1999A&A...344L..67H

We reduced the Hubble Space Telescope Space Telescope Imaging Spectrograph images of the gamma-ray burst GRB 990123 that were obtained on 8-9 February 1999 and find V₀=25.36±0.10, which corresponds to a flux of 0.258±0.023µJy for the optical transient 16.644 days after the burst's peak. The probable host galaxy has V₀=24.25±0.07 (0.716±0.046µJy) and the optical transient is located 0.65" (=5.5kpc) south of the galaxy's nucleus. We fit and subtracted a scaled point-spread function to the optical transient and found evidence for three bright knots situated within 0.5" (=4.3kpc) of the optical transient. Each knot has V₀∼28.1±0.3, a rest-frame V-band luminosity of L_V∼5-8x10⁸L_☉, and a star-formation rate of at least ∼0.1-0.2M_☉/yr. The knots are centrally concentrated with full-width at half-maximum of ∼0.17" (=1.5kpc). Their sizes and luminosities are consistent with their being star-forming regions. The optical transient is located 0.15" (=1.3kpc) southeast of the centre of one of these knots.