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GCN Circular 151

GRB970508 optical observations
1998-08-21T17:21:17Z (26 years ago)
Andrew S. Fruchter at STScI <>
HST Observations of the Host Galaxy of GRB970508: 

A. Fruchter (STScI), E. Pian (ITESRE) and an international
collaboration report:

The field of GRB970508 was imaged by HST with the STIS CCD in open
filter mode (50CCD) on 1998 August 5.78-6.03 for a total exposure time
of 11,568 seconds.  An extended object, which we believe to be the host
galaxy of GRB970508, was detected at the astrometric position of the
optical transient of GRB970508.  The galaxy has high signal-to-noise in
our data and is clearly resolved, with a major axis of approximately 
0."5 .  Including an uncertain contribution from the optical transient, 
the galaxy has a magnitude of V= 25.25 +/- 0.20, where the primary 
source of uncertainty is due to the wide bandpass (and thus poor color 
resolution) of the detector.   This agrees reasonably well with previous 
photometric estimates (Bloom et al. 1998, Zharikov et al. 1998).

The image of the galaxy is well described by an exponential disk with a
scale length of approximately 0."07, and an ellipticity of 0.3.
However, the FWHM of the PSF is comparable in size to the observed
scale length; thus an accurate measurement of the surface brightness
profile of the galaxy will require modelling and/or deconvolution.

We have obtained an astrometric solution with an r.m.s. uncertainty of
approximatgely 0."01 which maps the previous STIS image (Fruchter et
al. 1997, Pian et al 1998) onto the new data.  The position of the OT
in the June 1997 STIS image, and the center of the galaxy on the August
1998 image, agree to within the astrometric error.

Sections of the images are available on the web from

Bloom, J.S., Djorgovski, S.G., Kulkarni, S.R. and Frail, D.A., 1998,
	      Ap. J. (Lett.), submitted, astro-ph/9807315.
Fruchter, A., Bergeron, E., and Pian, E., 1997, IAUC 6674.  
Pian, E., et al., 1998, Ap. J. (Lett.), 492, L103.  
Zharikov, S.V., Sokolov, V.V.  and Baryshev, Yu.V., 1998, GCN 31.
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