TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  2243
SUBJECT: GRB 030329:  HST ACS Observations of the Host
DATE:    03/05/23 18:34:25 GMT
FROM:    Andrew S. Fruchter at STScI  <fruchter@stsci.edu>

A. Fruchter (STScI), A. Levan (U. Leicester/STScI), R. Hook
(ST-ECF/STScI), N. Pirzkal (ST-ECF), J. Gorosabel (STScI/IAA-CSIC),
P. Garnavich (Notre Dame), P. Nugent (LBNL), S. Thorsett (UCSB),
A. Castro-Tirado (IAA), J.M. Castro Ceron (STScI), C. Kouveliotou
(USRA), S.T. Holland (Notre Dame), J. Hjorth (U.  Copenhagen), and
N. Tanvir (U. Hertfordshire) report for the GOSH collaboration:

GRB 030329 has been observed at three epochs by HST:  15/16 April,
21/23 April and 12/13 May.  During all epochs UV (ACS/HRC), optical
(ACS/WFC) and NIR (NICMOS) images were obtained.  Additionally, in
the first epoch an ACS grism spectrum was taken, and in the second
epoch a STIS optical spectrum, delayed from the first epoch by a
gyrocope failure, was also obtained.  Here we report on aspects of
the imaging and grism spectroscopy which provide a unique contribution
from HST:  information on the nature of the host.

All the ACS images show the host extending to approximately ~0."5
from the OT to the west, with a PA of approximately 230 degrees.
In the most recent ACS/WFC images the OT has faded sufficiently to
allow a reasonable subtraction of the OT from the host, particularly
in the F435W and F606W filters where the blue color of the host
provides greater contrast against the redder OT than in the F814W
image.   We find the magnitude of the host to be approximately
V=22.7 +/- 0.3, where the uncertainty is dominated by the subtraction
of the OT and the unknown contribution to the host magnitude by
faint outerlying areas.  This apparent magnitude corresponds to an
absolute magnitude of about -16.5, very similar to that of the SMC.

The host can also be seen clearly in the UV ACS/HRC F250W images
taken on May 12.  The OT appears to lie at the end of a bar-like
structure approximately 0."25 across, which at the ~600 Mpc
angular-diameter distance of the GRB corresponds to a length of
about 800 pc.

The separation of the host from the OT can also be discerned in
the grism spectrum, where the emission lines of the host are seen
to be offset from spectral continuum of the OT.   Although the
spatial projection of the grism makes this somewhat uncertain, the
Halpha emission of the host appears to be fairly well centered on
host's blue light.

Images of the OT and host and the 2-d grism spectrum can be seen
at

http://www.stsci.edu/~fruchter/GRB/030329

[GCN OPS NOTE (23may03):  The extra zero in the URL was removed.]