The Afterglow and Complex Environment of the Optically Dim Burst GRB 980613

We report the identification of the optical afterglow of GRB 980613 in R- andI-band images obtained between 16 and 48 hours after the gamma-ray burst. Earlynear-infrared (NIR) H and K' observations are also reported. The afterglow wasoptically faint (R ~ 23) at discovery but did not exhibit an unusually rapiddecay (power-law decay slope alpha < 1.8 at 2 sigma). The optical/NIR spectralindex (beta_RH < 1.1) was consistent with the optical-to-X-ray spectral index(beta_RX ~ 0.6), indicating a maximal reddening of the afterglow of ~0.45 magin R. Hence the dimness of the optical afterglow was mainly due to the fairlyflat spectral shape rather than internal reddening in the host galaxy. We alsopresent late-time HST/STIS images of the field in which GRB 980613 occurred,obtained 799 days after the burst. These images show that GRB 980613 waslocated close to a very compact, blue V = 26.1 object inside a complex regionconsisting of star-forming knots and/or interacting galaxy fragments.Therefore, GRB 980613 constitutes a strong case for the association ofcosmological gamma-ray bursts with star-forming regions.Comment: 22 pages, 5 figures, ApJ, in pres