[vsnet-grb-info 395] GRB 020305: HST/STIS observations

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

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1542
SUBJECT: GRB 020305: HST/STIS observations
DATE:    02/09/16 15:27:24 GMT
FROM:    Ingunn Burud at Space Telescope Science Inst  <burud@stsci.edu>

J. Gorosabel (IAA-CSIC), J. Fynbo (Univ. of Aarhus), I. Burud (STScI),
A. Fruchter (STScI), J. Hjorth (Univ. of Copenhagen), report for the
larger GOSH (GRB Optical Studies with HST) collaboration:

We have observed the position of the optical afterglow (GCN 1267) of
GRB 020305 (GCN 1262) with HST/STIS. The GRB field was visited four
times; on April 12.7-12.9 UT (Clear filter), April 14.1-14.4 UT
(Long-Pass), June 16.4-16.5 UT (Clear filter), and June 16.6-16.8 UT
(Long-Pass).

Subtraction of the two epoch HST/STIS images taken for each filter
reveals a fading point source. The centroids of the Long-Pass and
Clear filter residuals are consistent with the afterglow position
independently derived registering an afterglow NOT image (taken on
March 16) on our HST/STIS frames.

The magnitudes of the afterglow (AB system) are: 24.693+/-0.008
(Clear, April 12.7-12.9 UT), 24.452+/-0.013 (Long-Pass, April
14.1-14.4 UT), 26.197+/-0.027 (Clear, June 16.4-16.5 UT) and
25.951+/-0.047 (Long-Pass, June 16.6-16.8 UT). The decay indexes
associated to the Clear and Long-Pass filters are, respectively;
Alpha_Clear = 1.40+/-0.03, Alpha_Long-Pass = 1.45+/-0.05. So, the
optical decay is consistent with being achromatic.

The afterglow position is consistent with an extended (~1.2"x0.5")
faint galaxy which shows a complex morphology. A preliminary aperture
photometry (AB system, considering a 0.675" aperture radius and the
corresponding aperture correction) of the whole host galaxy system
yields 25.771+/-0.054 (Clear filter) and 25.464+/-0.087 (Long-Pass
filter), respectively.

Several gifs of the HST/STIS images and the afterglow light curve can
be seen at:

http://www.stsci.edu/~fruchter/GRB/020305/

The optical measurements reported to date (our HST/STIS data points
and the ones given in the GCNs; 1264, 1265, 1267, 1270, 1271, 1275,
1279, 1283) might suggest the presence of a break ~10 days after the
burst.  However, a final confirmation would require a more careful
analysis.

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