HST/STIS Observations of the Optical Counterpart to GRB 970228

Andrew S. Fruchter, Elena Pian, Stephen E. Thorsett, Rosa Gonzalez, Kailash C. Sahu, Max Mutchler, Filippo Frontera, Titus Galama, Paul Groot, Richard Hook, Chryssa Kouveliotou, Mario Livio, Duccio Macchetto, Jan van Paradijs, Eliana Palazzi, Larry Petro, Marco Tavani

Published 1998-01-16, updated 1998-01-20Version 2

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made on 4~September~1997 using the STIS CCD on the Hubble Space Telescope. The unresolved counterpart is detected at V=28 +/- 0.25, consistent with a continued power-law decline with exponent -1.14 +/- 0.05. No proper motion is detected, in contradiction of an earlier claim. The counterpart is located within, but near the edge of, a faint extended source with diameter ~0."8 and integrated magnitude 25.7 +/- 0.25. Comparison with WFPC2 data taken one month after the initial burst and NTT data taken on March 13 shows no evidence for fading of the extended emission. After adjusting for the probable Galactic extinction in the direction of GRB 970228 of A_v=0.7, we find that the observed nebula is consistent with the sizes of galaxies of comparable magnitude found in the Hubble Deep Field and other deep HST images, and that only 2% of the sky is covered by galaxies of similar or greater surface brightness. We therefore conclude that the extended source observed about GRB 970228 is most likely a galaxy at moderate redshift, and is almost certainly the host of the gamma-ray burst.