[vsnet-grb-info 368] GRB 020813 : HST observations

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

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1518
SUBJECT: GRB 020813 : HST observations
DATE:    02/08/22 17:04:55 GMT
FROM:    Andrew Levan at U.of Leicester  <anl@star.le.ac.uk>

A.J. Levan, A.S. Fruchter, I. Burud and J.E. Rhoads (STScI)
report for the larger GOSH collaboration:

The field containing the optical counterpart of GRB 020813 was imaged by
HST using ACS/HRC, ACS/WFC and NICMOS on 17th August beginning at 04:00 UT
approximately 98 hours after the burst.

The optical counterpart is clearly visible in our images. Using the
zeropoints for the filters on ACS/WFC, as given below, we find the
magnitude of the counterpart to be:

FILTER    Z_P(AB) OT MAG(AB)
==========================
F435W     25.47     23.20
F606W     26.35     22.71
F814W     25.86     22.33

This is consistent with a spectral slope of beta =-0.8.

Comparing to early, 3 hour I Band observations (GCN 1513) gives
an approximate slope of ~t^-1.07. Comparing to observations taken after
23 hours (also GCN 1513) indicates a somewhat faster (broken) slope
of t^-1.35.

The OT appears largely point like, however a PSF subtraction reveals some
evidence for a possible 25th magnitude host galaxy underlying the OT.

Images of the field of GRB 020813 may be found at

http://www.stsci.edu/~fruchter/GRB/020813

Analysis is continuing and further observations are planned.

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