Hubble Space Telescope Imaging of the field of GRB970508

Abstract

We report on Hubble Space Telescope (HST) observations of the optical transient (OT) discovered in the error box of GRB970508. The object was imaged on 1997 June 2 with the Space Telescope Imaging Spectrograph (STIS) and Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). The observations reveal a point-like source with R=23.1±0.2 and H=20.6±0.3, in agreement with the power-law temporal decay seen in previous ground-based monitoring. Unlike the case of GRB970228, no nebulosity is detected surrounding the OT of GRB970508, although Mg I absorption and [O II] emission seen in Keck spectra at a redshift of z=0.835 suggest the presence of a dense, star-forming medium. The HST observations 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. Subsequent photometry suggests a flattening of the light curve at later epochs. Assuming the OT decline follows a pure power-law and ascribing the flattening to the presence of an underlying component of constant flux, we find that this must have R=25.4, consistent with the upper limits determined by HST. At z=0.8, this would correspond to an absolute magnitude in the U band of ∼−18, similar to that of the Large Magellanic Cloud (LMC). We propose a scenario in which the host galaxy of the GRB is of Magellanic type, possibly being a “satellite” of one of the bright galaxies located at few arcseconds from the OT.