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GCN Circular 1029

Subject
GRB 980326: Late-time HST/STIS observations
Date
2001-03-28T19:53:03Z (23 years ago)
From
Andrew S. Fruchter at STScI <fruchter@stsci.edu>
Andrew Fruchter (STScI), Paul Vreeswijk (University of Amsterdam) and
Peter Nugent (LBNL) report on behalf of a larger HST GRB collaboration:

We observed the field of GRB 980326 with HST/STIS on 31 December 2000,
just over 1000 days after the burst. The total exposure time was 7080
seconds in the 50CCD aperture. The images were reduced by the standard
HST pipeline, and drizzled to a combined image with a pixelscale of
0."0254.

We projected the location of the early afterglow from a KeckII/LRIS R
band image, taken by Filippenko, Leonard and Riess on March 27, 1998
(see GCN 33).  We estimate the 1-sigma error radius to be less than 1.5
drizzled HST pixels, which includes both the scatter in the
transformation (0.9 pixels, using 6 reference stars) and the
measurement error of the transient in the Keck image (0.8 pixels).

Within one pixel of the estimated position, there was evidence of a
small source.  We therefore convolved the image with a gaussian with a
FWHM equal to that of the PSF (3.5 drizzled pixels).  This procedure
uses the PSF as a matched filter to enhance the the signal-to-noise
ratio of unresolved sources.  In the convolved image an object is
found, again offset by one pixel from the expected position with a
significance of ~4.5 sigma.  We estimate a magnitude (in the broad STIS
50ccd filter) of V = 29.25 +/- 0.25.

Bloom et al., (1999, Nature, 401, 453) have suggested that the unusual
light curve of this GRB might be explained by an underlying supernova,
and on the basis of this estimated a redshift of about z~1.   A
supernova at this redshift (which in this model peaks around a
magnitude of 25) would be expected to be significantly fainter than
V=29 by now (as would the rapidly decaying GRB afterglow).  Therefore,
we may be detecting a faint host galaxy.  Deep HST images have in all
other cases detected a host underlying the GRB, when good astrometry
was available for the GRB.  If we are observing a host galaxy, then the
above magnitude could underestimate its brightness by as much as a few
tenths of a magnitude, due to extended emission missed in this
measurement.  A galaxy at a z~1 with this observed magnitude would lie
a rather remarkable 7 magnitudes below L*, the knee of the of the
galaxy luminosity function at that redshift.

Given the apparent point-source nature of the detected object, and the
lack of color information, we cannot exclude the possibility (which we
consider remote) that we are observing a light echo, from either the
GRB or the supernova.

Images of the field can be found at

http://www.stsci.edu/~fruchter/GRB/980326
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