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IAUC 6643: GRB 970228; 1997by

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                                                 Circular No. 6643
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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GRB 970228
     D. W. Fox, J. M. Kommers, and W. H. G. Lewin, Massachusetts
Institute of Technology; and E. A. Magnier, University of
Washington, report:  "We have analyzed the Hubble Space Telescope
(HST) Planetary Camera images (Sahu et al., IAUC 6606, 6619), taken
with the F606W (wide V) and F814W (I) filters, of the optical
transient associated with GRB 970228 (IAUC 6584) in an attempt to
characterize the extended emission first reported on IAUC 6588 and
later found to have faded below detection level (IAUC 6631).  We
find that the profile of the transient is well characterized by a
synthetic HST point-spread function (see
http://scivax.stsci.edu/~krist/tinytim.html); subtracting the
fitted contribution of the optical transient, and excluding the
region within 3 pixel lengths of its peak, we performed aperture
photometry on an ellipse containing the bulk of the apparent
emission, with semimajor axis 0".73 and semiminor axis 0".55 (16x12
pixels), centered at R.A. = 5h01m46s.65, Decl. = +11 46'54".0
(equinox 2000.0), and having major axis oriented 16 deg west of
north.  The sky background is determined from a Gaussian fit to a
histogram of pixel intensities from the central 5".8 x 5".8 of each
image.  The extended source is detected at > 3-sigma significance
in all but one image (Mar. 26.5 UT, F814W); the excess countrates
(in DN/s; see Holtzman et al. 1995, PASP 107, 1065, for conversion
to physical units) that we associate with it are as follows:  0.078
+/- 0.015 (Mar. 26.4, F606W), 0.027 +/- 0.019 (Mar. 26.5, F814W),
0.074 +/- 0.016 (Apr. 7.2, F606W), 0.064 +/- 0.019 (Apr. 7.3,
F814W).  We thus find no evidence for a change in the intensity or
F606W-F814W color of the extended emission between the two HST
observations.  Assuming a normal distribution of our errors, we
find that any change in the intensity of the extended emission over
this time interval lies (with 90-percent confidence) between -0.48
and +0.60 mag in F606W, and between -2.42 and +0.40 mag in F814W."

     R. Kirshner and S. Jha, Center for Astrophysics, report that a
spectrum of SN 1997by, taken by P. Berlind at the Whipple
Observatory's 1.5-m telescope on Apr. 30.28 UT, confirms that it is
a type-Ia supernova near maximum at a redshift of z = 0.045 (cf.
IAUC 6640).  This matches well the Abell Cluster 1736 redshift of z
= 0.046.

                      (C) Copyright 1997 CBAT
1997 May 6                     (6643)            Daniel W. E. Green

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