\documentstyle{preprint}
\input pub.sty
\begin{document}
\title{THE OPTICAL COUNTERPART TO \boldmath{$\gamma$}-RAY BURST GRB970228\\
~\\
OBSERVED USING THE HUBBLE SPACE TELESCOPE}
\author{Kailash C.\ Sahu, Mario Livio, Larry Petro, F.\ Duccio Macchetto\\
Space Telescope Science Institute\\
3700 San Martin Drive, Baltimore, MD 21218\\
\and
Jan van Paradijs\/\thanks{Physics Department, University of Alabama in 
Huntsville, Huntsville, AL 35899.}\\
Astronomical Institute ``Anton Pannekoek,'' University of Amsterdam \&\\
Center for High Energy Astrophysics\\
Kruislaan 403, 1098 SJ Amsterdam, The Netherlands\\
\and
Chryssa Kouveliotou\\
Universities Space Research Association\\
NASA Marshall Space Flight Center, ES-84, Huntsville, AL 35812\\
\and
Gerald J.\ Fishman, Charles A.\ Meegan\\
NASA Marshall Space Flight Center\\
ES-81, Huntsville, AL 35812\\
\and
Paul J.\ Groot, Titus Galama\\
Astronomical Institute ``Anton Pannekoek,'' University of Amsterdam \&\\
Center for High Energy Astrophysics\\
Kruislaan 403, 1098 SJ Amsterdam, The Netherlands}

\tobe{29 May 1997}{Nature}
\recacc{9 April 1997}{24 April 1997}
\maketitle
\begin{abstract}
Although more than 2,000 astronomical gamma-ray bursts (GRBs) have been 
detected, and numerous models proposed to explain their occurrence, they have 
remained enigmatic owing to the lack of an obvious counterpart at other 
wavelengths.  The recent ground-based detection of a transient source in the 
vicinity of GRB~970228 may therefore have provided a breakthrough.  The 
optical counterpart appears to be embedded in an extended source which, if a 
galaxy as has been suggested, would lend weight to those models that place 
GRBs at cosmological distances.  Here we report the observations using the 
Hubble Space Telescope of the transient counterpart and extended source 26 and 
39 days after the initial $\gamma$-ray outburst.  We find that the counterpart 
has faded since the initial detection (and continues to fade), but the extended 
source exhibits no significant change in brightness between the two dates of 
observations reported here.  The size and apparent constancy between the two 
epochs of HST observations imply that it is extragalactic, but its faintness 
makes a definitive statement about its nature difficult.  Nevertheless, the 
decay profile of the transient source is consistent with a popular 
impulsive-fireball model, which assumes a merger between two neutron stars in 
a distant galaxy.
\end{abstract}
\end{document}