Star-forming regions near GRB 990123

Abstract

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 V_0 = 25.36 +/- 0.10, which corresponds to a flux of 0.258 +/- 0.023 mu Jy for the optical transient 16.644 days after the burst's peak. The probable host galaxy has V_0 = 24.25 +/- 0.07 (0.716 +/- 0.046 mu Jy) and the optical transient is located 0farcs65 (= 5.5 kpc) 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 0farcs5 (= 4.3 kpc) of the optical transient. Each knot has V_0 ~ 28.1 +/- 0.3, a rest-frame V-band luminosity of L_V ~ 5-8 x 10(8) L_⊙, and a star-formation rate of at least ~ 0.1-0.2 {cal M}_⊙ yr(-1) . The knots are centrally concentrated with full-width at half-maximum of ~ 0farcs17 (= 1.5 kpc). Their sizes and luminosities are consistent with their being star-forming regions. The optical transient is located 0farcs15 (= 1.3 kpc) southeast of the centre of one of these knots. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.