GCN Circular 386

A. Fruchter, H. Ferguson, J. Pepper, R. Gibbons, K. Sahu (STScI) and E.
Pian (ITESRE-CNR, Bologna) report for the HST GRB collaboration:

The field of GRB 990510 was imaged by HST on 8.1 June 1999 (UT) and
17.9 June 1999 (UT) during one and two orbits, respectively, with the
STIS/CCD camera in Open Filter (50CCD) mode.  The OT of GRB 990510 was
detected in both cases.  Assuming the color of the OT has not changed
from the first few days when it could be modeled as f(nu) = nu^{-0.6}
passing through a Galactic extinction of E(B-V)=0.22 (Stanek et al.
1999, Harrison et al. 1999), we find magnitudes of V = 27.0 +/- 0.2  on
8.1 June and  27.8 +/- 0.3  on 17.9 June.

The STIS CCD in open filter mode is sensitive to wavelengths between
300 and 900 nm, and therefore both the conversion from counts to flux
density, and the estimated PSF (which is diffraction limited) depend
strongly on the assumed spectrum of the observed emission.  This is of
particular concern in the 17.9 June observation, where we observe some
sign of extended emission about the PSF.  Even though the extended
emission appears to be asymmetric, subtracting a red PSF (rather than
the relatively blue PSF implied by the early color) largely removes
this emission.   In any event, the GRB does not have a host galaxy
brighter than V~28.

The fits of Stanek et al. and Harrison et al. to the early time light
curve predict, at both epochs, an OT fainter than observed by at least
several tenths of a magnitude.  However,  the excess counts above
predicted are between a factor of three (first epoch) to seven (second
epoch) less than would exist were a supernova Type Ic of the luminosity
of SN1998bw at the probable redshift of the GRB (z=1.6, Vreeswijk et
al. 1999) superposed on the decaying power-law light curve.

Sections of the images in gif format can be retrieved from
http://www.stsci.edu/~fruchter/GRB/990510 .

References:  
Harrison, F.A. et al. 1999, astro-ph/9905306.  
Stanek, K., Garnavich, P.M., Kaluzny, J., Pych, W. and Thompson, I. 1999,
     Ap. J. (Letter,submitted), astro-ph/990534.  
Vreeswijk, P. et al.  1999, GCN 324
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