GCN Circular 757

A. Fruchter (STScI), R. Hook (ST-ECF), E. Pian (ITESRE-CNR, Bologna) report 
for a larger HST GRB collaboration:

The field of GRB 990510 was imaged by HST for a third time on 
29 April 2000 using the STIS/CCD camera in Open Filter (50CCD) mode,
as part of our program to study this GRB (see GCN 386
Not found
 
 
 for earlier results.
A total exposure time of 5840 s, divided between 8 dithered observations 
was obtained.

Based on our previous observations, we would expect the GRB to be well
below the threshold of detectability -- which is for this 
image a V=29 point source.   As predicted the OT was not detected. 
However, our June 1999 image had suggested the possible presence
of an underlying host galaxy (see GCN 386
Not found
 
 
).  We therefore convolved
the image with a gaussian of FWHM ~ 0."3, which is roughly comparable 
to the typical size of galaxies in other deep fields near the detection limit 
of the image for an extended source (V~28).  The convolved image reveals 
a >3 sigma source lying under the position of the OT, with its peak 
lying ~0."075 East of the position of the OT.  This we believe to 
most likely be the host of the GRB.

Although the formal significance of the object is above 3 sigma, it lies 
in a region of significant scattered light from bright nearby stars and it
is difficult to quantify the possible errors in the sky subtraction.
We therefore regard the detection of this host as tentative.   Our
best estimate for the host's magnitude, assuming its correct
identification, is V=28 +/-0.3.  Deeper integrations, particularly
ones taken at different roll angles, would be able to unambiguously
determine the reality of this detection.

The images of the field of GRB 990510 giving a clear idea of the significance 
of the detection can be found at:

http://www.stsci.edu/~fruchter/GRB/990510