Hubble Space Telescope Imaging of the Optical Transient Associated with GRB 970508
Piro, L.; Pian, E.; Palazzi, E.; Frontera, F.; Halpern, J.; Hurley, K.; Costa, E.; Fruchter, A. S.; Pedersen, H.; Thorsett, S. E.; Lucas, R. A.; Nicastro, L.; Feroci, M.; Castro-Tirado, A. J.; Sparks, W.; Tavani, M.; Bergeron, L. E.; Gull, T.
Italy, United States, Spain, Denmark
Abstract
We report on Hubble Space Telescope observations of the optical transient (OT) discovered in the error box of the gamma-ray burst GRB 970508. The object was imaged on 1997 June 2 with the Space Telescope Imaging Spectrograph and Near-Infrared Camera and Multiobject Spectrometer. The observations reveal a pointlike source with R = 23.1+/-0.2 and H = 20.6+/-0.3, in agreement with the power-law temporal decay seen in ground-based monitoring. Unlike the case of GRB 970228, no nebulosity is detected surrounding the OT of GRB 970508. We set very conservative upper limits of R~24.5 and H~22.2 on the brightness of any underlying extended source. If this subtends a substantial fraction of an arcsecond, then the R-band limit is ~25.5. In combination with Keck spectra that show Mg I absorption and [O II] emission at a redshift of z=0.835, our observations suggest that the OT is located in a star-forming galaxy with total luminosity 1 order of magnitude lower than the knee of the galaxy luminosity function, L*. Such galaxies are now thought to harbor the majority of star formation at z~1 therefore, these observations may provide support for a link between GRBs and star formation.