20.87+-0.04
Aug 2.020   R       600s     20.94+-0.05

(in this calibration, we obtain B=20.95, V=19.74, R=19.63 for the star
at: (RA, Dec) = (13:36:33.657, -21:55:29.55)).

Fitting a power-law to our three R-band data points gives a lightcurve,
which intersects the R=17.29 ROTSE-III (Rykoff et al. GCN#3726) data
point at t-t0=1590s, to within the errors of that data point.
Including the ROTSE-III data point in the fit, we find an R-band decay
slope of -1.29 +-0.05.

A findingchart of the field is shown at:
http://www.astro.ku.dk/~brian_j/grb/grb050801.769/ "